[PATCH] x86/debug: Fix stack recursion caused by DR7 accesses

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

In kernels compiled with CONFIG_PARAVIRT=n the compiler re-orders the
DR7 read in exc_nmi() to happen before the call to sev_es_ist_enter().

This is problematic when running as an SEV-ES guest because in this
environemnt the DR7 read might cause a #VC exception, and taking #VC
exceptions is not safe in exc_nmi() before sev_es_ist_enter() has run.

The result is stack recursion if the NMI was caused on the #VC IST
stack, because a subsequent #VC exception in the NMI handler will
overwrite the stack frame of the interrupted #VC handler.

As there are no compiler barriers affecting the ordering of DR7
reads/writes, make the accesses to this register volatile, forbidding
the compiler to re-order them.

Cc: Alexey Kardashevskiy <aik@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/include/asm/debugreg.h | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index b049d950612f..eb6238a5f60c 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -39,7 +39,18 @@ static __always_inline unsigned long native_get_debugreg(int regno)
 		asm("mov %%db6, %0" :"=r" (val));
 		break;
 	case 7:
-		asm("mov %%db7, %0" :"=r" (val));
+		/*
+		 * Make DR7 reads volatile to forbid re-ordering them with other
+		 * code. This is needed because a DR7 access can cause a #VC
+		 * exception when running under SEV-ES. But taking a #VC
+		 * exception is not safe at everywhere in the code-flow and
+		 * re-ordering might place the access into an unsafe place.
+		 *
+		 * This happened in the NMI handler, where the DR7 read was
+		 * re-ordered to happen before the call to sev_es_ist_enter(),
+		 * causing stack recursion.
+		 */
+		asm volatile ("mov %%db7, %0" : "=r" (val));
 		break;
 	default:
 		BUG();
@@ -66,7 +77,21 @@ static __always_inline void native_set_debugreg(int regno, unsigned long value)
 		asm("mov %0, %%db6"	::"r" (value));
 		break;
 	case 7:
-		asm("mov %0, %%db7"	::"r" (value));
+		/*
+		 * Make DR7 writes volatile to forbid re-ordering them with
+		 * other code. This is needed because a DR7 access can cause a
+		 * #VC exception when running under SEV-ES.  But taking a #VC
+		 * exception is not safe at everywhere in the code-flow and
+		 * re-ordering might place the access into an unsafe place.
+		 *
+		 * This happened in the NMI handler, where the DR7 read was
+		 * re-ordered to happen before the call to sev_es_ist_enter(),
+		 * causing stack recursion.
+		 *
+		 * While is didn't happen with a DR7 write, add the volatile
+		 * here too to avoid similar problems in the future.
+		 */
+		asm volatile ("mov %0, %%db7"	::"r" (value));
 		break;
 	default:
 		BUG();
-- 
2.39.0