[PATCH v6 26/26] x86/fpu/xstate: Add a sanity check for XFD state when saving XSTATE

[Chang S. Bae]

Add a DEBUG sanity check that XFD state matches with XINUSE state.

Instead of reading MSR IA32_XFD directly, read a per-cpu value that is
recorded at every MSR write.

Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v5:
* Added as a new patch. (Dave Hansen)
---
 arch/x86/include/asm/fpu/internal.h | 15 +++++++++++++++
 arch/x86/kernel/fpu/core.c          | 13 +++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index a7541d0e5057..06e5d7553aeb 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -570,10 +570,25 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 
 /* The Extended Feature Disable (XFD) helpers: */
 
+#ifdef CONFIG_X86_DEBUG_FPU
+DECLARE_PER_CPU(u64, xfd_shadow);
+static inline u64 xfd_debug_shadow(void)
+{
+	return this_cpu_read(xfd_shadow);
+}
+
+static inline void xfd_write(u64 value)
+{
+	wrmsrl_safe(MSR_IA32_XFD, value);
+	this_cpu_write(xfd_shadow, value);
+}
+#else
+#define xfd_debug_shadow()	0
 static inline void xfd_write(u64 value)
 {
 	wrmsrl_safe(MSR_IA32_XFD, value);
 }
+#endif
 
 static inline u64 xfd_read(void)
 {
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 82d08d22b4ce..b761c017589e 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -82,6 +82,10 @@ bool irq_fpu_usable(void)
 }
 EXPORT_SYMBOL(irq_fpu_usable);
 
+#ifdef CONFIG_X86_DEBUG_FPU
+DEFINE_PER_CPU(u64, xfd_shadow);
+#endif
+
 /*
  * Save the FPU register state in fpu->state. The register state is
  * preserved.
@@ -99,6 +103,15 @@ EXPORT_SYMBOL(irq_fpu_usable);
 void save_fpregs_to_fpstate(struct fpu *fpu)
 {
 	if (likely(use_xsave())) {
+		/*
+		 * If XFD is armed for an xfeature, XSAVE* will not save
+		 * its state. Verify XFD is clear for all features that
+		 * are in use before XSAVE*.
+		 */
+		if (IS_ENABLED(CONFIG_X86_DEBUG_FPU) && xfd_capable() &&
+		    boot_cpu_has(X86_FEATURE_XGETBV1))
+			WARN_ON_FPU(xgetbv(1) & xfd_debug_shadow());
+
 		os_xsave(&fpu->state->xsave, fpu->state_mask);
 
 		/*
-- 
2.17.1