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
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. ]
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 315562c9af3d ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler")
Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
arch/x86/include/asm/debugreg.h | 27 ++++++++++++++++++++++++---
1 file changed, 24 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index b049d950612f..d54d5dbfb4b9 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/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("mov %%db7, %0" : "=r" (val) : __FORCE_ORDER);
break;
default:
BUG();
@@ -66,8 +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));
- break;
+ /*
+ * 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("mov %0, %%db7" ::"r" (value), __FORCE_ORDER); break;
default:
BUG();
}
--
2.39.0
