[PATCH net-next v2 1/7] af_iucv: take socket lock around SO_MSGSIZE getsockopt

[Breno Leitao <leitao@debian.org>]

Mirror the locking used by the SO_MSGLIMIT case directly above: take
lock_sock() before reading iucv->hs_dev and dereferencing hs_dev->mtu,
and release it afterwards. This keeps the two adjacent getsockopt arms
consistent and matches the lock held by iucv_sock_close() when it
clears hs_dev.

This is not an exploitable bug. iucv_sock_close() is the only writer
of iucv->hs_dev and only runs from the protocol release callback,
which the socket layer invokes after the last file reference drops.
The getsockopt() syscall holds an fd reference for its entire
duration via fdget()/fdput(), so iucv_sock_close() cannot run
concurrently with the SO_MSGSIZE read on the same socket. There is
no other writer of hs_dev, and the aligned pointer load cannot tear
on any architecture Linux supports, so the existing code cannot
observe a NULL dereference or use-after-free in practice.

The change is purely defensive: making the locking pattern uniform
across the function avoids surprising the next reader and removes a
foot-gun should the close path ever grow a new caller that does not
hold the fd reference.

Note: For the reason above, it doesn't contain a "Fixes" tag, and is
aiming at net-next instead of net.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 net/iucv/af_iucv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index 72dfccd4e3d58..3dd11d7a967c8 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c
@@ -1566,9 +1566,11 @@ static int iucv_sock_getsockopt(struct socket *sock, int level, int optname,
 	case SO_MSGSIZE:
 		if (sk->sk_state == IUCV_OPEN)
 			return -EBADFD;
+		lock_sock(sk);
 		val = (iucv->hs_dev) ? iucv->hs_dev->mtu -
 				sizeof(struct af_iucv_trans_hdr) - ETH_HLEN :
 				0x7fffffff;
+		release_sock(sk);
 		break;
 	default:
 		return -ENOPROTOOPT;

-- 
2.53.0-Meta