[Xen-changelog] [xen-4.0-testing] x86: tighten filter on ptwr_do_page_fault()

["Xen patchbot-4.0-testing" <patchbot-4.0-testing@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir@xxxxxxx>
# Date 1291042003 0
# Node ID 465c81b077009ba3cb986caef2676c086718d71c
# Parent  2901cbe2eccc41ebd552bf6c829c8f0c46ba396c
x86: tighten filter on ptwr_do_page_fault()

Even not-so-recent Linux may, due to post-2.6.18 changes to the
process creation code, cause quite a number (depending on environment
and argument size) of faulting accesses to user space originating from
kernel mode. Generally those happen for non-present pages and would
lead to a nested page fault from guest_get_eff_l1e(). They can be
avoided by checking for PFEC_page_present as long as the guest isn't
running on shadow page tables.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
Signed-off-by: Keir Fraser <keir@xxxxxxx>
xen-unstable changeset:   22449:3afb5ecbf69f
xen-unstable date:        Mon Nov 29 14:40:55 2010 +0000
---
 xen/arch/x86/traps.c |   21 ++++++++++++++-------
 1 files changed, 14 insertions(+), 7 deletions(-)

diff -r 2901cbe2eccc -r 465c81b07700 xen/arch/x86/traps.c
--- a/xen/arch/x86/traps.c      Mon Nov 29 14:46:01 2010 +0000
+++ b/xen/arch/x86/traps.c      Mon Nov 29 14:46:43 2010 +0000
@@ -1237,13 +1237,20 @@ static int fixup_page_fault(unsigned lon
     }
 
     if ( VM_ASSIST(d, VMASST_TYPE_writable_pagetables) &&
-         guest_kernel_mode(v, regs) &&
-         /* Do not check if access-protection fault since the page may 
-            legitimately be not present in shadow page tables */
-         ((regs->error_code & (PFEC_write_access|PFEC_reserved_bit)) ==
-          PFEC_write_access) &&
-         ptwr_do_page_fault(v, addr, regs) )
-        return EXCRET_fault_fixed;
+         guest_kernel_mode(v, regs) )
+    {
+        unsigned int mbs = PFEC_write_access;
+        unsigned int mbz = PFEC_reserved_bit | PFEC_insn_fetch;
+
+        /* Do not check if access-protection fault since the page may 
+           legitimately be not present in shadow page tables */
+        if ( !paging_mode_enabled(d) )
+            mbs |= PFEC_page_present;
+
+        if ( ((regs->error_code & (mbs | mbz)) == mbs) &&
+             ptwr_do_page_fault(v, addr, regs) )
+            return EXCRET_fault_fixed;
+    }
 
     /* For non-external shadowed guests, we fix up both their own 
      * pagefaults and Xen's, since they share the pagetables. */

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