Subject: Re: [PATCH 1/7] fs/exec: make __set_task_comm always set a nul terminated string

From: Kees Cook <[email-masked]>
Date: Wed, 10 Nov 2021 12:17:34 -0800

On Wed, Nov 10, 2021 at 09:28:12AM +0100, David Hildenbrand wrote:
> On 08.11.21 09:38, Yafang Shao wrote:
> > Make sure the string set to task comm is always nul terminated.
> > 
> 
> strlcpy: "the result is always a valid NUL-terminated string that fits
> in the buffer"
> 
> The only difference seems to be that strscpy_pad() pads the remainder
> with zeroes.
> 
> Is this description correct and I am missing something important?

Yes, this makes sure it's zero padded just to be robust against full
tsk->comm copies that got noticed in other places.

The only other change is that we want to remove strlcpy() from the
kernel generally since it can trigger out-of-bound reads on the source
string[1].

So, in this case, the most robust version is to use strscpy_pad().

-Kees

[1] https://github.com/KSPP/linux/issues/89

> 
> > Signed-off-by: Yafang Shao <[email-masked]>
> > Reviewed-by: Kees Cook <[email-masked]>
> > Cc: Mathieu Desnoyers <[email-masked]>
> > Cc: Arnaldo Carvalho de Melo <[email-masked]>
> > Cc: Alexei Starovoitov <[email-masked]>
> > Cc: Andrii Nakryiko <[email-masked]>
> > Cc: Michal Miroslaw <[email-masked]> 
> > Cc: Peter Zijlstra <[email-masked]>
> > Cc: Steven Rostedt <[email-masked]>
> > Cc: Matthew Wilcox <[email-masked]>
> > Cc: David Hildenbrand <[email-masked]>
> > Cc: Al Viro <[email-masked]>
> > Cc: Kees Cook <[email-masked]>
> > Cc: Petr Mladek <[email-masked]>
> > ---
> >  fs/exec.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/exec.c b/fs/exec.c
> > index a098c133d8d7..404156b5b314 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1224,7 +1224,7 @@ void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
> >  {
> >  	task_lock(tsk);
> >  	trace_task_rename(tsk, buf);
> > -	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> > +	strscpy_pad(tsk->comm, buf, sizeof(tsk->comm));
> >  	task_unlock(tsk);
> >  	perf_event_comm(tsk, exec);
> >  }
> > 
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Kees Cook