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- 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
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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
This commit is contained in:
Linus Torvalds 2023-02-05 11:28:42 -08:00
parent de506eec89 9d2c7203ff
commit 9e482602c5
1 changed files with 24 additions and 2 deletions
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26
arch/x86/include/asm/debugreg.h
View File

@ -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();