linux-kernel - Re: [PATCH] ocfs2: use get_task

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Message-ID: <CAK8P3a3Ucm587Ah57-4fKwxkJwK6RvE_4Tgdf_cnKpU6PpoZhg@mail.gmail.com>
Date:   Tue, 5 Dec 2017 22:44:17 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mark Fasheh <mfasheh@...sity.com>,
        Joel Becker <jlbec@...lplan.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joseph Qi <jiangqi903@...il.com>, piaojun <piaojun@...wei.com>,
        ocfs2-devel@....oracle.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ocfs2: use get_task_comm

On Tue, Dec 5, 2017 at 9:32 PM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Dec 5, 2017 at 12:27 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>
>> More generally speaking though, how exactly do we guarantee that
>> there is NUL-termination on tsk->comm during a concurrent update?
>> Could we ever get into a situation where overwrite the NUL byte
>> while setting tsk->comm to a longer string, and read the new start
>> of the string together with an unterminated end, or do we strictly
>> guarantee that the last byte is still NUL? I assume the latter is
>> true, just haven't found exactly where that guarantee is made.
>
> strncpy will zero pad with the trailing NULL, so it's supposed to
> always be safe... still gives me the creeps, though.

But set_task_comm uses strlcpy(), not strncpy(), so you might
get some of the old data back, the question is just whether it could
leak uninitialized data or part of the task_struct up to the next
NUL byte. I could not come up with any code path that would leave
a non-NUL byte in at the end of task->comm though, so it's
probably still safe.

       Arnd