We might not strictly have to make modifictions to
access_error() to check the VMA here.
If we do not, we will do this:
1. app sets VMA pkey to K
2. app touches a !present page
3. do_page_fault(), allocates and maps page, sets pte.pkey=K
4. return to userspace
5. touch instruction reexecutes, but triggers PF_PK
6. do PKEY signal
What happens with this patch applied:
1. app sets VMA pkey to K
2. app touches a !present page
3. do_page_fault() notices that K is inaccessible
4. do PKEY signal
We basically skip the fault that does an allocation.
So what this lets us do is protect areas from even being
*populated* unless it is accessible according to protection
keys. That seems handy to me and makes protection keys work
more like an mprotect()'d mapping.
---
b/arch/x86/mm/fault.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff -puN arch/x86/mm/fault.c~pkeys-15-access_error arch/x86/mm/fault.c
--- a/arch/x86/mm/fault.c~pkeys-15-access_error 2015-09-16 10:48:18.012271934 -0700
+++ b/arch/x86/mm/fault.c 2015-09-16 10:48:18.016272115 -0700
@@ -889,6 +889,9 @@ static inline bool bad_area_access_from_
return false;
if (error_code & PF_PK)
return true;
+ /* this checks permission keys on the VMA: */
+ if (!arch_vma_access_permitted(vma, (error_code & PF_WRITE)))
+ return true;
return false;
}
@@ -1075,6 +1078,13 @@ access_error(unsigned long error_code, s
*/
if (error_code & PF_PK)
return 1;
+ /*
+ * Make sure to check the VMA so that we do not perform
+ * faults just to hit a PF_PK as soon as we fill in a
+ * page.
+ */
+ if (!arch_vma_access_permitted(vma, (error_code & PF_WRITE)))
+ return 1;
if (error_code & PF_WRITE) {
/* write, present and write, not present: */
_
--
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