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

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Message-ID: <CALCETrXqjNNWUaZQi4XNgABzWY0+w7Y7VcqeVht5SDrp6se+GQ@mail.gmail.com>
Date:	Tue, 24 Feb 2015 15:03:54 -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 3:01 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/24/2015 02:56 PM, Andy Lutomirski wrote:
>> 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?
>>
>
> Actually this part should have been broken up.  The word "several" in
> the patch description is by itself a cause to NAK the patch.

Point taken.

Denys, can you fix this and send a v2 of the entire series with the
traps.c fix as well?

Thanks,
Andy

>
>         -hpa
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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