lkml.org 
[lkml]   [2014]   [Apr]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH 7/9] x86: skip check for spurious faults for non-present faults
Date
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.

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@citrix.com>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
x86 maintainers, this is a prerequisite for removing Xen's usage of
_PAGE_IOMAP so I think this is best merged via the Xen tree.
---
arch/x86/mm/fault.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 8e57229..c39e249 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -936,8 +936,10 @@ 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 check for spurious faults on supervisor write or
+ instruction faults. */
+ if (error_code != (PF_WRITE | PF_PROT)
+ && error_code != (PF_INSTR | PF_PROT))
return 0;

pgd = init_mm.pgd + pgd_index(address);
--
1.7.2.5


\
 
 \ /
  Last update: 2014-04-15 17:21    [W:0.127 / U:17.944 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site