Re: [PATCH v7 5/8] mm/util: Fix possible race condition in kstrdup()

[Kees Cook <kees@kernel.org>]

On Sat, Aug 17, 2024 at 10:48:15AM +0200, Alejandro Colomar wrote:
> Hi Yafang,
> 
> On Sat, Aug 17, 2024 at 10:56:21AM GMT, Yafang Shao wrote:
> > In kstrdup(), it is critical to ensure that the dest string is always
> > NUL-terminated. However, potential race condidtion can occur between a
> > writer and a reader.
> > 
> > Consider the following scenario involving task->comm:
> > 
> >     reader                    writer
> > 
> >   len = strlen(s) + 1;
> >                              strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> >   memcpy(buf, s, len);
> > 
> > In this case, there is a race condition between the reader and the
> > writer. The reader calculate the length of the string `s` based on the
> > old value of task->comm. However, during the memcpy(), the string `s`
> > might be updated by the writer to a new value of task->comm.
> > 
> > If the new task->comm is larger than the old one, the `buf` might not be
> > NUL-terminated. This can lead to undefined behavior and potential
> > security vulnerabilities.
> > 
> > Let's fix it by explicitly adding a NUL-terminator.
> > 
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > ---
> >  mm/util.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/util.c b/mm/util.c
> > index 983baf2bd675..4542d8a800d9 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -62,8 +62,14 @@ char *kstrdup(const char *s, gfp_t gfp)
> >  
> >  	len = strlen(s) + 1;
> >  	buf = kmalloc_track_caller(len, gfp);
> > -	if (buf)
> > +	if (buf) {
> >  		memcpy(buf, s, len);
> > +		/* During memcpy(), the string might be updated to a new value,
> > +		 * which could be longer than the string when strlen() is
> > +		 * called. Therefore, we need to add a null termimator.
> > +		 */
> > +		buf[len - 1] = '\0';
> > +	}
> 
> I would compact the above to:
> 
> 	len = strlen(s);
> 	buf = kmalloc_track_caller(len + 1, gfp);
> 	if (buf)
> 		strcpy(mempcpy(buf, s, len), "");
> 
> It allows _FORTIFY_SOURCE to track the copy of the NUL, and also uses
> less screen.  It also has less moving parts.  (You'd need to write a
> mempcpy() for the kernel, but that's as easy as the following:)
> 
> 	#define mempcpy(d, s, n)  (memcpy(d, s, n) + n)
> 
> In shadow utils, I did a global replacement of all buf[...] = '\0'; by
> strcpy(..., "");.  It ends up being optimized by the compiler to the
> same code (at least in the experiments I did).

Just to repeat what's already been said: no, please, don't complicate
this with yet more wrappers. And I really don't want to add more str/mem
variants -- we're working really hard to _remove_ them. :P

-Kees

-- 
Kees Cook