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Date: Tue, 24 Feb 2015 15:01:30 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns
References: <1424803895-4420-1-git-send-email-dvlasenk@redhat.com> <CALCETrVretXX1Wgx9TNQH67XH=M6fGA+L+7agyp0Fm8503Ey9w@mail.gmail.com> <54ED00B5.3020203@zytor.com> <CALCETrV5PkXo4TXx0de-tYs2reZmKKRzN04NOrhv+FSmzKs=hg@mail.gmail.com>
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On 02/24/2015 02:56 PM, Andy Lutomirski wrote:
> 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?
> 

Actually this part should have been broken up.  The word "several" in
the patch description is by itself a cause to NAK the patch.

	-hpa