kvm: x86: Check memopp before dereference (CVE-2016-8630)

Commit 41061cdb98 ("KVM: emulate: do not initialize memopp") removes a
check for non-NULL under incorrect assumptions. An undefined instruction
with a ModR/M byte with Mod=0 and R/M-5 (e.g. 0xc7 0x15) will attempt
to dereference a null pointer here.

Fixes: 41061cdb98a0bec464278b4db8e894a3121671f5
Message-Id: <1477592752-126650-2-git-send-email-osh@google.com>
Signed-off-by: Owen Hofmann <osh@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 4e95d3eb29557bcb99219fddb7b909e23ff3b090..cbd7b92585bbbaabad95811f2a139e24283a4c35 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5045,7 +5045,7 @@ done_prefixes:
        /* Decode and fetch the destination operand: register or memory. */
        rc = decode_operand(ctxt, &ctxt->dst, (ctxt->d >> DstShift) & OpMask);
 
-       if (ctxt->rip_relative)
+       if (ctxt->rip_relative && likely(ctxt->memopp))
                ctxt->memopp->addr.mem.ea = address_mask(ctxt,
                                        ctxt->memopp->addr.mem.ea + ctxt->_eip);