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Fix a race in pmap_emulate_accessed_dirty() that could trigger a EPT

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misconfiguration VM-exit.

An EPT misconfiguration is triggered when the processor encounters a PTE
that is writable but not readable (WR=10). On processors that require A/D
bit emulation PG_M and PG_A map to EPT_PG_WRITE and EPT_PG_READ respectively.

If the PTE is updated as in the following code snippet:
	*pte |= PG_M;
	*pte |= PG_A;
then it is possible for another processor to observe the PTE after the PG_M
(aka EPT_PG_WRITE) bit is set but before PG_A (aka EPT_PG_READ) bit is set.

This will trigger an EPT misconfiguration VM-exit on the other processor.

Reported by:	rodrigc
Reviewed by:	grehan
MFC after:	3 days
This commit is contained in:
Neel Natu 2014-10-21 01:06:58 +00:00
parent 52d99877ba
commit a5045426db
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=273356
1 changed files with 12 additions and 2 deletions
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14
sys/amd64/amd64/pmap.c
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@ -6810,9 +6810,19 @@ pmap_emulate_accessed_dirty(pmap_t pmap, vm_offset_t va, int ftype)
if (ftype == VM_PROT_WRITE) {
if ((*pte & PG_RW) == 0)
goto done;
*pte |= PG_M;
/*
* Set the modified and accessed bits simultaneously.
*
* Intel EPT PTEs that do software emulation of A/D bits map
* PG_A and PG_M to EPT_PG_READ and EPT_PG_WRITE respectively.
* An EPT misconfiguration is triggered if the PTE is writable
* but not readable (WR=10). This is avoided by setting PG_A
* and PG_M simultaneously.
*/
*pte |= PG_M | PG_A;
} else {
*pte |= PG_A;
}
*pte |= PG_A;
/* try to promote the mapping */
if (va < VM_MAXUSER_ADDRESS)