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[PATCH 16/25] x86, pkeys: optimize fault handling in access_error()

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From: Dave Hansen <dave.hansen@linux.intel.com>

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@linux.intel.com>
---

 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	2015-09-28 11:39:48.287289263 -0700
+++ b/arch/x86/mm/fault.c	2015-09-28 11:39:48.290289400 -0700
@@ -904,6 +904,9 @@ 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;
 }
 
@@ -1091,6 +1094,13 @@ 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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Thread

[PATCH 00/25] x86: Memory Protection Keys Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
  [PATCH 21/25] mm: implement new mprotect_key() system call Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
    Re: [PATCH 21/25] mm: implement new mprotect_key() system call Michael Ellerman <mpe@ellerman.id.au> - 2015-09-29 08:40 +0200
      Re: [PATCH 21/25] mm: implement new mprotect_key() system call Dave Hansen <dave@sr71.net> - 2015-09-29 16:20 +0200
  [PATCH 07/25] x86, pkeys: new page fault error code bit: PF_PK Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
    Re: [PATCH 07/25] x86, pkeys: new page fault error code bit: PF_PK Thomas Gleixner <tglx@linutronix.de> - 2015-10-01 14:00 +0200
      Re: [PATCH 07/25] x86, pkeys: new page fault error code bit: PF_PK Dave Hansen <dave@sr71.net> - 2015-10-01 19:20 +0200
  [PATCH 16/25] x86, pkeys: optimize fault handling in access_error() Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
  [PATCH 13/25] mm: factor out VMA fault permission checking Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
  [PATCH 08/25] x86, pkeys: store protection in high VMA flags Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
  [PATCH 06/25] x86, pkeys: PTE bits for storing protection key Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200
  [PATCH 14/25] mm: simplify get_user_pages() PTE bit handling Dave Hansen <dave@sr71.net> - 2015-09-28 21:30 +0200

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