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

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Message-ID: <CALCETrV5PkXo4TXx0de-tYs2reZmKKRzN04NOrhv+FSmzKs=hg@mail.gmail.com>
Date:	Tue, 24 Feb 2015 14:56:13 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

On Tue, Feb 24, 2015 at 2:52 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/24/2015 02:25 PM, Andy Lutomirski wrote:
>> On Tue, Feb 24, 2015 at 10:51 AM, Denys Vlasenko <dvlasenk@...hat.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
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