From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
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.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---
b/arch/x86/mm/fault.c | 15 +++++++++++++++
1 file changed, 15 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 2015-12-03 16:21:26.872727820 -0800
+++ b/arch/x86/mm/fault.c 2015-12-03 16:21:26.876728002 -0800
@@ -900,10 +900,16 @@ bad_area(struct pt_regs *regs, unsigned
static inline bool bad_area_access_from_pkeys(unsigned long error_code,
struct vm_area_struct *vma)
{
+ /* This code is always called on the current mm */
+ int foreign = 0;
+
if (!boot_cpu_has(X86_FEATURE_OSPKE))
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), foreign))
+ return true;
return false;
}
@@ -1091,6 +1097,8 @@ int show_unhandled_signals = 1;
static inline int
access_error(unsigned long error_code, struct vm_area_struct *vma)
{
+ /* This is only called for the current mm, so: */
+ int foreign = 0;
/*
* Access or read was blocked by protection keys. We do
* this check before any others because we do not want
@@ -1099,6 +1107,13 @@ 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), foreign))
+ return 1;
if (error_code & PF_WRITE) {
/* write, present and write, not present: */
_
--
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