linux-kernel - Re: [PATCH] kernel: tsacct: strncpy, always be sure of NUL terminated.

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Message-ID: <CAHGf_=rPcUWc=YvR4YTQJH=+SSEQ40nSYmSQXX_aJLDMquiPGw@mail.gmail.com>
Date:	Sun, 7 Apr 2013 23:52:34 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel: tsacct: strncpy, always be sure of NUL terminated.

On Sun, Apr 7, 2013 at 11:27 PM, Chen Gang <gang.chen@...anux.com> wrote:
>
>   for NUL terminated string, always set '\0' at the end.
>
> Signed-off-by: Chen Gang <gang.chen@...anux.com>
> ---
>  kernel/tsacct.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/tsacct.c b/kernel/tsacct.c
> index a1dd9a1..01bcc4e 100644
> --- a/kernel/tsacct.c
> +++ b/kernel/tsacct.c
> @@ -78,7 +78,8 @@ void bacct_add_tsk(struct user_namespace *user_ns,
>         stats->ac_minflt = tsk->min_flt;
>         stats->ac_majflt = tsk->maj_flt;
>
> -       strncpy(stats->ac_comm, tsk->comm, sizeof(stats->ac_comm));
> +       strncpy(stats->ac_comm, tsk->comm, sizeof(stats->ac_comm) - 1);
> +       stats->ac_comm[sizeof(stats->ac_comm) - 1] = '\0';

sizeof(tsk->comm) is 16 and sizeof(stats->ac_comm) is 32. then this statement
is strange. and set_task_comm ensure tsk->comm is nul-terminated.

so your code never change the behavior, right?

And, If buggy driver change tsk->comm not to use set_task_comm and tsk->comm
is not nul-terminated, strncpy() still touch unrelated memory and ac_comm may
expose kernel internal info. that's bad.
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