linux-kernel - Re: [PATCH v6 02/12] fs/exec: make __get_task

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Date:   Mon, 25 Oct 2021 14:08:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     akpm@...ux-foundation.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, arnaldo.melo@...il.com,
        pmladek@...e.com, peterz@...radead.org, viro@...iv.linux.org.uk,
        valentin.schneider@....com, qiang.zhang@...driver.com,
        robdclark@...omium.org, christian@...uner.io,
        dietmar.eggemann@....com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        dennis.dalessandro@...nelisnetworks.com,
        mike.marciniszyn@...nelisnetworks.com, dledford@...hat.com,
        jgg@...pe.ca, linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, oliver.sang@...el.com, lkp@...el.com,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH v6 02/12] fs/exec: make __get_task_comm always get a nul
 terminated string

On Mon, Oct 25, 2021 at 08:33:05AM +0000, Yafang Shao wrote:
> If the dest buffer size is smaller than sizeof(tsk->comm), the buffer
> will be without null ternimator, that may cause problem. We can make sure
> the buffer size not smaller than comm at the callsite to avoid that
> problem, but there may be callsite that we can't easily change.
> 
> Using strscpy_pad() instead of strncpy() in __get_task_comm() can make
> the string always nul ternimated.
> 
> Suggested-by: Kees Cook <keescook@...omium.org>
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Petr Mladek <pmladek@...e.com>
> ---
>  fs/exec.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 404156b5b314..bf2a7a91eeea 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1209,7 +1209,8 @@ static int unshare_sighand(struct task_struct *me)
>  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 null terminated */

This may could say "always NUL terminated and zero-padded"

> +	strscpy_pad(buf, tsk->comm, buf_size);
>  	task_unlock(tsk);
>  	return buf;
>  }
> -- 
> 2.17.1
> 

But for the replacement with strscpy_pad(), yes please:

Reviewed-by: Kees Cook <keescook@...omium.org>


-- 
Kees Cook