FAILED: patch "[PATCH] x86/fpu: Avoid writing LBR bit to IA32_XSS unless supported" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 2848ff28d180bd63a95da8e5dcbcdd76c1beeb7b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024090809-plaything-sash-1d57@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

2848ff28d180 ("x86/fpu: Avoid writing LBR bit to IA32_XSS unless supported") c33f0a81a2cf ("x86/fpu: Add fpu_state_config::legacy_features") d72c87018d00 ("x86/fpu/xstate: Move remaining xfeature helpers to core") eda32f4f93b4 ("x86/fpu: Rework restore_regs_from_fpstate()") daddee247319 ("x86/fpu: Mop up xfeatures_mask_uabi()")
1c253ff2287f ("x86/fpu: Move xstate feature masks to fpu_*_cfg") 2bd264bce238 ("x86/fpu: Move xstate size to fpu_*_cfg")
cd9ae7617449 ("x86/fpu/xstate: Cleanup size calculations") 617473acdfe4 ("x86/fpu: Cleanup fpu__init_system_xstate_size_legacy()") 578971f4e228 ("x86/fpu: Provide struct fpu_config")
5509cc78080d ("x86/fpu/signal: Use fpstate for size and features") ad6ede407aae ("x86/fpu: Use fpstate in fpu_copy_kvm_uabi_to_fpstate()") be31dfdfd75b ("x86/fpu: Use fpstate::size")
248452ce21ae ("x86/fpu: Add size and mask information to fpstate") 2dd8eedc80b1 ("x86/process: Move arch_thread_struct_whitelist() out of line") c20942ce5128 ("x86/fpu/core: Convert to fpstate")
7e049e8b7459 ("x86/fpu/signal: Convert to fpstate")
087df48c298c ("x86/fpu: Replace KVMs xstate component clearing") 18b3fa1ad15f ("x86/fpu: Convert restore_fpregs_from_fpstate() to struct fpstate") f83ac56acdad ("x86/fpu: Convert fpstate_init() to struct fpstate")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2848ff28d180bd63a95da8e5dcbcdd76c1beeb7b Mon Sep 17 00:00:00 2001
From: Mitchell Levy <[email protected]>
Date: Mon, 12 Aug 2024 13:44:12 -0700
Subject: [PATCH] x86/fpu: Avoid writing LBR bit to IA32_XSS unless supported

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 ]

index eb17f31b06d2..de16862bf230 100644
@@ -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 */

index c5a026fee5e0..1339f8328db5 100644
@@ -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.
 	 */

index 2ee0b9c53dcc..afb404cd2059 100644
@@ -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 */

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 ]

[ 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 ]

---
 arch/x86/include/asm/fpu/xstate.h | 5 +++--
 arch/x86/kernel/fpu/xstate.c      | 7 +++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

index d91df71f60fb..3bc08b5313b0 100644
@@ -85,6 +85,7 @@
 #endif
 
 extern u64 xfeatures_mask_all;
+extern u64 xfeatures_mask_indep;
 
 static inline u64 xfeatures_mask_supervisor(void)
 {

@@ -124,9 +125,9 @@ static inline u64 xfeatures_mask_fpstate(void)
 static inline u64 xfeatures_mask_independent(void)
 {
 	if (!boot_cpu_has(X86_FEATURE_ARCH_LBR))
-		return XFEATURE_MASK_INDEPENDENT & ~XFEATURE_MASK_LBR;
+		return xfeatures_mask_indep & ~XFEATURE_MASK_LBR;
 
-	return XFEATURE_MASK_INDEPENDENT;
+	return xfeatures_mask_indep;
 }
 
 extern u64 xstate_fx_sw_bytes[USER_XSTATE_FX_SW_WORDS];

index 81891f0fff6f..3772577462a0 100644
@@ -60,6 +60,11 @@ static short xsave_cpuid_features[] __initdata = {
  * XSAVE buffer, both supervisor and user xstates.
  */
 u64 xfeatures_mask_all __ro_after_init;
+/*
+ * This represents the "independent" xfeatures that are supported by XSAVES, but not managed as part
+ * of the FPU core, such as LBR.
+ */
+u64 xfeatures_mask_indep __ro_after_init;
 EXPORT_SYMBOL_GPL(xfeatures_mask_all);
 
 static unsigned int xstate_offsets[XFEATURE_MAX] __ro_after_init =

@@ -768,6 +773,8 @@ void __init fpu__init_system_xstate(void)
 		goto out_disable;
 	}
 
+	xfeatures_mask_indep = xfeatures_mask_all & XFEATURE_MASK_INDEPENDENT;
+
 	/*
 	 * Clear XSAVE features that are disabled in the normal CPUID.
 	 */

2.34.1

<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>