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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Thu, 14 Oct 2021 17:26:16 +0800
Message-ID: <CALOAHbA8OkrPfWJbYv7Kr988QGHsV0KaHxZwWWimRh6oJDy-8A@mail.gmail.com>
Subject: Re: [PATCH v4 2/5] connector: use __get_task_comm in proc_comm_connector
To:     Kees Cook <keescook@chromium.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Petr Mladek <pmladek@suse.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        qiang.zhang@windriver.com, robdclark@chromium.org,
        christian@brauner.io, Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        David Miller <davem@davemloft.net>, kuba@kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Vladimir Zapolskiy <vzapolskiy@gmail.com>,
        David Howells <dhowells@redhat.com>
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On Thu, Oct 14, 2021 at 12:50 PM Kees Cook <keescook@chromium.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@gmail.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