[Xen-changelog] [xen master] x86/nHVM: avoid NULL deref during INVLPG intercept handling

[patchbot@xxxxxxx]

commit 86c59615f4e7f38df24182f20d9dbdec3299c514
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Feb 9 13:22:13 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Feb 9 13:22:13 2016 +0100

    x86/nHVM: avoid NULL deref during INVLPG intercept handling

    When intercepting (or emulating) L1 guest INVLPG, the nested P2M
    pointer may be (is?) NULL, and hence there's no point in calling
    p2m_flush(). In fact doing so would cause a dereference of that NULL
    pointer at least in the ASSERT() right at the beginning of the
    function.

    While so far nothing supports hap_invlpg() being reachable from the
    INVLPG intercept paths (only INVLPG insn emulation would lead there),
    and hence the code in question (added by dd6de3ab99 ["Implement
    Nested-on-Nested"]) appears to be dead, this seems to be the change
    which can be agreed on as an immediate fix. Ideally, however, the
    problematic code would go away altogether. See thread at
    lists.xenproject.org/archives/html/xen-devel/2016-01/msg03762.html.

    Reported-by: å??令 <liuling-it@xxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 xen/arch/x86/mm/hap/hap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index ee87825..f173539 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -687,7 +687,8 @@ static bool_t hap_invlpg(struct vcpu *v, unsigned long va)
          * Must perform the flush right now or an other vcpu may
          * use it when we use the next VMRUN emulation, otherwise.
          */
-        p2m_flush(v, vcpu_nestedhvm(v).nv_p2m);
+        if ( vcpu_nestedhvm(v).nv_p2m )
+            p2m_flush(v, vcpu_nestedhvm(v).nv_p2m);
         return 1;
     }

--
generated by git-patchbot for /home/xen/git/xen.git#master

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