[2/3] x86/speculation: Support Automatic IBRS

  The AMD Zen4 core supports a new feature called Automatic IBRS.

It is a "set-and-forget" feature that means that, unlike e.g.,
s/w-toggled SPEC_CTRL.IBRS, h/w manages its IBRS mitigation
resources automatically across CPL transitions.

The feature is advertised by CPUID_Fn80000021_EAX bit 8 and is
enabled by setting MSR C000_0080 (EFER) bit 21.

Enable Automatic IBRS by default if the CPU feature is present.
It typically provides greater performance over the incumbent
generic retpolines mitigation.  In addition:

 - Don't clear the RSB on VMEXIT when AutoIBRS is enabled:
   The internal return address stack used for return address
   predictions is automatically cleared on VMEXIT.

 - Automatic IBRS removes the need for toggling IBRS during
   firmware switches, so don't enable IBRS_FW when Automatic
   IBRS is enabled.

 - Allow for spectre_v2=autoibrs in the kernel command line,
   reverting to auto-selection if the feature isn't available.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
 .../admin-guide/kernel-parameters.txt         |  1 +
 arch/x86/include/asm/msr-index.h              |  2 ++
 arch/x86/include/asm/nospec-branch.h          |  1 +
 arch/x86/kernel/cpu/bugs.c                    | 34 +++++++++++++++++--
 4 files changed, 35 insertions(+), 3 deletions(-)
  

On Fri, Nov 04, 2022 at 04:36:50PM -0500, Kim Phillips wrote:
>  - Allow for spectre_v2=autoibrs in the kernel command line,
>    reverting to auto-selection if the feature isn't available.

Why?

What the whole logic here should do is enable autoibrs when detected
automatically, without the need for the user to even select it as it is
the superior mitigation.
  

On Fri, Nov 04, 2022 at 10:52:13PM +0100, Borislav Petkov wrote:
> On Fri, Nov 04, 2022 at 04:36:50PM -0500, Kim Phillips wrote:
> >  - Allow for spectre_v2=autoibrs in the kernel command line,
> >    reverting to auto-selection if the feature isn't available.
> 
> Why?
> 
> What the whole logic here should do is enable autoibrs when detected
> automatically, without the need for the user to even select it as it is
> the superior mitigation.

Well; perhaps the whole autoibrs thing should be mapped to the existing
eIBRS options. AFAICT this is the same thing under a new name, no need
to invent yet more options. bugs.c is quite insane enough already.
  

From: Borislav Petkov
> Sent: 04 November 2022 21:52
> 
> On Fri, Nov 04, 2022 at 04:36:50PM -0500, Kim Phillips wrote:
> >  - Allow for spectre_v2=autoibrs in the kernel command line,
> >    reverting to auto-selection if the feature isn't available.
> 
> Why?
> 
> What the whole logic here should do is enable autoibrs when detected
> automatically, without the need for the user to even select it as it is
> the superior mitigation.

The only useful option is to allow a different option be
selected for code testing.
So maybe you want an option for completeness - for when
an 'even better' option is available.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
  

On 11/5/22 6:10 AM, Peter Zijlstra wrote:
> On Fri, Nov 04, 2022 at 10:52:13PM +0100, Borislav Petkov wrote:
>> On Fri, Nov 04, 2022 at 04:36:50PM -0500, Kim Phillips wrote:
>>>   - Allow for spectre_v2=autoibrs in the kernel command line,
>>>     reverting to auto-selection if the feature isn't available.
>>
>> Why?
>>
>> What the whole logic here should do is enable autoibrs when detected
>> automatically, without the need for the user to even select it as it is
>> the superior mitigation.
> 
> Well; perhaps the whole autoibrs thing should be mapped to the existing
> eIBRS options. AFAICT this is the same thing under a new name, no need
> to invent yet more options. bugs.c is quite insane enough already.

I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
spectre_v2_mitigation enum, but, so far, it's change to bugs.c
looks bigger: 58 lines changed vs. 34 (see below).

Let me know if you want me to send it as a part of a v2 submission
after I take care of the kvm CPUID review.

Thanks,

Kim

Autoibrs-as-eibrs diff:

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 2e9dd8823244..3ab90f23e7f7 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -442,7 +442,6 @@ enum spectre_v2_mitigation {
  	SPECTRE_V2_EIBRS_RETPOLINE,
  	SPECTRE_V2_EIBRS_LFENCE,
  	SPECTRE_V2_IBRS,
-	SPECTRE_V2_AUTO_IBRS,
  };

  /* The indirect branch speculation control variants */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 31e5af78baa0..ccfd8fb12095 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1005,6 +1005,7 @@ static inline const char *spectre_v2_module_string(void) { return ""; }
  #endif

