From 18c390171a63c1be8df1a62172214efbd9e84813 Mon Sep 17 00:00:00 2001
From: Joe Lawrence <joe.lawrence@redhat.com>
Date: Thu, 20 Jan 2022 14:39:19 -0500
Subject: [KPATCH CVE-2021-0920] af_unix: kpatch fixes for CVE-2021-0920
Kernels:
4.18.0-348.el8
4.18.0-348.2.1.el8_5
4.18.0-348.7.1.el8_5
4.18.0-348.12.2.el8_5
Changes since last build:
arches: x86_64 ppc64le
af_unix.o: changed function: unix_dgram_recvmsg
af_unix.o: changed function: unix_stream_read_generic
---------------------------
Kpatch-MR: https://gitlab.com/redhat/prdsc/rhel/src/kpatch/rhel-8/-/merge_requests/17
Approved-by: Yannick Cote (@ycote1)
Approved-by: Artem Savkov (@artem.savkov)
Kernels:
4.18.0-348.el8
4.18.0-348.2.1.el8_5
4.18.0-348.7.1.el8_5
4.18.0-348.12.2.el8_5
Modifications: none
commit 49c79494c048e940be91a9454c2f507bc33680fc
Author: Patrick Talbert <ptalbert@redhat.com>
Date: Mon Jan 10 13:13:05 2022 +0100
af_unix: fix garbage collect vs MSG_PEEK
Bugzilla: https://bugzilla.redhat.com/2031974
CVE: CVE-2021-0920
Y-Commit: 35c0f6eeb4644e87e7f3c1198a9f31b76220053d
O-CVE: CVE-2021-0920
O-Bugzilla: https://bugzilla.redhat.com/2031975
Upstream status: main
Testing: Sanity only
commit cbcf01128d0a92e131bd09f1688fe032480b65ca
Author: Miklos Szeredi <mszeredi@redhat.com>
Date: Wed Jul 28 14:47:20 2021 +0200
af_unix: fix garbage collect vs MSG_PEEK
unix_gc() assumes that candidate sockets can never gain an external
reference (i.e. be installed into an fd) while the unix_gc_lock is
held. Except for MSG_PEEK this is guaranteed by modifying inflight
count under the unix_gc_lock.
MSG_PEEK does not touch any variable protected by unix_gc_lock (file
count is not), yet it needs to be serialized with garbage collection.
Do this by locking/unlocking unix_gc_lock:
1) increment file count
2) lock/unlock barrier to make sure incremented file count is visible
to garbage collection
3) install file into fd
This is a lock barrier (unlike smp_mb()) that ensures that garbage
collection is run completely before or completely after the barrier.
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Patrick Talbert <ptalbert@redhat.com>
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
net/unix/af_unix.c | 51 ++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 49 insertions(+), 2 deletions(-)
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 247e3138d1ef..d9c968caaf20 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1498,6 +1498,53 @@ static int unix_getname(struct socket *sock, struct sockaddr *uaddr, int peer)
return err;
}
+static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb)
+{
+ scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+
+ /*
+ * Garbage collection of unix sockets starts by selecting a set of
+ * candidate sockets which have reference only from being in flight
+ * (total_refs == inflight_refs). This condition is checked once during
+ * the candidate collection phase, and candidates are marked as such, so
+ * that non-candidates can later be ignored. While inflight_refs is
+ * protected by unix_gc_lock, total_refs (file count) is not, hence this
+ * is an instantaneous decision.
+ *
+ * Once a candidate, however, the socket must not be reinstalled into a
+ * file descriptor while the garbage collection is in progress.
+ *
+ * If the above conditions are met, then the directed graph of
+ * candidates (*) does not change while unix_gc_lock is held.
+ *
+ * Any operations that changes the file count through file descriptors
+ * (dup, close, sendmsg) does not change the graph since candidates are
+ * not installed in fds.
+ *
+ * Dequeing a candidate via recvmsg would install it into an fd, but
+ * that takes unix_gc_lock to decrement the inflight count, so it's
+ * serialized with garbage collection.
+ *
+ * MSG_PEEK is special in that it does not change the inflight count,
+ * yet does install the socket into an fd. The following lock/unlock
+ * pair is to ensure serialization with garbage collection. It must be
+ * done between incrementing the file count and installing the file into
+ * an fd.
+ *
+ * If garbage collection starts after the barrier provided by the
+ * lock/unlock, then it will see the elevated refcount and not mark this
+ * as a candidate. If a garbage collection is already in progress
+ * before the file count was incremented, then the lock/unlock pair will
+ * ensure that garbage collection is finished before progressing to
+ * installing the fd.
+ *
+ * (*) A -> B where B is on the queue of A or B is on the queue of C
+ * which is on the queue of listening socket A.
+ */
+ spin_lock(&unix_gc_lock);
+ spin_unlock(&unix_gc_lock);
+}
+
static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
{
int err = 0;
@@ -2124,7 +2171,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
sk_peek_offset_fwd(sk, size);
if (UNIXCB(skb).fp)
- scm.fp = scm_fp_dup(UNIXCB(skb).fp);
+ unix_peek_fds(&scm, skb);
}
err = (flags & MSG_TRUNC) ? skb->len - skip : size;
@@ -2365,7 +2412,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
/* It is questionable, see note in unix_dgram_recvmsg.
*/
if (UNIXCB(skb).fp)
- scm.fp = scm_fp_dup(UNIXCB(skb).fp);
+ unix_peek_fds(&scm, skb);
sk_peek_offset_fwd(sk, chunk);
--
2.34.1