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[Xen-changelog] [xen-unstable] x86, shadow: Fix read-to-use race conditi

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Subject: [Xen-changelog] [xen-unstable] x86, shadow: Fix read-to-use race condition
From: Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>
Date: Tue, 13 Apr 2010 14:55:37 -0700
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# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1271091116 -3600
# Node ID 78488a63bbc200095413824cc146134b54635da9
# Parent  b010b792c0f814de725f669d8a6bee738df68963
x86, shadow: Fix read-to-use race condition

If OOS mode is enabled, after last possible resync, read the guest l1e
one last time.  If it's different than the original read, start over

This fixes a race which can result in inconsistent in-sync shadow
tables, leading to corruption:

v1: take page fault, read gl1e from an out-of-sync PT.
v2: modify gl1e, lowering permissions
[v1,v3]: resync l1 which was just read.
v1: propagate change to l1 shadow using stale gl1e

Now we have an in-sync shadow with more permissions than the guest.

The resync can happen either as a result of a 3rd vcpu doing a cr3
update, or under certain conditions by v1 itself.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
 xen/arch/x86/mm/shadow/multi.c |   26 ++++++++++++++++++++++++++
 1 files changed, 26 insertions(+)

diff -r b010b792c0f8 -r 78488a63bbc2 xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c    Mon Apr 12 17:47:16 2010 +0100
+++ b/xen/arch/x86/mm/shadow/multi.c    Mon Apr 12 17:51:56 2010 +0100
@@ -240,6 +240,23 @@ shadow_check_gwalk(struct vcpu *v, unsig
     return !mismatch;
+static int
+shadow_check_gl1e(struct vcpu *v, walk_t *gw)
+    guest_l1e_t *l1p, nl1e;
+    if ( !mfn_valid(gw->l1mfn) )
+        return 0;
+    /* Can't just pull-through because mfn may have changed */
+    l1p = map_domain_page(mfn_x(gw->l1mfn));
+    nl1e.l1 = l1p[guest_l1_table_offset(gw->va)].l1;
+    unmap_domain_page(l1p);
+    return gw->l1e.l1 != nl1e.l1;
 /* Remove write access permissions from a gwalk_t in a batch, and
  * return OR-ed result for TLB flush hint and need to rewalk the guest
@@ -3236,6 +3253,15 @@ static int sh_page_fault(struct vcpu *v,
          * OOS but not in the hash table anymore. */
         return 0;
+    }
+    /* Final check: if someone has synced a page, it's possible that
+     * our l1e is stale.  Compare the entries, and rewalk if necessary. */
+    if ( shadow_check_gl1e(v, &gw)  )
+    {
+        perfc_incr(shadow_inconsistent_gwalk);
+        shadow_unlock(d);
+        goto rewalk;
 #endif /* OOS */

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