net/9p/mux.c: use-after-free

net/9p/mux.c: use-after-free

[Adrian Bunk]

The Coverity checker spotted the following use-after-free
in net/9p/mux.c:

<--  snip  -->

...
struct p9_conn *p9_conn_create(struct p9_transport *trans, int msize,
                                    unsigned char *extended)
{
...
        if (!m->tagpool) {
                kfree(m);
                return ERR_PTR(PTR_ERR(m->tagpool));
        }
...

<--  snip  -->


cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

Re: net/9p/mux.c: use-after-free

[Eric Van Hensbergen]

On 7/22/07, Adrian Bunk <bunk@stusta.de> wrote:
> The Coverity checker spotted the following use-after-free
> in net/9p/mux.c:
>
> <--  snip  -->
>
> ...
> struct p9_conn *p9_conn_create(struct p9_transport *trans, int msize,
>                                     unsigned char *extended)
> {
> ...
>         if (!m->tagpool) {
>                 kfree(m);
>                 return ERR_PTR(PTR_ERR(m->tagpool));
>         }
> ...
>
> <--  snip  -->
>

I've got a fix for this one:
        if (!m->tagpool) {
                mtmp = ERR_PTR(PTR_ERR(m->tagpool));
                kfree(m);
                return mtmp;
        }

but I was wondering about one of the other returns further down the function:

...
        memset(&m->poll_waddr, 0, sizeof(m->poll_waddr));
        m->poll_task = NULL;
        n = p9_mux_poll_start(m);
        if (n)
                return ERR_PTR(n);

        n = trans->poll(trans, &m->pt);
...

lucho: doesn't that constitute a leak?  Shouldn't we be doing:

        if (n) {
                kfree(m);
                return ERR_PTR(n);
        }

         -eric

Re: net/9p/mux.c: use-after-free

[Latchesar Ionkov]

Yep, it's a leak.

Thanks,
    Lucho

On 7/25/07, Eric Van Hensbergen <ericvh@gmail.com> wrote:
> On 7/22/07, Adrian Bunk <bunk@stusta.de> wrote:
> > The Coverity checker spotted the following use-after-free
> > in net/9p/mux.c:
> >
> > <--  snip  -->
> >
> > ...
> > struct p9_conn *p9_conn_create(struct p9_transport *trans, int msize,
> >                                     unsigned char *extended)
> > {
> > ...
> >         if (!m->tagpool) {
> >                 kfree(m);
> >                 return ERR_PTR(PTR_ERR(m->tagpool));
> >         }
> > ...
> >
> > <--  snip  -->
> >
>
> I've got a fix for this one:
>         if (!m->tagpool) {
>                 mtmp = ERR_PTR(PTR_ERR(m->tagpool));
>                 kfree(m);
>                 return mtmp;
>         }
>
> but I was wondering about one of the other returns further down the function:
>
> ...
>         memset(&m->poll_waddr, 0, sizeof(m->poll_waddr));
>         m->poll_task = NULL;
>         n = p9_mux_poll_start(m);
>         if (n)
>                 return ERR_PTR(n);
>
>         n = trans->poll(trans, &m->pt);
> ...
>
> lucho: doesn't that constitute a leak?  Shouldn't we be doing:
>
>         if (n) {
>                 kfree(m);
>                 return ERR_PTR(n);
>         }
>
>          -eric
>

Re: net/9p/mux.c: use-after-free

[Eric Van Hensbergen]

On 7/25/07, Latchesar Ionkov <lucho@ionkov.net> wrote:
> Yep, it's a leak.
>

Okay, I'll roll that into the patch as well.

           -eric

Re: net/9p/mux.c: use-after-free

[Eric Van Hensbergen]

On 7/25/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 25 Jul 2007 13:43:16 -0500 "Eric Van Hensbergen" <ericvh@gmail.com> wrote:
>
> >                 mtmp = ERR_PTR(PTR_ERR(m->tagpool));
>
> odd.  What does ERR_PTR(PTR_ERR(...)) do?
>

I kind of assumed it was a necessry evil to get the casting right.  A
quick grep shows it in 42 other places within the kernel.  Unpacking
the macros it looks like:

   (void *)(long)(struct p9_idpool *)

So all that you would really need is (void *) or ERR_PTR -- but that
might look confusing in the code.  Of course, broadening the context a
bit:

        m->tagpool = p9_idpool_create();
        if (!m->tagpool) {
                mtmp = ERR_PTR(PTR_ERR(m->tagpool));
                kfree(m);
                return mtmp;
        }

m->tagpool must be zero to enter the code at all, so we are returning
a NULL pointer, not really an error -- which is probably wrong (I
don't think it will properly trigger IS_ERR_VALUE) -- so we should
probably be returning -ENOMEM.

Of course, we really should be seeing an ERR_PTR returned from
p9_idpool_create, not 0 -- checking that code, it either returns
-ENOMEM or the correct value, never 0, so the check is wrong as well.
It should be:

        m->tagpool = p9_idpool_create();
        if (IS_ERR(m->tagpool)) {
                mtmp = ERR_PTR(-ENOMEM);
                kfree(m);
                return mtmp;
        }

We could have done:
   ERR_PTR(m->tagpool);
or kept the long:
   ERR_PTR(PTR_ERR(m->tagpool));
but I think returning an explicit error code keeps the code more clear.

So, which is the correct approach?

                   -eric