  #define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
+#define SPECTRE_V2_EIBRS_AMD_MSG "WARNING: AutoIBRS does not need additional RETPOLINE/LFENCE mitigations, not doing them\n"
  #define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
  #define SPECTRE_V2_EIBRS_LFENCE_EBPF_SMT_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS+LFENCE mitigation and SMT, data leaks possible via Spectre v2 BHB attacks!\n"
  #define SPECTRE_V2_IBRS_PERF_MSG "WARNING: IBRS mitigation selected on Enhanced IBRS CPU, this may cause unnecessary performance loss\n"
@@ -1125,7 +1126,7 @@ spectre_v2_parse_user_cmdline(void)
  	return SPECTRE_V2_USER_CMD_AUTO;
  }

-/* Checks for Intel IBRS versions */
+/* Checks for IBRS versions */
  static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
  {
  	return mode == SPECTRE_V2_IBRS ||
@@ -1201,7 +1202,8 @@ spectre_v2_user_select_mitigation(void)
  	 */
  	if (!boot_cpu_has(X86_FEATURE_STIBP) ||
  	    !smt_possible ||
-	    spectre_v2_in_ibrs_mode(spectre_v2_enabled))
+	    (spectre_v2_in_ibrs_mode(spectre_v2_enabled) &&
+	     !boot_cpu_has(X86_FEATURE_AUTOIBRS)))
  		return;

  	/*
@@ -1231,11 +1233,10 @@ static const char * const spectre_v2_strings[] = {
  	[SPECTRE_V2_NONE]			= "Vulnerable",
  	[SPECTRE_V2_RETPOLINE]			= "Mitigation: Retpolines",
  	[SPECTRE_V2_LFENCE]			= "Mitigation: LFENCE",
-	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced IBRS",
+	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced / Automatic IBRS",
  	[SPECTRE_V2_EIBRS_LFENCE]		= "Mitigation: Enhanced IBRS + LFENCE",
  	[SPECTRE_V2_EIBRS_RETPOLINE]		= "Mitigation: Enhanced IBRS + Retpolines",
  	[SPECTRE_V2_IBRS]			= "Mitigation: IBRS",
-	[SPECTRE_V2_AUTO_IBRS]			= "Mitigation: Automatic IBRS",
  };

  static const struct {
@@ -1250,9 +1251,9 @@ static const struct {
  	{ "retpoline,lfence",	SPECTRE_V2_CMD_RETPOLINE_LFENCE,  false },
  	{ "retpoline,generic",	SPECTRE_V2_CMD_RETPOLINE_GENERIC, false },
  	{ "eibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
+	{ "autoibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
  	{ "eibrs,lfence",	SPECTRE_V2_CMD_EIBRS_LFENCE,	  false },
  	{ "eibrs,retpoline",	SPECTRE_V2_CMD_EIBRS_RETPOLINE,	  false },
-	{ "autoibrs",		SPECTRE_V2_CMD_AUTOIBRS,	  false },
  	{ "auto",		SPECTRE_V2_CMD_AUTO,		  false },
  	{ "ibrs",		SPECTRE_V2_CMD_IBRS,              false },
  };
@@ -1303,15 +1304,17 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
  	if ((cmd == SPECTRE_V2_CMD_EIBRS ||
  	     cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
  	     cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
-	    !boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
-		pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
+	    (!boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) ||
+	     !boot_cpu_has(X86_FEATURE_AUTOIBRS))) {
+		pr_err("%s selected but CPU doesn't have Enhanced or Automatic IBRS. Switching to AUTO select\n",
  		       mitigation_options[i].option);
  		return SPECTRE_V2_CMD_AUTO;
  	}

-	if (cmd == SPECTRE_V2_CMD_AUTOIBRS &&
-	    !boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
-		pr_err("%s selected but CPU doesn't have AMD Automatic IBRS. Switching to AUTO select\n",
+	if ((cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
+	     cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
+	    boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+		pr_err("%s selected but AMD Automatic IBRS doesn't need extra retpoline mitigations. Switching to AUTO select\n",
  		       mitigation_options[i].option);
  		return SPECTRE_V2_CMD_AUTO;
  	}
@@ -1403,7 +1406,6 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
  	 */
  	switch (mode) {
  	case SPECTRE_V2_NONE:
-	case SPECTRE_V2_AUTO_IBRS:
  		return;

  	case SPECTRE_V2_EIBRS_LFENCE:
@@ -1447,12 +1449,8 @@ static void __init spectre_v2_select_mitigation(void)

  	case SPECTRE_V2_CMD_FORCE:
  	case SPECTRE_V2_CMD_AUTO:
-		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
-			mode = SPECTRE_V2_AUTO_IBRS;
-			break;
-		}
-
-		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
+		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) ||
+		    boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
  			mode = SPECTRE_V2_EIBRS;
  			break;
  		}
@@ -1488,19 +1486,24 @@ static void __init spectre_v2_select_mitigation(void)
  		break;

