diff mbox series

[v7,34/41] x86/shstk: Support WRSS for userspace

Message ID 20230227222957.24501-35-rick.p.edgecombe@intel.com
State New
Headers
Series Shadow stacks for userspace |

Commit Message

Edgecombe, Rick P Feb. 27, 2023, 10:29 p.m. UTC 
  For the current shadow stack implementation, shadow stacks contents can't
easily be provisioned with arbitrary data. This property helps apps
protect themselves better, but also restricts any potential apps that may
want to do exotic things at the expense of a little security.

The x86 shadow stack feature introduces a new instruction, WRSS, which
can be enabled to write directly to shadow stack permissioned memory from
userspace. Allow it to get enabled via the prctl interface.

Only enable the userspace WRSS instruction, which allows writes to
userspace shadow stacks from userspace. Do not allow it to be enabled
independently of shadow stack, as HW does not support using WRSS when
shadow stack is disabled.

From a fault handler perspective, WRSS will behave very similar to WRUSS,
which is treated like a user access from a #PF err code perspective.

Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---
v6:
 - Make set_clr_bits_msrl() avoid side affects in 'msr'

v5:
 - Switch to EOPNOTSUPP
 - Move set_clr_bits_msrl() to patch where it is first used
 - Commit log formatting

v3:
 - Make wrss_control() static
 - Fix verbiage in commit log (Kees)

v2:
 - Add some commit log verbiage from (Dave Hansen)

v1:
 - New patch.
---
 arch/x86/include/asm/msr.h        | 11 +++++++++++
 arch/x86/include/uapi/asm/prctl.h |  1 +
 arch/x86/kernel/shstk.c           | 32 ++++++++++++++++++++++++++++++-
 3 files changed, 43 insertions(+), 1 deletion(-)

Comments

Borislav Petkov March 10, 2023, 4:44 p.m. UTC | #1 
On Mon, Feb 27, 2023 at 02:29:50PM -0800, Rick Edgecombe wrote:
> For the current shadow stack implementation, shadow stacks contents can't
> easily be provisioned with arbitrary data. This property helps apps
> protect themselves better, but also restricts any potential apps that may
> want to do exotic things at the expense of a little security.
> 
> The x86 shadow stack feature introduces a new instruction, WRSS, which
> can be enabled to write directly to shadow stack permissioned memory from

s/permissioned //

By now it is clear that shadow stack memory is a special thing anyway.

> userspace. Allow it to get enabled via the prctl interface.
> 
> Only enable the userspace WRSS instruction, which allows writes to
> userspace shadow stacks from userspace. Do not allow it to be enabled
> independently of shadow stack, as HW does not support using WRSS when
> shadow stack is disabled.
> 
> From a fault handler perspective, WRSS will behave very similar to WRUSS,
> which is treated like a user access from a #PF err code perspective.

...

> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 65ec1965cd28..2d3b35c957ad 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -310,6 +310,17 @@ void msrs_free(struct msr *msrs);
>  int msr_set_bit(u32 msr, u8 bit);
>  int msr_clear_bit(u32 msr, u8 bit);
>  
> +/* Helper that can never get accidentally un-inlined. */
> +#define set_clr_bits_msrl(msr, set, clear)	do {	\

Uff, pls kill this thing.

Our MSR interfaces universe is already insane and arch/x86/lib/msr.c
already has similar attempts to what you're doing here in addition to
all the other gunk in msr.h.

I highly doubt this can't be done the usual way, lemme see...

> +	u64 __val, __new_val, __msr = msr;		\
> +							\
> +	rdmsrl(__msr, __val);				\
> +	__new_val = (__val & ~(clear)) | (set);		\
> +							\
> +	if (__new_val != __val)				\
> +		wrmsrl(__msr, __new_val);		\
> +} while (0)
> +
>  #ifdef CONFIG_SMP
>  int rdmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h);
>  int wrmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 l, u32 h);
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 7dfd9dc00509..e31495668056 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -28,5 +28,6 @@
>  
>  /* ARCH_SHSTK_ features bits */
>  #define ARCH_SHSTK_SHSTK		(1ULL <<  0)
> +#define ARCH_SHSTK_WRSS			(1ULL <<  1)
>  
>  #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 0a3decab70ee..009cb3fa0ae5 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -363,6 +363,36 @@ void shstk_free(struct task_struct *tsk)
>  	unmap_shadow_stack(shstk->base, shstk->size);
>  }
>  
> +static int wrss_control(bool enable)
> +{
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
> +		return -EOPNOTSUPP;
> +
> +	/*
> +	 * Only enable wrss if shadow stack is enabled. If shadow stack is not

"WRSS". Insns in all caps pls.

> +	 * enabled, wrss will already be disabled, so don't bother clearing it

Ditto.

