On Thu, Oct 10, 2024 at 04:57:31PM -0700, Mitchell Levy wrote:
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@linutronix.de [ Mitchell Levy: Backport to 5.15, since struct fpu_config is not introduced until 578971f4e228 and feature masks are not included in said struct until 1c253ff2287f ] 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.c...
arch/x86/include/asm/fpu/xstate.h | 5 +++-- arch/x86/kernel/fpu/xstate.c | 7 +++++++ 2 files changed, 10 insertions(+), 2 deletions(-)
<formletter>
This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.
</formletter>