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