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: f0dccc9da4c0 ("x86/fpu/xstate: Support dynamic supervisor feature for LBR")
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Mitchell Levy <levymitchell0@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/all/20240812-xsave-lbr-fix-v3-1-95bac1bf62f4@xxxxxxxxx
diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index eb17f31b06d2..de16862bf230 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -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 */
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c5a026fee5e0..1339f8328db5 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -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.
*/
diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index 2ee0b9c53dcc..afb404cd2059 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -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 */
