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

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Date:   Mon, 31 Jul 2017 15:06:59 -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 2:52 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Mon, Jul 31, 2017 at 11:18 PM, Kees Cook <keescook@...omium.org> wrote:
>> 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.
>
> To clarify: this happens in inetaddr_event(), which has a sockaddr_in
> on the stack, not a sockaddr_storage. I tried casting the sockaddr_in
> pointer to sockaddr_storage, but that did not help. Changing the

Oooh, I see now. Yeah, addr_event() sees it directly as struct
sockaddr and even with the resulting inlining into inetaddr_event(),
the dead-code analysis doesn't eliminate the AF_INET6 case, which is a
shame.

> type of the stack variable to sockaddr_storage does help.

That seems like an unfortunate waste of stack space for a false
positive. :) I think your original fix is fine. (In fact, I think it's
actually more robust since there isn't a hard-coded "16" -- not that
it'll ever change, of course.)

-Kees

-- 
Kees Cook
Pixel Security