  	case SPECTRE_V2_CMD_EIBRS:
+	case SPECTRE_V2_CMD_AUTOIBRS:
  		mode = SPECTRE_V2_EIBRS;
  		break;

  	case SPECTRE_V2_CMD_EIBRS_LFENCE:
-		mode = SPECTRE_V2_EIBRS_LFENCE;
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
+			mode = SPECTRE_V2_EIBRS;
+		} else
+			mode = SPECTRE_V2_EIBRS_LFENCE;
  		break;

  	case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
-		mode = SPECTRE_V2_EIBRS_RETPOLINE;
-		break;
-
-	case SPECTRE_V2_CMD_AUTOIBRS:
-		mode = SPECTRE_V2_AUTO_IBRS;
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
+			mode = SPECTRE_V2_EIBRS;
+		} else
+			mode = SPECTRE_V2_EIBRS_RETPOLINE;
  		break;
  	}

@@ -1508,8 +1511,13 @@ static void __init spectre_v2_select_mitigation(void)
  		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);

  	if (spectre_v2_in_ibrs_mode(mode)) {
-		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			rdmsrl(MSR_EFER, efer);
+			wrmsrl(MSR_EFER, efer | EFER_AUTOIBRS);
+		} else {
+			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
+			write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		}
  	}

  	switch (mode) {
@@ -1517,11 +1525,6 @@ static void __init spectre_v2_select_mitigation(void)
  	case SPECTRE_V2_EIBRS:
  		break;

-	case SPECTRE_V2_AUTO_IBRS:
-		rdmsrl(MSR_EFER, efer);
-		wrmsrl(MSR_EFER, efer | EFER_AUTOIBRS);
-		break;
-
  	case SPECTRE_V2_IBRS:
  		setup_force_cpu_cap(X86_FEATURE_KERNEL_IBRS);
  		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED))
@@ -1616,8 +1619,8 @@ static void __init spectre_v2_select_mitigation(void)
  			pr_info("Enabling Speculation Barrier for firmware calls\n");
  		}

-	} else if (boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode) &&
-		   mode != SPECTRE_V2_AUTO_IBRS) {
+	} else if ((boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode)) ||
+		   (boot_cpu_has(X86_FEATURE_AUTOIBRS) && !spectre_v2_in_ibrs_mode(mode))) {
  		setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
  		pr_info("Enabling Restricted Speculation for firmware calls\n");
  	}
@@ -2353,7 +2356,8 @@ static ssize_t mmio_stale_data_show_state(char *buf)

  static char *stibp_state(void)
  {
-	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled))
+	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled) &&
+	    !boot_cpu_has(X86_FEATURE_AUTOIBRS))
  		return "";

  	switch (spectre_v2_user_stibp) {
  

On 11/5/22 6:39 AM, David Laight wrote:
> From: Borislav Petkov
>> Sent: 04 November 2022 21:52
>>
>> On Fri, Nov 04, 2022 at 04:36:50PM -0500, Kim Phillips wrote:
>>>   - Allow for spectre_v2=autoibrs in the kernel command line,
>>>     reverting to auto-selection if the feature isn't available.
>>
>> Why?
>>
>> What the whole logic here should do is enable autoibrs when detected
>> automatically, without the need for the user to even select it as it is
>> the superior mitigation.
> 
> The only useful option is to allow a different option be
> selected for code testing.
> So maybe you want an option for completeness - for when
> an 'even better' option is available.

This is true.

Kim
  

On 11/7/22 14:39, Kim Phillips wrote:
> I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
> spectre_v2_mitigation enum, but, so far, it's change to bugs.c
> looks bigger: 58 lines changed vs. 34 (see below).
> 
> Let me know if you want me to send it as a part of a v2 submission
> after I take care of the kvm CPUID review.

Thanks for putting that together.  I generally like how this looks.

I think it probably goes to a _bit_ too much trouble to turn off
"eibrs,lfence/retpoline".  If someone goes to the trouble of specifying
those, a warning or pr_info() is probably enough.  You don't need to
actively override it.

> -    } else if (boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode) &&
> -           mode != SPECTRE_V2_AUTO_IBRS) {
> +    } else if ((boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode)) ||
> +           (boot_cpu_has(X86_FEATURE_AUTOIBRS) && !spectre_v2_in_ibrs_mode(mode))) {
>          setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
>          pr_info("Enabling Restricted Speculation for firmware calls\n"); 

Did the "mode != SPECTRE_V2_AUTO_IBRS" check get dropped accidentally?
Or is it unnecessary now?
  

On Mon, Nov 07, 2022 at 03:41:03PM -0800, Dave Hansen wrote:
> On 11/7/22 14:39, Kim Phillips wrote:
> > I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
> > spectre_v2_mitigation enum, but, so far, it's change to bugs.c
> > looks bigger: 58 lines changed vs. 34 (see below).
> > 
> > Let me know if you want me to send it as a part of a v2 submission
> > after I take care of the kvm CPUID review.
> 
> Thanks for putting that together.  I generally like how this looks.
> 
> I think it probably goes to a _bit_ too much trouble to turn off
> "eibrs,lfence/retpoline".  If someone goes to the trouble of specifying
> those, a warning or pr_info() is probably enough.  You don't need to
> actively override it.

