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

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Date:   Tue, 5 Dec 2017 21:27:08 +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 8:19 PM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Dec 5, 2017 at 7:20 AM, Arnd Bergmann <arnd@...db.de> wrote:
>> While reviewing all callers of get_task_comm(), I stumbled
>> over this one that claimed it was not exported, when in fact
>> it is. Accessing task->comm directly is not safe, so better
>> convert this one to using get_task_comm as well.
>
> Using get_task_comm() in cases like this is actually overkill (i.e.
> using up stack space), since there's (currently) no benefit. Nothing
> protects getting a "correct" view of task->comm (i.e. it could get
> updated in the middle of a copy), but it _is_ always NULL terminated,
> so it's safe to use with %s like this. While it does make me slightly
> uncomfortable to _depend_ on this NULL termination, but there are lots
> of open-coded %s users of task->comm. When we're trying to save a
> _copy_ of task->comm, then we want get_task_comm(), just to make sure
> we're doing it right.
>
> So, while I don't oppose this patch, it might be seen as a wasteful
> use of stack space.

It's only a few bytes of stack space in a leaf function, I'd not be
worried about that.

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.

      Arnd