Re: [Qemu-devel] [BUG] user-to-root privesc inside VM via bad translation caching

[Pranith Kumar]

On Thu, Mar 23, 2017 at 6:27 AM, Paolo Bonzini <address@hidden> wrote:
>
>
> On 22/03/2017 21:01, Richard Henderson wrote:
>>>
>>> Ah, OK. Thanks for the explanation. May be we should check the size of
>>> the instruction while decoding the prefixes and error out once we
>>> exceed the limit. We would not generate any IR code.
>>
>> Yes.
>>
>> It would not enforce a true limit of 15 bytes, since you can't know that
>> until you've done the rest of the decode.  But you'd be able to say that
>> no more than 14 prefix + 1 opc + 6 modrm+sib+ofs + 4 immediate = 25
>> bytes is used.
>>
>> Which does fix the bug.
>
> Yeah, that would work for 2.9 if somebody wants to put together a patch.
>  Ensuring that all instruction fetching happens before translation side
> effects is a little harder, but perhaps it's also the opportunity to get
> rid of s->rip_offset which is a little ugly.

How about the following?

diff --git a/target/i386/translate.c b/target/i386/translate.c
index 72c1b03a2a..67c58b8900 100644
--- a/target/i386/translate.c
+++ b/target/i386/translate.c
@@ -4418,6 +4418,11 @@ static target_ulong disas_insn(CPUX86State
*env, DisasContext *s,
     s->vex_l = 0;
     s->vex_v = 0;
  next_byte:
+    /* The prefixes can atmost be 14 bytes since x86 has an upper
+       limit of 15 bytes for the instruction */
+    if (s->pc - pc_start > 14) {
+        goto illegal_op;
+    }
     b = cpu_ldub_code(env, s->pc);
     s->pc++;
     /* Collect prefixes.  */

--
Pranith