Not, even, just do it. User told you to, it's not technically
impossible, so just go.
  

On Mon, Nov 07, 2022 at 04:39:02PM -0600, Kim Phillips wrote:
> I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
> spectre_v2_mitigation enum, but, so far, it's change to bugs.c
> looks bigger: 58 lines changed vs. 34 (see below).

It can be smaller. You simply do:

	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);

and the rest should just work - see below.

And yes, as Peter says, when the user requests something, the user
should get it. No matter whether it makes sense or not.

Thx.

---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 66d7addf1784..2b77eaee9bd2 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1005,6 +1005,7 @@ static inline const char *spectre_v2_module_string(void) { return ""; }
 #endif
 
 #define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
+#define SPECTRE_V2_EIBRS_AMD_MSG "WARNING: AutoIBRS does not need additional RETPOLINE/LFENCE mitigations, not doing them\n"
 #define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
 #define SPECTRE_V2_EIBRS_LFENCE_EBPF_SMT_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS+LFENCE mitigation and SMT, data leaks possible via Spectre v2 BHB attacks!\n"
 #define SPECTRE_V2_IBRS_PERF_MSG "WARNING: IBRS mitigation selected on Enhanced IBRS CPU, this may cause unnecessary performance loss\n"
@@ -1124,6 +1125,7 @@ spectre_v2_parse_user_cmdline(void)
 	return SPECTRE_V2_USER_CMD_AUTO;
 }
 
+/* Checks for IBRS versions */
 static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
 {
 	return mode == SPECTRE_V2_IBRS ||
@@ -1229,7 +1231,7 @@ static const char * const spectre_v2_strings[] = {
 	[SPECTRE_V2_NONE]			= "Vulnerable",
 	[SPECTRE_V2_RETPOLINE]			= "Mitigation: Retpolines",
 	[SPECTRE_V2_LFENCE]			= "Mitigation: LFENCE",
-	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced IBRS",
+	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced / Automatic IBRS",
 	[SPECTRE_V2_EIBRS_LFENCE]		= "Mitigation: Enhanced IBRS + LFENCE",
 	[SPECTRE_V2_EIBRS_RETPOLINE]		= "Mitigation: Enhanced IBRS + Retpolines",
 	[SPECTRE_V2_IBRS]			= "Mitigation: IBRS",
@@ -1247,6 +1249,7 @@ static const struct {
 	{ "retpoline,lfence",	SPECTRE_V2_CMD_RETPOLINE_LFENCE,  false },
 	{ "retpoline,generic",	SPECTRE_V2_CMD_RETPOLINE_GENERIC, false },
 	{ "eibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
+	{ "autoibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
 	{ "eibrs,lfence",	SPECTRE_V2_CMD_EIBRS_LFENCE,	  false },
 	{ "eibrs,retpoline",	SPECTRE_V2_CMD_EIBRS_RETPOLINE,	  false },
 	{ "auto",		SPECTRE_V2_CMD_AUTO,		  false },
@@ -1300,7 +1303,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	     cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
 	     cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
 	    !boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
-		pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
+		pr_err("%s selected but CPU doesn't have Enhanced or Automatic IBRS. Switching to AUTO select\n",
 		       mitigation_options[i].option);
 		return SPECTRE_V2_CMD_AUTO;
 	}
@@ -1474,11 +1477,19 @@ static void __init spectre_v2_select_mitigation(void)
 		break;
 
 	case SPECTRE_V2_CMD_EIBRS_LFENCE:
-		mode = SPECTRE_V2_EIBRS_LFENCE;
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
+			mode = SPECTRE_V2_EIBRS;
+		} else
+			mode = SPECTRE_V2_EIBRS_LFENCE;
 		break;
 
 	case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
-		mode = SPECTRE_V2_EIBRS_RETPOLINE;
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
+			mode = SPECTRE_V2_EIBRS;
+		} else
+			mode = SPECTRE_V2_EIBRS_RETPOLINE;
 		break;
 	}
 
@@ -1486,8 +1497,12 @@ static void __init spectre_v2_select_mitigation(void)
 		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
 
 	if (spectre_v2_in_ibrs_mode(mode)) {
-		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
+		} else {
+			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
+			write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		}
 	}
 
 	switch (mode) {
@@ -1571,8 +1586,8 @@ static void __init spectre_v2_select_mitigation(void)
 	/*
 	 * Retpoline protects the kernel, but doesn't protect firmware.  IBRS
 	 * and Enhanced IBRS protect firmware too, so enable IBRS around
-	 * firmware calls only when IBRS / Enhanced IBRS aren't otherwise
-	 * enabled.
+	 * firmware calls only when IBRS / Enhanced / Automatic IBRS aren't
+	 * otherwise enabled.
 	 *
 	 * Use "mode" to check Enhanced IBRS instead of boot_cpu_has(), because
 	 * the user might select retpoline on the kernel command line and if
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 73cc546e024d..45e3670bdaaf 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1341,6 +1344,10 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	if (ia32_cap & ARCH_CAP_IBRS_ALL)
 		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
 