> +	 * when disabling.
> +	 */
> +	if (!features_enabled(ARCH_SHSTK_SHSTK))
> +		return -EPERM;
> +
> +	/* Already enabled/disabled? */
> +	if (features_enabled(ARCH_SHSTK_WRSS) == enable)
> +		return 0;
> +
> +	fpregs_lock_and_load();
> +	if (enable) {
> +		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
> +		features_set(ARCH_SHSTK_WRSS);
> +	} else {
> +		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
> +		features_clr(ARCH_SHSTK_WRSS);
> +	}
> +	fpregs_unlock();

Yes, doing it the "usual" way is more readable because it is a common
code pattern which one encounters all around arch/x86/.

Diff ontop:

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 009cb3fa0ae5..914feff26b23 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -365,6 +365,8 @@ void shstk_free(struct task_struct *tsk)
 
 static int wrss_control(bool enable)
 {
+	u64 msrval;
+
 	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
 		return -EOPNOTSUPP;
 
@@ -381,13 +383,22 @@ static int wrss_control(bool enable)
 		return 0;
 
 	fpregs_lock_and_load();
+	rdmsrl(MSR_IA32_U_CET, msrval);
+
 	if (enable) {
-		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
 		features_set(ARCH_SHSTK_WRSS);
+		msrval |= CET_WRSS_EN;
 	} else {
-		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
 		features_clr(ARCH_SHSTK_WRSS);
+		if (!(msrval & CET_WRSS_EN))
+			goto unlock;
+
+		msrval &= ~CET_WRSS_EN;
 	}
+
+	wrmsrl(MSR_IA32_U_CET, msrval);
+
+unlock:
 	fpregs_unlock();
 
 	return 0;
Edgecombe, Rick P March 10, 2023, 5:16 p.m. UTC | #2 
On Fri, 2023-03-10 at 17:44 +0100, Borislav Petkov wrote:

Thanks on text edits.

> >   
> > +/* Helper that can never get accidentally un-inlined. */
> > +#define set_clr_bits_msrl(msr, set, clear)   do {    \
> 
> Uff, pls kill this thing.
> 
> Our MSR interfaces universe is already insane and arch/x86/lib/msr.c
> already has similar attempts to what you're doing here in addition to
> all the other gunk in msr.h.
> 
> I highly doubt this can't be done the usual way, lemme see...

Seems reasonable.
diff mbox series

Patch

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 65ec1965cd28..2d3b35c957ad 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -310,6 +310,17 @@  void msrs_free(struct msr *msrs);
 int msr_set_bit(u32 msr, u8 bit);
 int msr_clear_bit(u32 msr, u8 bit);
 
+/* Helper that can never get accidentally un-inlined. */
+#define set_clr_bits_msrl(msr, set, clear)	do {	\
+	u64 __val, __new_val, __msr = msr;		\
+							\
+	rdmsrl(__msr, __val);				\
+	__new_val = (__val & ~(clear)) | (set);		\
+							\
+	if (__new_val != __val)				\
+		wrmsrl(__msr, __new_val);		\
+} while (0)
+
 #ifdef CONFIG_SMP
 int rdmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h);
 int wrmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 l, u32 h);
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 7dfd9dc00509..e31495668056 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -28,5 +28,6 @@ 
 
 /* ARCH_SHSTK_ features bits */
 #define ARCH_SHSTK_SHSTK		(1ULL <<  0)
+#define ARCH_SHSTK_WRSS			(1ULL <<  1)
 
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 0a3decab70ee..009cb3fa0ae5 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -363,6 +363,36 @@  void shstk_free(struct task_struct *tsk)
 	unmap_shadow_stack(shstk->base, shstk->size);
 }
 
+static int wrss_control(bool enable)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return -EOPNOTSUPP;
+
+	/*
+	 * Only enable wrss if shadow stack is enabled. If shadow stack is not
+	 * enabled, wrss will already be disabled, so don't bother clearing it
+	 * when disabling.
+	 */
+	if (!features_enabled(ARCH_SHSTK_SHSTK))
+		return -EPERM;
+
+	/* Already enabled/disabled? */
+	if (features_enabled(ARCH_SHSTK_WRSS) == enable)
+		return 0;
+
+	fpregs_lock_and_load();
+	if (enable) {
+		set_clr_bits_msrl(MSR_IA32_U_CET, CET_WRSS_EN, 0);
+		features_set(ARCH_SHSTK_WRSS);
+	} else {
+		set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_WRSS_EN);
+		features_clr(ARCH_SHSTK_WRSS);
+	}
+	fpregs_unlock();
+
+	return 0;
+}
+
 static int shstk_disable(void)
 {
 	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
@@ -379,7 +409,7 @@  static int shstk_disable(void)
 	fpregs_unlock();
 
 	shstk_free(current);
-	features_clr(ARCH_SHSTK_SHSTK);
+	features_clr(ARCH_SHSTK_SHSTK | ARCH_SHSTK_WRSS);
 
 	return 0;
 }