linux-kernel - Re: [PATCH] infiniband: avoid overflow warning

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Message-ID: <CAGXu5j+EPe7csgFt5SnDoobQWkOGH5qaoSZ=QMHHoGFFsMV45Q@mail.gmail.com>
Date:   Mon, 31 Jul 2017 14:18:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Moni Shoua <monis@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        Sean Hefty <sean.hefty@...el.com>,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        Daniel Micay <danielmicay@...il.com>,
        "Kalderon, Michal" <Michal.Kalderon@...ium.com>,
        Ariel Elior <Ariel.Elior@...ium.com>,
        "David S. Miller" <davem@...emloft.net>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Parav Pandit <parav@...lanox.com>,
        Noa Osherovich <noaos@...lanox.com>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] infiniband: avoid overflow warning

On Mon, Jul 31, 2017 at 2:10 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Mon, Jul 31, 2017 at 10:58 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Mon, Jul 31, 2017 at 12:30 AM, Arnd Bergmann <arnd@...db.de> wrote:
>>> On Mon, Jul 31, 2017 at 9:08 AM, Moni Shoua <monis@...lanox.com> wrote:
>>>>>                 break;
>>>>>         default:
>>>>>                 return -EINVAL;
>>>> what happens if you replace 16 with sizeof(struct in6_addr)?
>>>
>>> Same thing: the problem is that gcc already knows the size of the structure we
>>> pass in here, and it is in fact shorter.
>>
>> So gcc is ignoring both the cast (to 16 byte struct in6_addr) and the
>> caller's actual 128 byte struct sockaddr_storage, and looking only at
>> struct sockaddr? That seems really weird.
>
> Using a sockaddr_storage on the stack would address the warning, but
> the question was about just changing the hardcoded 16 to a sizeof()
> operation, and that has no effect.

Right, I didn't mean that; I was curious why the fortify macro
resulted in an error at all. The callers are casting from struct
sockaddr_storage (large enough) to struct sockaddr (not large enough),
and then the inline is casting back to sockaddr_in6 (large enough). I
would have expected fortify to check either sockaddr_storage or
sockaddr_in6, but not sockaddr.

-Kees

-- 
Kees Cook
Pixel Security