[PATCH 1/3] x86: skip check for spurious faults for non-present faults

From: David Vrabel
Date: Tue Sep 02 2014 - 12:39:09 EST


If a fault on a kernel address is due to a non-present page, then it
cannot be the result of stale TLB entry from a protection change (RO
to RW or NX to X). Thus the pagetable walk in spurious_fault() can be
skipped.

See the initial if in spurious_fault() and the tests in
spurious_fault_check()) for the set of possible error codes checked
for spurious faults. These are:

IRUWP
Before x00xx && ( 1xxxx || xxx1x )
After ( 10001 || 00011 ) && ( 1xxxx || xxx1x )

Thus the new condition is a subset of the previous one, excluding only
non-present faults (I == 1 and W == 1 are mutually exclusive).

This avoids spurious_fault() oopsing in some cases if the pagetables
it attempts to walk are not accessible. This obscures the location of
the original fault.

This also fixes a crash with Xen PV guests when they access entries in
the M2P corresponding to device MMIO regions. The M2P is mapped
(read-only) by Xen into the kernel address space of the guest and this
mapping may contains holes for non-RAM regions. Read faults will
result in calls to spurious_fault(), but because the page tables for
the M2P mappings are not accessible by the guest the pagetable walk
would fault.

This was not normally a problem as MMIO mappings would not normally
result in a M2P lookup because of the use of the _PAGE_IOMAP bit the
PTE. However, removing the _PAGE_IOMAP bit requires M2P lookups for
MMIO mappings as well.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
---
arch/x86/mm/fault.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index a241946..83bb03b 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -933,8 +933,17 @@ static int spurious_fault_check(unsigned long error_code, pte_t *pte)
* cross-processor TLB flush, even if no stale TLB entries exist
* on other processors.
*
+ * Spurious faults may only occur if the TLB contains an entry with
+ * fewer permission than the page table entry. Non-present (P = 0)
+ * and reserved bit (R = 1) faults are never spurious.
+ *
* There are no security implications to leaving a stale TLB when
* increasing the permissions on a page.
+ *
+ * Returns non-zero if a spurious fault was handled, zero otherwise.
+ *
+ * See Intel Developer's Manual Vol 3 Section 4.10.4.3, bullet 3
+ * (Optional Invalidation).
*/
static noinline int
spurious_fault(unsigned long error_code, unsigned long address)
@@ -945,8 +954,17 @@ spurious_fault(unsigned long error_code, unsigned long address)
pte_t *pte;
int ret;

- /* Reserved-bit violation or user access to kernel space? */
- if (error_code & (PF_USER | PF_RSVD))
+ /*
+ * Only writes to RO or instruction fetches from NX may cause
+ * spurious faults.
+ *
+ * These could be from user or supervisor accesses but the TLB
+ * is only lazily flushed after a kernel mapping protection
+ * change, so user accesses are not expected to cause spurious
+ * faults.
+ */
+ if (error_code != (PF_WRITE | PF_PROT)
+ && error_code != (PF_INSTR | PF_PROT))
return 0;

pgd = init_mm.pgd + pgd_index(address);
--
1.7.10.4

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