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Groups > linux.kernel > #1303178

[PATCH 19/31] 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>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
---

 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	2016-01-06 15:50:11.270411174 -0800
+++ b/arch/x86/mm/fault.c	2016-01-06 15:50:11.274411354 -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 */
+	bool foreign = false;
+
 	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: */
+	bool foreign = false;
 	/*
 	 * 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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Thread

[PATCH 00/31] x86: Memory Protection Keys (v8) Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 22/31] x86, pkeys: dump pkey from VMA in /proc/pid/smaps Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 28/31] x86, fpu: allow setting of XSAVE state Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 06/31] x86, pkeys: add PKRU xsave fields and data structure(s) Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 07/31] x86, pkeys: PTE bits for storing protection key Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 01/31] mm, gup: introduce concept of "foreign" get_user_pages() Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
    Re: [PATCH 01/31] mm, gup: introduce concept of "foreign"  get_user_pages() Vlastimil Babka <vbabka@suse.cz> - 2016-01-13 20:10 +0100
      Re: [PATCH 01/31] mm, gup: introduce concept of "foreign"  get_user_pages() Dave Hansen <dave@sr71.net> - 2016-01-13 20:20 +0100
  [PATCH 17/31] x86, pkeys: check VMAs and PTEs for protection keys Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 03/31] x86, pkeys: Add Kconfig option Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 29/31] x86, pkeys: allow kernel to modify user pkey rights register Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 21/31] x86, pkeys: dump PKRU with other kernel registers Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 11/31] x86, pkeys: pass VMA down in to fault signal generation code Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 10/31] x86, pkeys: arch-specific protection bits Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 19/31] x86, pkeys: optimize fault handling in access_error() Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 05/31] x86, pkeys: define new CR4 bit Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 09/31] x86, pkeys: store protection in high VMA flags Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 16/31] x86, mm: simplify get_user_pages() PTE bit handling Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 27/31] x86: separate out LDT init from context init Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 14/31] x86, pkeys: add functions to fetch PKRU Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
    Re: [PATCH 14/31] x86, pkeys: add functions to fetch PKRU Thomas Gleixner <tglx@linutronix.de> - 2016-01-08 20:40 +0100
  [PATCH 31/31] x86, pkeys: execute-only support Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
    Re: [PATCH 31/31] x86, pkeys: execute-only support Kees Cook <keescook@google.com> - 2016-01-07 22:10 +0100
      Re: [PATCH 31/31] x86, pkeys: execute-only support Dave Hansen <dave@sr71.net> - 2016-01-07 23:30 +0100
    Re: [PATCH 31/31] x86, pkeys: execute-only support Andy Lutomirski <luto@amacapital.net> - 2016-01-07 22:20 +0100
      Re: [PATCH 31/31] x86, pkeys: execute-only support Dave Hansen <dave@sr71.net> - 2016-01-07 23:20 +0100
        Re: [PATCH 31/31] x86, pkeys: execute-only support Andy Lutomirski <luto@amacapital.net> - 2016-01-07 23:50 +0100
    Re: [PATCH 31/31] x86, pkeys: execute-only support Thomas Gleixner <tglx@linutronix.de> - 2016-01-08 21:00 +0100
  [PATCH 24/31] x86, pkeys: actually enable Memory Protection Keys in CPU Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 30/31] x86, pkeys: create an x86 arch_calc_vm_prot_bits() for VMA flags Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 18/31] mm: add gup flag to indicate "foreign" mm access Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 26/31] x86, pkeys: add arch_validate_pkey() Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
    Re: [PATCH 26/31] x86, pkeys: add arch_validate_pkey() Thomas Gleixner <tglx@linutronix.de> - 2016-01-08 20:40 +0100
  [PATCH 08/31] x86, pkeys: new page fault error code bit: PF_PK Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 13/31] x86, pkeys: fill in pkey field in siginfo Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 12/31] signals, pkeys: notify userspace about protection key faults Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 23/31] x86, pkeys: add Kconfig prompt to existing config option Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 15/31] mm: factor out VMA fault permission checking Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 04/31] x86, pkeys: cpuid bit definition Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 20/31] x86, pkeys: differentiate instruction fetches Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 25/31] mm, multi-arch: pass a protection key in to calc_vm_flag_bits() Dave Hansen <dave@sr71.net> - 2016-01-07 01:10 +0100
  [PATCH 02/31] x86, fpu: add placeholder for Processor Trace XSAVE state Dave Hansen <dave@sr71.net> - 2016-01-07 01:20 +0100

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