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

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

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:

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, 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 
        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);

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