Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

Andy Lutomirski
2015-02-24T22:56:36Z

On Tue, Feb 24, 2015 at 2:52 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 02/24/2015 02:25 PM, Andy Lutomirski wrote:
>> On Tue, Feb 24, 2015 at 10:51 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>> In all three 32-bit entry points, %eax is zero-extended to %rax.
>>> It is safe to do 32-bit compare when checking that syscall#
>>> is not too large.
>>
>> Applied. Thanks!
>>
>
> NAK NAK NAK NAK NAK!!!!
>
> We have already had this turn into a security issue not just once but
> TWICE, because someone decided to "optimize" the path by taking out the
> zero extend.
>
> The use of a 64-bit compare here is an intentional "belts and
> suspenders" safety issue.

Fair enough. OK if I just undo that part of this patch?

--Andy