On 17/06/21 12:15, Nadav Amit wrote:
From: Nadav Amit <nadav.amit@xxxxxxxxx>
According to Intel SDM 4.10.4.3 "Optional Invalidation": "If CR4.SMEP =
0 and a paging-structure entry is modified to change the U/S flag from 0
to 1, failure to perform an invalidation may result in a "spurious"
page-fault exception (e.g., in response to an attempted user-mode
access) but no other adverse behavior."
The access test actually causes in certain environments a spurious
page-fault. So invalidate the relevant PTE after setting the user bit.
Signed-off-by: Nadav Amit <nadav.amit@xxxxxxxxx>
---
x86/access.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/x86/access.c b/x86/access.c
index 0ad677e..47807cc 100644
--- a/x86/access.c
+++ b/x86/access.c
@@ -216,8 +216,12 @@ static unsigned set_cr4_smep(int smep)
if (smep)
ptl2[2] &= ~PT_USER_MASK;
r = write_cr4_checking(cr4);
- if (r || !smep)
+ if (r || !smep) {
ptl2[2] |= PT_USER_MASK;
+
+ /* Flush to avoid spurious #PF */
+ invlpg((void *)(2 << 21));
+ }
if (!r)
shadow_cr4 = cr4;
return r;
Queued, thanks.
Paolo
