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

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Message-ID: <54ED00B5.3020203@zytor.com>
Date:	Tue, 24 Feb 2015 14:52:37 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>
CC:	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 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.

	-hpa

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