linux-kernel - Re: [PATCH] exec: avoid gcc-8 warning for get_task

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Date:   Tue, 5 Dec 2017 14:38:28 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <james.l.morris@...cle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] exec: avoid gcc-8 warning for get_task_comm

On Mon, Dec 4, 2017 at 7:37 PM, Kees Cook <keescook@...omium.org> wrote:
> On Mon, Dec 4, 2017 at 6:49 AM, Arnd Bergmann <arnd@...db.de> wrote:
>> gcc-8 warns about using strncpy() with the source size as the limit:
>>
>> fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
>>
>> This is indeed slightly suspicious, as it protects us from source
>> arguments without NUL-termination, but does not guarantee that the
>> destination is terminated.
>>
>> This changes it to strlcpy with a hardcoded length, to guarantee
>> a properly terminated string. Since we already use strlcpy() for
>> __set_task_comm(), the source should always be terminated properly,
>
> It's terminated but it's not padded. This change _might_ result in
> leaving a string buffer uninitialized (and if that buffer is passed as
> a whole instead of just its strlen(), this could be a problem:
> proc_comm_connector() e.g. may suffer from this, I haven't checked
> closely).
>
>> so this patch won't change the behavior, but make it a bit more robust.
>
> I would rather we change get_task_comm() to include the size of buf...
> maybe just convert it to a macro:
>
> -char *get_task_comm(char *buf, struct task_struct *tsk)
> +char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>
> ...
>
> #define get_task_comm(buf, tsk) __get_task_comm(buf, sizeof(buf), tsk)
>
> ?
>
> That will only work for preallocated arrays though. Maybe a build bug
> when sizeof(buf) != TASK_COMM_LEN ?

Interesting idea, I'll give that a try.

> I think the strncpy() should stay, but for other conversions where
> NULL-padding isn't an issue, strscpy() is likely preferred to
> strlcpy() to avoid the potential "overread" due to the strlen() call
> in strlcpy().

Ok, I'll keep that in mind if I do more of those conversions.
gcc-8 has started warning about over 100 drivers that use
strncpy() incorrectly, and I suppose a lot of them should
actually use strscpy().

       Arnd