patch "ANDROID: binder: remove WARN() for redundant txn error" added to char-misc-linus

This is a note to let you know that I've just added the patch titled

ANDROID: binder: remove WARN() for redundant txn error

to my char-misc git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
in the char-misc-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From: Todd Kjos <[email protected]>
Date: Wed, 7 Feb 2018 12:38:47 -0800
Subject: ANDROID: binder: remove WARN() for redundant txn error

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().

---
 drivers/android/binder.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

index ad5e662e3e14..31322e9a235d 100644
@@ -1991,8 +1991,14 @@ static void binder_send_failed_reply(struct binder_transaction *t,
 					&target_thread->reply_error.work);
 				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);

2.16.1