From: Joerg Roedel jroedel@suse.de

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@vger.kernel.org Fixes: 315562c9af3d ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler") Signed-off-by: Joerg Roedel jroedel@suse.de --- 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(); }