+	/* AMDs AutoIBRS is equivalent to Intel's eIBRS - use the Intel flag. */
+	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
+		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
+
 	if (!cpu_matches(cpu_vuln_whitelist, NO_MDS) &&
 	    !(ia32_cap & ARCH_CAP_MDS_NO)) {
 		setup_force_cpu_bug(X86_BUG_MDS);
  

On Fri, Nov 11, 2022 at 01:09:37PM +0100, Borislav Petkov wrote:
> On Mon, Nov 07, 2022 at 04:39:02PM -0600, Kim Phillips wrote:
> > I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
> > spectre_v2_mitigation enum, but, so far, it's change to bugs.c
> > looks bigger: 58 lines changed vs. 34 (see below).
> 
> It can be smaller. You simply do:
> 
> 	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
> 		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
> 
> and the rest should just work - see below.
> 
> And yes, as Peter says, when the user requests something, the user
> should get it. No matter whether it makes sense or not.
> 
> Thx.
> 
> ---
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 66d7addf1784..2b77eaee9bd2 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1005,6 +1005,7 @@ static inline const char *spectre_v2_module_string(void) { return ""; }
>  #endif
>  
>  #define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
> +#define SPECTRE_V2_EIBRS_AMD_MSG "WARNING: AutoIBRS does not need additional RETPOLINE/LFENCE mitigations, not doing them\n"
>  #define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
>  #define SPECTRE_V2_EIBRS_LFENCE_EBPF_SMT_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS+LFENCE mitigation and SMT, data leaks possible via Spectre v2 BHB attacks!\n"
>  #define SPECTRE_V2_IBRS_PERF_MSG "WARNING: IBRS mitigation selected on Enhanced IBRS CPU, this may cause unnecessary performance loss\n"
> @@ -1124,6 +1125,7 @@ spectre_v2_parse_user_cmdline(void)
>  	return SPECTRE_V2_USER_CMD_AUTO;
>  }
>  
> +/* Checks for IBRS versions */
>  static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
>  {
>  	return mode == SPECTRE_V2_IBRS ||
> @@ -1229,7 +1231,7 @@ static const char * const spectre_v2_strings[] = {
>  	[SPECTRE_V2_NONE]			= "Vulnerable",
>  	[SPECTRE_V2_RETPOLINE]			= "Mitigation: Retpolines",
>  	[SPECTRE_V2_LFENCE]			= "Mitigation: LFENCE",
> -	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced IBRS",
> +	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced / Automatic IBRS",
>  	[SPECTRE_V2_EIBRS_LFENCE]		= "Mitigation: Enhanced IBRS + LFENCE",
>  	[SPECTRE_V2_EIBRS_RETPOLINE]		= "Mitigation: Enhanced IBRS + Retpolines",
>  	[SPECTRE_V2_IBRS]			= "Mitigation: IBRS",
> @@ -1247,6 +1249,7 @@ static const struct {
>  	{ "retpoline,lfence",	SPECTRE_V2_CMD_RETPOLINE_LFENCE,  false },
>  	{ "retpoline,generic",	SPECTRE_V2_CMD_RETPOLINE_GENERIC, false },
>  	{ "eibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
> +	{ "autoibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
>  	{ "eibrs,lfence",	SPECTRE_V2_CMD_EIBRS_LFENCE,	  false },
>  	{ "eibrs,retpoline",	SPECTRE_V2_CMD_EIBRS_RETPOLINE,	  false },
>  	{ "auto",		SPECTRE_V2_CMD_AUTO,		  false },
> @@ -1300,7 +1303,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
>  	     cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
>  	     cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
>  	    !boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
> -		pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
> +		pr_err("%s selected but CPU doesn't have Enhanced or Automatic IBRS. Switching to AUTO select\n",
>  		       mitigation_options[i].option);
>  		return SPECTRE_V2_CMD_AUTO;
>  	}
> @@ -1474,11 +1477,19 @@ static void __init spectre_v2_select_mitigation(void)
>  		break;
>  
>  	case SPECTRE_V2_CMD_EIBRS_LFENCE:
> -		mode = SPECTRE_V2_EIBRS_LFENCE;
> +		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> +			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
> +			mode = SPECTRE_V2_EIBRS;
> +		} else
> +			mode = SPECTRE_V2_EIBRS_LFENCE;
>  		break;
>  
>  	case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
> -		mode = SPECTRE_V2_EIBRS_RETPOLINE;
> +		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> +			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
> +			mode = SPECTRE_V2_EIBRS;
> +		} else
> +			mode = SPECTRE_V2_EIBRS_RETPOLINE;
>  		break;
>  	}
>  

I am confused here. Isn't the agreement that the user should get what they
asked for? That is, instead of warning and changing the mode to
SPECTRE_V2_EIBRS, the kernel should still use lfence or retpoline as requested?

