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

[Dan Carpenter]

On Tue, Jun 23, 2020 at 11:04:04AM +0200, Christian Brauner wrote:
> On Tue, Jun 23, 2020 at 11:50:21AM +0300, Dan Carpenter 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@xxxxxxxxxx>
> > > ---
> > >  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);
> > > +	}
> > 
> > Where is device allocated?
> > 
> > It looks to me like they are allocated in init_binder_device().  So why
> > are calling misc_deregister?  And it looks like the kfree(proc->context->name);
> > is wrong as well because that's from the
> > "device_names = kstrdup(binder_devices_param, GFP_KERNEL);" in
> > binder_init().
> 
> This whole codepath is only hit for binderfs binder devices which are
> allocated in binderfs.c.

Ah.  I see that now.  Thanks!

regards,
dan carpenter