x86/debug: Fix stack recursion caused by wrongly ordered DR7 accesses

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
environment 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.

  [ bp: Massage text, make them volatile too, to make sure some
  aggressive compiler optimization pass doesn't discard them. ]

Fixes: 315562c ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler")
Reported-by: Alexey Kardashevskiy <[email protected]>
Signed-off-by: Joerg Roedel <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

Author: joergroedel

arch/x86/include/asm/debugreg.h

@@ -39,7 +39,20 @@ 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));
+		/*
+		 * Apply __FORCE_ORDER to DR7 reads 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. Taking a #VC exception is not a
+		 * safe thing to do just anywhere in the entry code and
+		 * re-ordering might place the access into an unsafe location.
+		 *
+		 * 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) : __FORCE_ORDER);
 		break;
 	default:
 		BUG();
@@ -66,7 +79,16 @@ 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));
+		/*
+		 * Apply __FORCE_ORDER to DR7 writes to forbid re-ordering them
+		 * with other code.
+		 *
+		 * While is didn't happen with a DR7 write (see the DR7 read
+		 * comment above which explains where it happened), add the
+		 * __FORCE_ORDER here too to avoid similar problems in the
+		 * future.
+		 */
+		asm volatile("mov %0, %%db7"	::"r" (value), __FORCE_ORDER);
 		break;
 	default:
 		BUG();