binder: fix UAF in binder_free_transaction()

In binder_free_transaction(), the t->to_proc is read under the t->lock.
However, once the t->lock is dropped, the to_proc can die in parallel.
This leads to a use-after-free error when we attempt to acquire its
inner lock right afterwards:

  ==================================================================
  BUG: KASAN: slab-use-after-free in _raw_spin_lock+0xe4/0x1a0
  Write of size 4 at addr ffff00001125da70 by task B/672

  CPU: 20 UID: 0 PID: 672 Comm: B Not tainted 7.1.0-rc6-00284-g8e65320d91cd #4 PREEMPT
  Hardware name: linux,dummy-virt (DT)
  Call trace:
   _raw_spin_lock+0xe4/0x1a0
   binder_free_transaction+0x8c/0x320
   binder_send_failed_reply+0x21c/0x2f8
   binder_thread_release+0x488/0x7e0
   binder_ioctl+0x12c0/0x29a0
  [...]

  Allocated by task 675:
   __kmalloc_cache_noprof+0x174/0x444
   binder_open+0x118/0xb70
   do_dentry_open+0x374/0x1040
   vfs_open+0x58/0x3bc
  [...]

  Freed by task 212:
   __kasan_slab_free+0x58/0x80
   kfree+0x1a0/0x4a4
   binder_proc_dec_tmpref+0x32c/0x5e0
   binder_deferred_func+0xc48/0x104c
   process_one_work+0x53c/0xbc0
  [...]
  ==================================================================

To prevent this, pin the target thread (t->to_thread) to guarantee the
target process remains alive. Undelivered transactions without a target
thread are already safe, as the target process can only be the current
context in those paths.

Cc: stable <stable@kernel.org>
Reported-by: Alice Ryhl <aliceryhl@google.com>
Closes: https://lore.kernel.org/all/aikJKVuny_eOivwN@google.com/
Fixes: a370003 ("binder: fix possible UAF when freeing buffer")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260619185233.2194678-2-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Carlos Llamas

drivers/android/binder.c

@@ -1658,10 +1658,19 @@ static void binder_txn_latency_free(struct binder_transaction *t)
 
 static void binder_free_transaction(struct binder_transaction *t)
 {
+	struct binder_thread *target_thread;
 	struct binder_proc *target_proc;
 
 	spin_lock(&t->lock);
 	target_proc = t->to_proc;
+	target_thread = t->to_thread;
+	/*
+	 * Pin target_thread to keep target_proc alive. Undelivered
+	 * transactions with !target_thread are safe, as target_proc
+	 * can only be the current context there.
+	 */
+	if (target_thread)
+		atomic_inc(&target_thread->tmp_ref);
 	spin_unlock(&t->lock);
 
 	if (target_proc) {
@@ -1676,6 +1685,10 @@ static void binder_free_transaction(struct binder_transaction *t)
 			t->buffer->transaction = NULL;
 		binder_inner_proc_unlock(target_proc);
 	}
+
+	if (target_thread)
+		binder_thread_dec_tmpref(target_thread);
+
 	if (trace_binder_txn_latency_free_enabled())
 		binder_txn_latency_free(t);
 	/*