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

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Date:	Mon, 08 Apr 2013 12:03:55 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.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 2013年04月08日 11:52, KOSAKI Motohiro wrote:
> 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?
> 

  right.

  thank you for your information:
    originally, I really did not check the sizeof details.


> 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.
> 
> 

  really, that's bad !

  thank you for your information:
    originally, I did not think of a buggy driver can change tsk->comm.


  :-)

-- 
Chen Gang

Asianux Corporation
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