[Xen-changelog] [xen stable-4.6] x86/shadow: Avoid overflowing sh_ctxt->seg_reg[]

[patchbot@xxxxxxx]

commit 26352b6344ce5d5a2ee78e56ae631e156fbdce7f
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Sep 8 14:26:01 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Sep 8 14:26:01 2016 +0200

    x86/shadow: Avoid overflowing sh_ctxt->seg_reg[]
    
    hvm_get_seg_reg() does not perform a range check on its input segment, calls
    hvm_get_segment_register() and writes straight into sh_ctxt->seg_reg[].
    
    x86_seg_none is outside the bounds of sh_ctxt->seg_reg[], and will hit a 
BUG()
    in {vmx,svm}_get_segment_register().
    
    HVM guests running with shadow paging can end up performing a virtual to
    linear translation with x86_seg_none.  This is used for addresses which are
    already linear.  However, none of this is a legitimate pagetable update, so
    fail the emulation in such a case.
    
    This is XSA-187 / CVE-2016-7094.
    
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
    master commit: a9f3b3bad17d91e2067fc00d51b0302349570d08
    master date: 2016-09-08 14:16:26 +0200
---
 xen/arch/x86/mm/shadow/common.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index 0eeffb4..83f6938 100644
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -142,9 +142,18 @@ static int hvm_translate_linear_addr(
     struct sh_emulate_ctxt *sh_ctxt,
     unsigned long *paddr)
 {
-    struct segment_register *reg = hvm_get_seg_reg(seg, sh_ctxt);
+    struct segment_register *reg;
     int okay;
 
+    /*
+     * Can arrive here with non-user segments.  However, no such cirucmstance
+     * is part of a legitimate pagetable update, so fail the emulation.
+     */
+    if ( !is_x86_user_segment(seg) )
+        return X86EMUL_UNHANDLEABLE;
+
+    reg = hvm_get_seg_reg(seg, sh_ctxt);
+
     okay = hvm_virtual_to_linear_addr(
         seg, reg, offset, bytes, access_type, sh_ctxt->ctxt.addr_size, paddr);
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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