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Date: Wed, 26 Oct 2016 14:52:45 -0700
From: Kees Cook <keescook@...omium.org>
To: David Windsor <dwindsor@...il.com>
Cc: "Reshetova, Elena" <elena.reshetova@...el.com>, AKASHI Takahiro <takahiro.akashi@...aro.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Hans Liljestrand <ishkamiel@...il.com>
Subject: Re: [RFC v2 PATCH 12/13] x86: implementation for HARDENED_ATOMIC
On Wed, Oct 26, 2016 at 2:48 PM, David Windsor <dwindsor@...il.com> wrote:
> On Wed, Oct 26, 2016 at 4:51 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Wed, Oct 26, 2016 at 4:15 AM, Reshetova, Elena
>> <elena.reshetova@...el.com> wrote:
>>>>> +static __always_inline int __atomic_add_unless_wrap(atomic_wrap_t *v,
>>>>> + int a, int u)
>>>>> +{
>>>>> + int c, old, new;
>>>>> + c = atomic_read_wrap(v);
>>>>> + for (;;) {
>>>>> + if (unlikely(c == (u)))
>>>>> + break;
>>>>> +
>>>>> + asm volatile("addl %2,%0\n"
>>>>> +
>>>>> +#ifdef CONFIG_HARDENED_ATOMIC
>>>>> + "jno 0f\n"
>>>>> + "subl %2,%0\n"
>>>>> + "int $4\n0:\n"
>>>>> + _ASM_EXTABLE(0b, 0b)
>>>>> +#endif
>>>>
>>>> Is this a mistake? We don't need a check here.
>>>
>>>>Yes, this appears to be a mistake.
>>>
>>> Clear copy paste mistake. Fixed now. Thanks again!
>>
>> What was the mistake here?
>>
>
> It's an function that allows wrapping to occur: overflow protection
> shouldn't happen here.
Oh! Yes, of course. My eyes missed the _wrap part. :)
-Kees
--
Kees Cook
Nexus Security
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