The point of those options was to protect against Branch History Injection
attacks and Intra-Mode Branch Target Injection attacks. The first one might not
affect the CPUs that support AUTOIBRS, though we haven't heard that.

The second one (IMBTI) is very likely still possible with AUTOIBRS and
retpolines should still protect against those attacks. So users who want to be
paranoid should still be able to opt for "eibrs,retpoline" and have retpolines
enabled.

Cascardo.

> @@ -1486,8 +1497,12 @@ static void __init spectre_v2_select_mitigation(void)
>  		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
>  
>  	if (spectre_v2_in_ibrs_mode(mode)) {
> -		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
> -		write_spec_ctrl_current(x86_spec_ctrl_base, true);
> +		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> +			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
> +		} else {
> +			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
> +			write_spec_ctrl_current(x86_spec_ctrl_base, true);
> +		}
>  	}
>  
>  	switch (mode) {
> @@ -1571,8 +1586,8 @@ static void __init spectre_v2_select_mitigation(void)
>  	/*
>  	 * Retpoline protects the kernel, but doesn't protect firmware.  IBRS
>  	 * and Enhanced IBRS protect firmware too, so enable IBRS around
> -	 * firmware calls only when IBRS / Enhanced IBRS aren't otherwise
> -	 * enabled.
> +	 * firmware calls only when IBRS / Enhanced / Automatic IBRS aren't
> +	 * otherwise enabled.
>  	 *
>  	 * Use "mode" to check Enhanced IBRS instead of boot_cpu_has(), because
>  	 * the user might select retpoline on the kernel command line and if
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 73cc546e024d..45e3670bdaaf 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1341,6 +1344,10 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
>  	if (ia32_cap & ARCH_CAP_IBRS_ALL)
>  		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
>  
> +	/* AMDs AutoIBRS is equivalent to Intel's eIBRS - use the Intel flag. */
> +	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
> +		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
> +
>  	if (!cpu_matches(cpu_vuln_whitelist, NO_MDS) &&
>  	    !(ia32_cap & ARCH_CAP_MDS_NO)) {
>  		setup_force_cpu_bug(X86_BUG_MDS);
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette
  

On 11/11/22 6:40 AM, Thadeu Lima de Souza Cascardo wrote:
> On Fri, Nov 11, 2022 at 01:09:37PM +0100, Borislav Petkov wrote:
>> On Mon, Nov 07, 2022 at 04:39:02PM -0600, Kim Phillips wrote:
>>> I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
>>> spectre_v2_mitigation enum, but, so far, it's change to bugs.c
>>> looks bigger: 58 lines changed vs. 34 (see below).
>>
>> It can be smaller. You simply do:
>>
>> 	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
>> 		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
>>
>> and the rest should just work - see below.
>>
>> And yes, as Peter says, when the user requests something, the user
>> should get it. No matter whether it makes sense or not.

OK & thanks.

>> @@ -1474,11 +1477,19 @@ static void __init spectre_v2_select_mitigation(void)
>>   		break;
>>   
>>   	case SPECTRE_V2_CMD_EIBRS_LFENCE:
>> -		mode = SPECTRE_V2_EIBRS_LFENCE;
>> +		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
>> +			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
>> +			mode = SPECTRE_V2_EIBRS;
>> +		} else
>> +			mode = SPECTRE_V2_EIBRS_LFENCE;
>>   		break;
>>   
>>   	case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
>> -		mode = SPECTRE_V2_EIBRS_RETPOLINE;
>> +		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
>> +			pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
>> +			mode = SPECTRE_V2_EIBRS;
>> +		} else
>> +			mode = SPECTRE_V2_EIBRS_RETPOLINE;
>>   		break;
>>   	}
>>   
> 
> I am confused here. Isn't the agreement that the user should get what they
> asked for? That is, instead of warning and changing the mode to
> SPECTRE_V2_EIBRS, the kernel should still use lfence or retpoline as requested?
> 
> The point of those options was to protect against Branch History Injection
> attacks and Intra-Mode Branch Target Injection attacks. The first one might not
> affect the CPUs that support AUTOIBRS, though we haven't heard that.
> 
> The second one (IMBTI) is very likely still possible with AUTOIBRS and
> retpolines should still protect against those attacks. So users who want to be
> paranoid should still be able to opt for "eibrs,retpoline" and have retpolines
> enabled.

I've removed the above and have the complete diff below.  It includes patch 1/3 and
drops 3/3 for now due to Jim Mattson's comments.  After some more testing, I'll
resubmit.

