[PATCH 15/26] x86, pkeys: optimize fault handling in access_error()

[Dave Hansen]

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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