Patch "drbd: Fix kernel_sendmsg() usage - potential NULL deref" has been added to the 4.4-stable tree

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

drbd: Fix kernel_sendmsg() usage - potential NULL deref

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drbd-fix-kernel_sendmsg-usage-potential-null-deref.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Richard Weinberger <[email protected]>
Date: Wed, 9 Nov 2016 22:52:58 +0100
Subject: drbd: Fix kernel_sendmsg() usage - potential NULL deref

From: Richard Weinberger <[email protected]>

commit d8e9e5e80e882b4f90cba7edf1e6cb7376e52e54 upstream.

Don't pass a size larger than iov_len to kernel_sendmsg(). Otherwise it will cause a NULL pointer deref when kernel_sendmsg() returns with rv < size.

DRBD as external module has been around in the kernel 2.4 days already. We used to be compatible to 2.4 and very early 2.6 kernels, we used to use
rv = sock_sendmsg(sock, &msg, iov.iov_len);
then later changed to
rv = kernel_sendmsg(sock, &msg, &iov, 1, size);
when we should have used
rv = kernel_sendmsg(sock, &msg, &iov, 1, iov.iov_len);

tcp_sendmsg() used to totally ignore the size parameter.
57be5bd ip: convert tcp_sendmsg() to iov_iter primitives
changes that, and exposes our long standing error.

Even with this error exposed, to trigger the bug, we would need to have an environment (config or otherwise) causing us to not use sendpage() for larger transfers, a failing connection, and have it fail "just at the right time". Apparently that was unlikely enough for most, so this went unnoticed for years.

Still, it is known to trigger at least some of these,
and suspected for the others:
[0] http://lists.linbit.com/pipermail/drbd-user/2016-July/023112.html
[1] http://lists.linbit.com/pipermail/drbd-dev/2016-March/003362.html
[2] https://forums.grsecurity.net/viewtopic.php?f=3&t=4546
[3] https://ubuntuforums.org/showthread.php?t=2336150
[4] http://e2.howsolveproblem.com/i/1175162/

This should go into 4.9,
and into all stable branches since and including v4.0, which is the first to contain the exposing change.

It is correct for all stable branches older than that as well (which contain the DRBD driver; which is 2.6.33 and up).

It requires a small "conflict" resolution for v4.4 and earlier, with v4.5 we dropped the comment block immediately preceding the kernel_sendmsg().

---
 drivers/block/drbd/drbd_main.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -1802,7 +1802,7 @@ int drbd_send(struct drbd_connection *co
  * do we need to block DRBD_SIG if sock == &meta.socket ??
  * otherwise wake_asender() might interrupt some send_*Ack !
  */
-		rv = kernel_sendmsg(sock, &msg, &iov, 1, size);
+		rv = kernel_sendmsg(sock, &msg, &iov, 1, iov.iov_len);
 		if (rv == -EAGAIN) {
 			if (we_should_drop_the_connection(connection, sock))
 				break;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/drbd-fix-kernel_sendmsg-usage-potential-null-deref.patch