binder: fix null deref of proc->context

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 f0fe2c0f ("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: f0fe2c0f ("binder: prevent UAF for binderfs devices II")
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200622200715.114382-1-tkjos@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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