Thanks,

Kim


diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a465d5242774..b260a36dc3ef 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5698,9 +5698,10 @@
  			retpoline,generic - Retpolines
  			retpoline,lfence  - LFENCE; indirect branch
  			retpoline,amd     - alias for retpoline,lfence
-			eibrs		  - enhanced IBRS
-			eibrs,retpoline   - enhanced IBRS + Retpolines
-			eibrs,lfence      - enhanced IBRS + LFENCE
+			eibrs		  - Enhanced/Auto IBRS
+			autoibrs	  - Enhanced/Auto IBRS
+			eibrs,retpoline   - Enhanced/Auto IBRS + Retpolines
+			eibrs,lfence      - Enhanced/Auto IBRS + LFENCE
  			ibrs		  - use IBRS to protect kernel
  
  			Not specifying this option is equivalent to
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 97669aaf1202..ec9a4eb8e7b9 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -84,7 +84,7 @@
  
  /* CPU types for specific tunings: */
  #define X86_FEATURE_K8			( 3*32+ 4) /* "" Opteron, Athlon64 */
-/* FREE, was #define X86_FEATURE_K7			( 3*32+ 5) "" Athlon */
+#define X86_FEATURE_AUTOIBRS		( 3*32+ 5) /* AMD Automatic IBRS */
  #define X86_FEATURE_P3			( 3*32+ 6) /* "" P3 */
  #define X86_FEATURE_P4			( 3*32+ 7) /* "" P4 */
  #define X86_FEATURE_CONSTANT_TSC	( 3*32+ 8) /* TSC ticks at a constant rate */
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index a3eb4d3e70b8..56e4f3aab31c 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -30,6 +30,7 @@
  #define _EFER_SVME		12 /* Enable virtualization */
  #define _EFER_LMSLE		13 /* Long Mode Segment Limit Enable */
  #define _EFER_FFXSR		14 /* Enable Fast FXSAVE/FXRSTOR */
+#define _EFER_AUTOIBRS		21 /* Enable Automatic IBRS */
  
  #define EFER_SCE		(1<<_EFER_SCE)
  #define EFER_LME		(1<<_EFER_LME)
@@ -38,6 +39,7 @@
  #define EFER_SVME		(1<<_EFER_SVME)
  #define EFER_LMSLE		(1<<_EFER_LMSLE)
  #define EFER_FFXSR		(1<<_EFER_FFXSR)
+#define EFER_AUTOIBRS		(1<<_EFER_AUTOIBRS)
  
  /* Intel MSRs. Some also available on other CPUs */
  
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 66d7addf1784..4060ca8c2c60 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1229,7 +1229,7 @@ static const char * const spectre_v2_strings[] = {
  	[SPECTRE_V2_NONE]			= "Vulnerable",
  	[SPECTRE_V2_RETPOLINE]			= "Mitigation: Retpolines",
  	[SPECTRE_V2_LFENCE]			= "Mitigation: LFENCE",
-	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced IBRS",
+	[SPECTRE_V2_EIBRS]			= "Mitigation: Enhanced / Automatic IBRS",
  	[SPECTRE_V2_EIBRS_LFENCE]		= "Mitigation: Enhanced IBRS + LFENCE",
  	[SPECTRE_V2_EIBRS_RETPOLINE]		= "Mitigation: Enhanced IBRS + Retpolines",
  	[SPECTRE_V2_IBRS]			= "Mitigation: IBRS",
@@ -1247,6 +1247,7 @@ static const struct {
  	{ "retpoline,lfence",	SPECTRE_V2_CMD_RETPOLINE_LFENCE,  false },
  	{ "retpoline,generic",	SPECTRE_V2_CMD_RETPOLINE_GENERIC, false },
  	{ "eibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
+	{ "autoibrs",		SPECTRE_V2_CMD_EIBRS,		  false },
  	{ "eibrs,lfence",	SPECTRE_V2_CMD_EIBRS_LFENCE,	  false },
  	{ "eibrs,retpoline",	SPECTRE_V2_CMD_EIBRS_RETPOLINE,	  false },
  	{ "auto",		SPECTRE_V2_CMD_AUTO,		  false },
@@ -1300,7 +1301,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
  	     cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
  	     cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
  	    !boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
-		pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
+		pr_err("%s selected but CPU doesn't have Enhanced or Automatic IBRS. Switching to AUTO select\n",
  		       mitigation_options[i].option);
  		return SPECTRE_V2_CMD_AUTO;
  	}
@@ -1486,8 +1487,12 @@ static void __init spectre_v2_select_mitigation(void)
  		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
  
  	if (spectre_v2_in_ibrs_mode(mode)) {
-		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-		write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
+		} else {
+			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
+			write_spec_ctrl_current(x86_spec_ctrl_base, true);
+		}
  	}
  
