[Xen-devel] [PATCH] x86/svm: Adjust ModRM Mode check in is_invlpg()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Coverity points out that x86_insn_modrm() returns -EINVAL for instructions not
encoded with a ModRM byte.  A consequence is that checking != 3 is
insufficient to confirm that &ext was actually written to.

In practice, this check is only used after decode has been successful, and
0f01 will have a ModRM byte.

Use an unsigned < comparison to exclude the -EINVAL case, guaranteeing that
ext is only read if it was filled in by x86_insn_modrm(), which should placate
Coverity.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>

RFC.  I haven't actually checked that this fixes the issue.

An alternative would be to ASSERT() that x86_insn_modrm() is non-negative, but
I can't nice way of integrating that into the existing logic (without using
the comma operator, and that isn't nice to read).
---
 xen/arch/x86/hvm/svm/svm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index ae8e2c4..ff134a5 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2162,7 +2162,7 @@ static bool is_invlpg(const struct x86_emulate_state 
*state,
     unsigned int ext;
 
     return ctxt->opcode == X86EMUL_OPC(0x0f, 0x01) &&
-           x86_insn_modrm(state, NULL, &ext) != 3 &&
+           (unsigned int)x86_insn_modrm(state, NULL, &ext) < 3 &&
            (ext & 7) == 7;
 }
 
-- 
2.1.4


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