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x86/fpu: Avoid writing LBR bit to IA32_XSS unless supported

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There are two distinct CPU features related to the use of XSAVES and LBR:
whether LBR is itself supported and whether XSAVES supports LBR. The LBR
subsystem correctly checks both in intel_pmu_arch_lbr_init(), but the
XSTATE subsystem does not.

The LBR bit is only removed from xfeatures_mask_independent when LBR is not
supported by the CPU, but there is no validation of XSTATE support.

If XSAVES does not support LBR the write to IA32_XSS causes a #GP fault,
leaving the state of IA32_XSS unchanged, i.e. zero. The fault is handled
with a warning and the boot continues.

Consequently the next XRSTORS which tries to restore supervisor state fails
with #GP because the RFBM has zero for all supervisor features, which does
not match the XCOMP_BV field.

As XFEATURE_MASK_FPSTATE includes supervisor features setting up the FPU
causes a #GP, which ends up in fpu_reset_from_exception_fixup(). That fails
due to the same problem resulting in recursive #GPs until the kernel runs
out of stack space and double faults.

Prevent this by storing the supported independent features in
fpu_kernel_cfg during XSTATE initialization and use that cached value for
retrieving the independent feature bits to be written into IA32_XSS.

[ tglx: Massaged change log ]

Fixes: f0dccc9da4 ("x86/fpu/xstate: Support dynamic supervisor feature for LBR")
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240812-xsave-lbr-fix-v3-1-95bac1bf62f4@gmail.com
This commit is contained in:
Mitchell Levy 2024-08-12 13:44:12 -07:00 committed by Thomas Gleixner
parent 0ecc5be200
commit 2848ff28d1
3 changed files with 12 additions and 2 deletions
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7
arch/x86/include/asm/fpu/types.h
View File

@ -591,6 +591,13 @@ struct fpu_state_config {
* even without XSAVE support, i.e. legacy features FP + SSE
*/
u64 legacy_features;
/*
* @independent_features:
*
* Features that are supported by XSAVES, but not managed as part of
* the FPU core, such as LBR
*/
u64 independent_features;
};
/* FPU state configuration information */

3
arch/x86/kernel/fpu/xstate.c
View File

@ -788,6 +788,9 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
goto out_disable;
}
fpu_kernel_cfg.independent_features = fpu_kernel_cfg.max_features &
XFEATURE_MASK_INDEPENDENT;
/*
* Clear XSAVE features that are disabled in the normal CPUID.
*/

4
arch/x86/kernel/fpu/xstate.h
View File

@ -62,9 +62,9 @@ static inline u64 xfeatures_mask_supervisor(void)
static inline u64 xfeatures_mask_independent(void)
{
if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR))
return XFEATURE_MASK_INDEPENDENT & ~XFEATURE_MASK_LBR;
return fpu_kernel_cfg.independent_features & ~XFEATURE_MASK_LBR;
return XFEATURE_MASK_INDEPENDENT;
return fpu_kernel_cfg.independent_features;
}
/* XSAVE/XRSTOR wrapper functions */