linux-kernel - Re: [PATCH] drbd: Fix kernel

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Date:   Tue, 8 Nov 2016 14:43:43 +0100
From:   Lars Ellenberg <lars.ellenberg@...bit.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        drbd-dev@...ts.linbit.com, lars.ellenberg@...bit.com,
        philipp.reisner@...bit.com, viro@...iv.linux.org.uk,
        Richard Weinberger <richard@....at>,
        christoph.lechleitner@...g.at, wolfgang.glas@...g.at
Subject: Re: [PATCH] drbd: Fix kernel_sendmsg() usage

>From 3a5859e696178e31a25e65de58c461046fc52beb Mon Sep 17 00:00:00 2001
From: Richard Weinberger <richard@....at>
Date: Tue, 8 Nov 2016 11:43:09 +0100
Subject: [PATCH] drbd: Fix kernel_sendmsg() usage - potential NULL deref
drbd: Fix kernel_sendmsg() usage - potential NULL deref

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 flaky 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().

Cc: stable@...r.kernel.org
Cc: viro@...iv.linux.org.uk
Cc: christoph.lechleitner@...g.at
Cc: wolfgang.glas@...g.at
Reported-by: Christoph Lechleitner <christoph.lechleitner@...g.at>
Tested-by: Christoph Lechleitner <christoph.lechleitner@...g.at>
Signed-off-by: Richard Weinberger <richard@....at>
Signed-off-by: Lars Ellenberg <lars.ellenberg@...bit.com>
---
 drivers/block/drbd/drbd_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index 100be55..8348272 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -1871,7 +1871,7 @@ int drbd_send(struct drbd_connection *connection, struct socket *sock,
 		drbd_update_congested(connection);
 	}
 	do {
-		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;
-- 
2.7.4