Merge tag 'x86_urgent_for_v6.2_rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Borislav Petkov:

 - Prevent the compiler from reordering accesses to debug regs which
   could cause a #VC exception in SEV-ES guests at the wrong place in
   the NMI handling path

* tag 'x86_urgent_for_v6.2_rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/debug: Fix stack recursion caused by wrongly ordered DR7 accesses

Author: torvalds

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();