[1/3] x86/bugs: Create an option to disable MDS
Commit Message
There is no way to disable MDS mitigation today at compilation time. MDS
is enabled even if CONFIG_SPECULATION_MITIGATIONS is unset.
Create a new KCONFIG option that allow MDS mitigations to be disabled in
compilation time.
Signed-off-by: Breno Leitao <leitao@debian.org>
---
arch/x86/Kconfig | 11 +++++++++++
arch/x86/kernel/cpu/bugs.c | 9 ++++++++-
2 files changed, 19 insertions(+), 1 deletion(-)
Comments
On Thu, Jun 15, 2023 at 09:44:12AM -0700, Breno Leitao wrote:
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 182af64387d0..50f12829dce9 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -269,6 +269,7 @@ static void x86_amd_ssb_disable(void)
> /* Default mitigation for MDS-affected CPUs */
> static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
> static bool mds_nosmt __ro_after_init = false;
> +#define MDS_WARN_MSG "WARNING: Microarchitectural Data Sampling (MDS) speculative mitigation disabled!\n"
>
> static const char * const mds_strings[] = {
> [MDS_MITIGATION_OFF] = "Vulnerable",
> @@ -278,11 +279,17 @@ static const char * const mds_strings[] = {
>
> static void __init mds_select_mitigation(void)
> {
> - if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
> + if (!boot_cpu_has_bug(X86_BUG_MDS)) {
> mds_mitigation = MDS_MITIGATION_OFF;
> return;
> }
>
> + if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
> + mds_mitigation = MDS_MITIGATION_OFF;
> + pr_err(MDS_WARN_MSG);
> + return;
Why does compile time config needs to be so restrictive that it does not
allow runtime override with mds= cmdline?
I believe Kconfig should only be setting the mitigation default,
allowing users to select mitigation at runtime:
---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 182af64387d0..50e1ca4ea68b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -267,7 +267,11 @@ static void x86_amd_ssb_disable(void)
#define pr_fmt(fmt) "MDS: " fmt
/* Default mitigation for MDS-affected CPUs */
+#if IS_ENABLED(CONFIG_MITIGATE_MDS)
static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
+#else
+static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_OFF;
+#endif
static bool mds_nosmt __ro_after_init = false;
static const char * const mds_strings[] = {
On Thu, Jun 15, 2023 at 03:13:47PM -0700, Pawan Gupta wrote:
> On Thu, Jun 15, 2023 at 09:44:12AM -0700, Breno Leitao wrote:
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 182af64387d0..50f12829dce9 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -269,6 +269,7 @@ static void x86_amd_ssb_disable(void)
> > /* Default mitigation for MDS-affected CPUs */
> > static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
> > static bool mds_nosmt __ro_after_init = false;
> > +#define MDS_WARN_MSG "WARNING: Microarchitectural Data Sampling (MDS) speculative mitigation disabled!\n"
> >
> > static const char * const mds_strings[] = {
> > [MDS_MITIGATION_OFF] = "Vulnerable",
> > @@ -278,11 +279,17 @@ static const char * const mds_strings[] = {
> >
> > static void __init mds_select_mitigation(void)
> > {
> > - if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
> > + if (!boot_cpu_has_bug(X86_BUG_MDS)) {
> > mds_mitigation = MDS_MITIGATION_OFF;
> > return;
> > }
> >
> > + if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
> > + mds_mitigation = MDS_MITIGATION_OFF;
> > + pr_err(MDS_WARN_MSG);
> > + return;
>
> Why does compile time config needs to be so restrictive that it does not
> allow runtime override with mds= cmdline?
>
> I believe Kconfig should only be setting the mitigation default,
> allowing users to select mitigation at runtime:
Sure, that is doable as well. If no one has any opposition to it, I will
implemented as suggested.
@@ -2649,6 +2649,17 @@ config SLS
against straight line speculation. The kernel image might be slightly
larger.
+config MITIGATE_MDS
+ bool "Mitigate Microarchitectural Data Sampling (MDS) hardware bug"
+ depends on CPU_SUP_INTEL && X86_64
+ default y
+ help
+ Enable mitigation for Microarchitectural Data Sampling (MDS). MDS is
+ a hardware vulnerability which allows unprivileged speculative access
+ to data which is available in various CPU internal buffer. Deeper
+ technical information is available in the MDS specific x86 architecture
+ section: Documentation/arch/x86/mds.rst.
+
endif
config ARCH_HAS_ADD_PAGES
@@ -269,6 +269,7 @@ static void x86_amd_ssb_disable(void)
/* Default mitigation for MDS-affected CPUs */
static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
static bool mds_nosmt __ro_after_init = false;
+#define MDS_WARN_MSG "WARNING: Microarchitectural Data Sampling (MDS) speculative mitigation disabled!\n"
static const char * const mds_strings[] = {
[MDS_MITIGATION_OFF] = "Vulnerable",
@@ -278,11 +279,17 @@ static const char * const mds_strings[] = {
static void __init mds_select_mitigation(void)
{
- if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
+ if (!boot_cpu_has_bug(X86_BUG_MDS)) {
mds_mitigation = MDS_MITIGATION_OFF;
return;
}
+ if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
+ mds_mitigation = MDS_MITIGATION_OFF;
+ pr_err(MDS_WARN_MSG);
+ return;
+ }
+
if (mds_mitigation == MDS_MITIGATION_FULL) {
if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
mds_mitigation = MDS_MITIGATION_VMWERV;