From: Todd Kjos tkjos@android.com
binder_send_failed_reply() is called when a synchronous transaction fails. It reports an error to the thread that is waiting for the completion. Given that the transaction is synchronous, there should never be more than 1 error response to that thread -- this was being asserted with a WARN().
However, when exercising the driver with syzbot tests, cases were observed where multiple "synchronous" requests were sent without waiting for responses, so it is possible that multiple errors would be reported to the thread. This testing was conducted with panic_on_warn set which forced the crash.
This is easily reproduced by sending back-to-back "synchronous" transactions without checking for any response (eg, set read_size to 0):
bwr.write_buffer = (uintptr_t)&bc1; bwr.write_size = sizeof(bc1); bwr.read_buffer = (uintptr_t)&br; bwr.read_size = 0; ioctl(fd, BINDER_WRITE_READ, &bwr); sleep(1); bwr2.write_buffer = (uintptr_t)&bc2; bwr2.write_size = sizeof(bc2); bwr2.read_buffer = (uintptr_t)&br; bwr2.read_size = 0; ioctl(fd, BINDER_WRITE_READ, &bwr2); sleep(1);
The first transaction is sent to the servicemanager and the reply fails because no VMA is set up by this client. After binder_send_failed_reply() is called, the BINDER_WORK_RETURN_ERROR is sitting on the thread's todo list since the read_size was 0 and the client is not waiting for a response.
The 2nd transaction is sent and the BINDER_WORK_RETURN_ERROR has not been consumed, so the thread's reply_error.cmd is still set (normally cleared when the BINDER_WORK_RETURN_ERROR is handled). Therefore when the servicemanager attempts to reply to the 2nd failed transaction, the error is already set and it triggers this warning.
This is a user error since it is not waiting for the synchronous transaction to complete. If it ever does check, it will see an error.
Changed the WARN() to a pr_warn().
Signed-off-by: Todd Kjos tkjos@android.com Reported-by: syzbot syzkaller@googlegroups.com Cc: stable stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/android/binder.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/drivers/android/binder.c b/drivers/android/binder.c index c88582a..abf05aa 100644 --- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -1919,8 +1919,14 @@ static void binder_send_failed_reply(struct binder_transaction *t, &target_thread->todo); wake_up_interruptible(&target_thread->wait); } else { - WARN(1, "Unexpected reply error: %u\n", - target_thread->reply_error.cmd); + /* + * Cannot get here for normal operation, but + * we can if multiple synchronous transactions + * are sent without blocking for responses. + * Just ignore the 2nd error in this case. + */ + pr_warn("Unexpected reply error: %u\n", + target_thread->reply_error.cmd); } binder_inner_proc_unlock(target_thread->proc); binder_thread_dec_tmpref(target_thread);