linux-kernel - Re: [PATCH v4 2/5] connector: use __get_task_comm in proc_comm

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Date:   Thu, 14 Oct 2021 17:26:16 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        qiang.zhang@...driver.com, robdclark@...omium.org,
        christian@...uner.io, Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        David Miller <davem@...emloft.net>, kuba@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Vladimir Zapolskiy <vzapolskiy@...il.com>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v4 2/5] connector: use __get_task_comm in proc_comm_connector

On Thu, Oct 14, 2021 at 12:50 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Wed, Oct 13, 2021 at 10:24:18PM -0400, Steven Rostedt wrote:
> > On Thu, 14 Oct 2021 09:48:09 +0800
> > Yafang Shao <laoar.shao@...il.com> wrote:
> >
> > > > __get_task_comm() uses strncpy() which my understanding is, does not add
> > > > the nul terminating byte when truncating. Which changes the functionality
> > > > here. As all task comms have a terminating byte, the old method would copy
> > > > that and include it. This won't add the terminating byte if the buffer is
> > > > smaller than the comm, and that might cause issues.
> > > >
> > >
> > > Right, that is a problem.
> > > It seems that we should add a new helper get_task_comm_may_truncated().
> >
> > Or simply change __get_task_comm() to:
> >
> > char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> > {
> >       task_lock(tsk);
> >       strncpy(buf, tsk->comm, buf_size);
> >       /* The copied value is always nul terminated */
> >       buf[buf_size - 1] = '\0';
> >       task_unlock(tsk);
> >       return buf;
> > }
> >
> > But that should probably be a separate patch.
>
> strscpy_pad() is the right thing here -- it'll retain the NUL-fill
> properties of strncpy and terminate correctly.
>

strscpy_pad() can also work, and seems more simple.

> The use of non-terminating issue with strncpy() wasn't a problem here
> because get_task_comm() would always make sure task->comm was
> terminated. (It uses strlcpy(), which I think needs to be changed to
> strscpy_pad() too...)
>
> --
> Kees Cook



-- 
Thanks
Yafang