On Mon, Jun 22, 2020 at 1:18 PM Todd Kjos tkjos@google.com wrote:

On Mon, Jun 22, 2020 at 1:09 PM Christian Brauner christian.brauner@ubuntu.com wrote:

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On Mon, Jun 22, 2020 at 01:07:15PM -0700, Todd Kjos wrote:

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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

Turns out the patch with the issue was also backported to 5.4.y, so the fix is needed there too.

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Thanks, looks good to me! Acked-by: Christian Brauner christian.brauner@ubuntu.com

Christian

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---

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