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

H. Peter Anvin
2015-02-24T22:53:15Z

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.

-hpa