linux-kernel - [PATCH 7/9] x86: skip check for spurious faults for non-present faults

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Message-ID: <1397571337-20409-8-git-send-email-david.vrabel@citrix.com>
Date:	Tue, 15 Apr 2014 15:15:35 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	<xen-devel@...ts.xenproject.org>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	<x86@...nel.org>, Mel Gorman <mgorman@...e.de>
Subject: [PATCH 7/9] 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.

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@...rix.com>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.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

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