Re: [PATCH] binder: fix null deref of proc->context

[Todd Kjos <tkjos@google.com>]

On Mon, Jun 22, 2020 at 1:09 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Mon, Jun 22, 2020 at 01:07:15PM -0700, Todd Kjos wrote:
> > The binder driver makes the assumption proc->context pointer is invariant after
> > initialization (as documented in the kerneldoc header for struct proc).
> > However, in commit f0fe2c0f050d ("binder: prevent UAF for binderfs devices II")
> > proc->context is set to NULL during binder_deferred_release().
> >
> > Another proc was in the middle of setting up a transaction to the dying
> > process and crashed on a NULL pointer deref on "context" which is a local
> > set to &proc->context:
> >
> >     new_ref->data.desc = (node == context->binder_context_mgr_node) ? 0 : 1;
> >
> > Here's the stack:
> >
> > [ 5237.855435] Call trace:
> > [ 5237.855441] binder_get_ref_for_node_olocked+0x100/0x2ec
> > [ 5237.855446] binder_inc_ref_for_node+0x140/0x280
> > [ 5237.855451] binder_translate_binder+0x1d0/0x388
> > [ 5237.855456] binder_transaction+0x2228/0x3730
> > [ 5237.855461] binder_thread_write+0x640/0x25bc
> > [ 5237.855466] binder_ioctl_write_read+0xb0/0x464
> > [ 5237.855471] binder_ioctl+0x30c/0x96c
> > [ 5237.855477] do_vfs_ioctl+0x3e0/0x700
> > [ 5237.855482] __arm64_sys_ioctl+0x78/0xa4
> > [ 5237.855488] el0_svc_common+0xb4/0x194
> > [ 5237.855493] el0_svc_handler+0x74/0x98
> > [ 5237.855497] el0_svc+0x8/0xc
> >
> > The fix is to move the kfree of the binder_device to binder_free_proc()
> > so the binder_device is freed when we know there are no references
> > remaining on the binder_proc.
> >
> > Fixes: f0fe2c0f050d ("binder: prevent UAF for binderfs devices II")
> > Signed-off-by: Todd Kjos <tkjos@google.com>

Forgot to include stable. The issue was introduced in 5.6, so fix needed in 5.7.
Cc: stable@vger.kernel.org # 5.7


>
>
> Thanks, looks good to me!
> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
>
> Christian
>
> > ---
> >  drivers/android/binder.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > index e47c8a4c83db..f50c5f182bb5 100644
> > --- a/drivers/android/binder.c
> > +++ b/drivers/android/binder.c
> > @@ -4686,8 +4686,15 @@ static struct binder_thread *binder_get_thread(struct binder_proc *proc)
> >
> >  static void binder_free_proc(struct binder_proc *proc)
> >  {
> > +     struct binder_device *device;
> > +
> >       BUG_ON(!list_empty(&proc->todo));
> >       BUG_ON(!list_empty(&proc->delivered_death));
> > +     device = container_of(proc->context, struct binder_device, context);
> > +     if (refcount_dec_and_test(&device->ref)) {
> > +             kfree(proc->context->name);
> > +             kfree(device);
> > +     }
> >       binder_alloc_deferred_release(&proc->alloc);
> >       put_task_struct(proc->tsk);
> >       binder_stats_deleted(BINDER_STAT_PROC);
> > @@ -5406,7 +5413,6 @@ static int binder_node_release(struct binder_node *node, int refs)
> >  static void binder_deferred_release(struct binder_proc *proc)
> >  {
> >       struct binder_context *context = proc->context;
> > -     struct binder_device *device;
> >       struct rb_node *n;
> >       int threads, nodes, incoming_refs, outgoing_refs, active_transactions;
> >
> > @@ -5423,12 +5429,6 @@ static void binder_deferred_release(struct binder_proc *proc)
> >               context->binder_context_mgr_node = NULL;
> >       }
> >       mutex_unlock(&context->context_mgr_node_lock);
> > -     device = container_of(proc->context, struct binder_device, context);
> > -     if (refcount_dec_and_test(&device->ref)) {
> > -             kfree(context->name);
> > -             kfree(device);
> > -     }
> > -     proc->context = NULL;
> >       binder_inner_proc_lock(proc);
> >       /*
> >        * Make sure proc stays alive after we
> > --
> > 2.27.0.111.gc72c7da667-goog
> >