From 31668511424110ad470315c6a63dec9a10f1a7ba Mon Sep 17 00:00:00 2001 From: David Vrabel Date: Thu, 10 Apr 2014 18:46:45 +0100 Subject: [PATCH] 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 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 Reported-by: Konrad Rzeszutek Wilk Tested-by: Konrad Rzeszutek Wilk Acked-by: Dave Hansen --- 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 a24194681513..83bb03bfa259 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);