  	switch (mode) {
@@ -1571,8 +1576,8 @@ static void __init spectre_v2_select_mitigation(void)
  	/*
  	 * Retpoline protects the kernel, but doesn't protect firmware.  IBRS
  	 * and Enhanced IBRS protect firmware too, so enable IBRS around
-	 * firmware calls only when IBRS / Enhanced IBRS aren't otherwise
-	 * enabled.
+	 * firmware calls only when IBRS / Enhanced / Automatic IBRS aren't
+	 * otherwise enabled.
  	 *
  	 * Use "mode" to check Enhanced IBRS instead of boot_cpu_has(), because
  	 * the user might select retpoline on the kernel command line and if
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 423a760fa9de..287b356ccf92 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1340,6 +1340,10 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
  	if (ia32_cap & ARCH_CAP_IBRS_ALL)
  		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
  
+	/* AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel flag. */
+	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
+		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
+
  	if (!cpu_matches(cpu_vuln_whitelist, NO_MDS) &&
  	    !(ia32_cap & ARCH_CAP_MDS_NO)) {
  		setup_force_cpu_bug(X86_BUG_MDS);
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index f53944fb8f7f..cef8c3e688b4 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -45,8 +45,10 @@ static const struct cpuid_bit cpuid_bits[] = {
  	{ X86_FEATURE_CPB,		CPUID_EDX,  9, 0x80000007, 0 },
  	{ X86_FEATURE_PROC_FEEDBACK,    CPUID_EDX, 11, 0x80000007, 0 },
  	{ X86_FEATURE_MBA,		CPUID_EBX,  6, 0x80000008, 0 },
+	{ X86_FEATURE_AUTOIBRS,		CPUID_EAX, 20, 0x80000021, 0 },
  	{ X86_FEATURE_PERFMON_V2,	CPUID_EAX,  0, 0x80000022, 0 },
  	{ X86_FEATURE_AMD_LBR_V2,	CPUID_EAX,  1, 0x80000022, 0 },
+
  	{ 0, 0, 0, 0, 0 }
  };
  

On Fri, Nov 11, 2022 at 4:46 PM Kim Phillips <kim.phillips@amd.com> wrote:
>
> On 11/11/22 6:40 AM, Thadeu Lima de Souza Cascardo wrote:
> > On Fri, Nov 11, 2022 at 01:09:37PM +0100, Borislav Petkov wrote:
> >> On Mon, Nov 07, 2022 at 04:39:02PM -0600, Kim Phillips wrote:
> >>> I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
> >>> spectre_v2_mitigation enum, but, so far, it's change to bugs.c
> >>> looks bigger: 58 lines changed vs. 34 (see below).
> >>
> >> It can be smaller. You simply do:
> >>
> >>      if (cpu_has(c, X86_FEATURE_AUTOIBRS))
> >>              setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
> >>
> >> and the rest should just work - see below.
> >>
> >> And yes, as Peter says, when the user requests something, the user
> >> should get it. No matter whether it makes sense or not.
>
> OK & thanks.
>
> >> @@ -1474,11 +1477,19 @@ static void __init spectre_v2_select_mitigation(void)
> >>              break;
> >>
> >>      case SPECTRE_V2_CMD_EIBRS_LFENCE:
> >> -            mode = SPECTRE_V2_EIBRS_LFENCE;
> >> +            if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> >> +                    pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
> >> +                    mode = SPECTRE_V2_EIBRS;
> >> +            } else
> >> +                    mode = SPECTRE_V2_EIBRS_LFENCE;
> >>              break;
> >>
> >>      case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
> >> -            mode = SPECTRE_V2_EIBRS_RETPOLINE;
> >> +            if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> >> +                    pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
> >> +                    mode = SPECTRE_V2_EIBRS;
> >> +            } else
> >> +                    mode = SPECTRE_V2_EIBRS_RETPOLINE;
> >>              break;
> >>      }
> >>
> >
> > I am confused here. Isn't the agreement that the user should get what they
> > asked for? That is, instead of warning and changing the mode to
> > SPECTRE_V2_EIBRS, the kernel should still use lfence or retpoline as requested?
> >
> > The point of those options was to protect against Branch History Injection
> > attacks and Intra-Mode Branch Target Injection attacks. The first one might not
> > affect the CPUs that support AUTOIBRS, though we haven't heard that.
> >
> > The second one (IMBTI) is very likely still possible with AUTOIBRS and
> > retpolines should still protect against those attacks. So users who want to be
> > paranoid should still be able to opt for "eibrs,retpoline" and have retpolines
> > enabled.
>
> I've removed the above and have the complete diff below.  It includes patch 1/3 and
> drops 3/3 for now due to Jim Mattson's comments.  After some more testing, I'll
> resubmit.

I bought the argument that AutoIBRS => Same Mode IBRS, so L2 should
not be able to steer L1's indirect branches, even if they share a
predictor mode.