[PATCH v2 05/10] mm/util: Fix possible race condition in kstrdup()

[Yafang Shao]

On Fri, Jun 14, 2024 at 5:14 AM Andrew Morton <akpm at linux-foundation.org> wrote:
>
> On Thu, 13 Jun 2024 10:30:39 +0800 Yafang Shao <laoar.shao at gmail.com> 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.
>
> The concept sounds a little strange.  If some code takes a copy of a
> string while some other code is altering it, yes, the result will be a
> mess.  This is why get_task_comm() exists, and why it uses locking.
>
> I get that "your copy is a mess" is less serious than "your string
> isn't null-terminated" but still.  Whichever outcome we get, the
> calling code is buggy and should be fixed.
>
> Are there any other problematic scenarios we're defending against here?
>
> >
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -60,8 +60,10 @@ 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);
> > +             buf[len - 1] = '\0';
> > +     }
> >       return buf;
> >  }
>
> Now I'll start receiving patches to remove this again.  Let's have a
> code comment please.

I will add a comment for it.

>
> And kstrdup() is now looking awfully similar to kstrndup().  Perhaps
> there's a way to reduce duplication?

Yes, I believe we can add a common helper for them :

  static char *__kstrndup(const char *s, size_t max, gfp_t gfp)

-- 
Regards
Yafang