From 10bde99768d3c92f5fde1ec51f74e926fc4cf779 Mon Sep 17 00:00:00 2001

From: Artem Savkov <asavkov@redhat.com>

Date: Thu, 27 Jan 2022 11:44:06 +0100

Subject: [KPATCH CVE-2021-0920] af_unix: fix garbage collect vs MSG_PEEK

Kernels:

- 3.10.0-1160.15.2.el7

- 3.10.0-1160.21.1.el7

- 3.10.0-1160.24.1.el7

- 3.10.0-1160.25.1.el7

- 3.10.0-1160.31.1.el7

- 3.10.0-1160.36.2.el7

- 3.10.0-1160.41.1.el7

- 3.10.0-1160.42.2.el7

- 3.10.0-1160.45.1.el7

- 3.10.0-1160.49.1.el7

- 3.10.0-1160.53.1.el7

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

- garbage.o: new function: unix_peek_fds

----

Kpatch-MR: https://gitlab.com/redhat/prdsc/rhel/src/kpatch/rhel-7/-/merge_requests/18

Approved-by: Joe Lawrence (@joe.lawrence)

Approved-by: Yannick Cote (@ycote1)

Kernels:

3.10.0-1160.21.1.el7

3.10.0-1160.24.1.el7

3.10.0-1160.25.1.el7

3.10.0-1160.31.1.el7

3.10.0-1160.36.2.el7

3.10.0-1160.41.1.el7

3.10.0-1160.42.2.el7

3.10.0-1160.45.1.el7

3.10.0-1160.49.1.el7

3.10.0-1160.53.1.el7

Modifications: moved unix_peek_fds() to net/unix/garbage.c to avoid

changing unix_gc_lock scope.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2031986

Z-MR: https://gitlab.com/redhat/rhel/src/kernel/rhel-7/-/merge_requests/379

commit ab0fd1713f1efeb2c859d32721625ea98cd9e663

Author: William Zhao <wizhao@redhat.com>

Date:   Wed Jan 19 09:29:17 2022 -0500

    af_unix: fix garbage collect vs MSG_PEEK

    Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2031970

    Upstream Status: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

    CVE: CVE-2021-0920

    Conflicts: The code still uses the old "sock_iocb" structure since rhel-7

    does not have the commit 7cc05662682da ("net: remove sock_iocb"). Thus

    passing the "scm_cookie" pointer to the "unix_peek_fds" was slightly

    modified to use "siocb->scm" instead of just "&scm". Additionally

    the "unix_gc_lock" is not exposed to "af_unix.c" via a header file since

    rhel-7 does not have the commit f4e65870e5ce ("net: split out functions

    related to registering inflight socket files"). Bringing in the commit

    will add a new UNIX_SCM kernel config; this can be avoided by adding

    extern-ing the "unix_gc_lock".

    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: William Zhao <wizhao@redhat.com>

Signed-off-by: Artem Savkov <asavkov@redhat.com>

---

 net/unix/af_unix.c |  7 +++++--

 net/unix/garbage.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++

 2 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c

index bcb0ad54b9b3..a264b4598872 100644

--- a/net/unix/af_unix.c

+++ b/net/unix/af_unix.c

@@ -1468,6 +1468,8 @@ out:

 	return err;

 }

+void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb);

+

 static void unix_detach_fds(struct scm_cookie *scm, struct sk_buff *skb)

 {

 	int i;

@@ -2182,7 +2184,8 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,

 		sk_peek_offset_fwd(sk, size);

 		if (UNIXCB(skb).fp)

-			siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);

+			unix_peek_fds(siocb->scm, skb);

+

 	}

 	err = (flags & MSG_TRUNC) ? skb->len - skip : size;

@@ -2432,7 +2435,7 @@ unlock:

 			/* It is questionable, see note in unix_dgram_recvmsg.

 			 */

 			if (UNIXCB(skb).fp)

-				siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);

+				unix_peek_fds(siocb->scm, skb);

 			sk_peek_offset_fwd(sk, chunk);

diff --git a/net/unix/garbage.c b/net/unix/garbage.c

index c36757e72844..f242268477ba 100644

--- a/net/unix/garbage.c

+++ b/net/unix/garbage.c

@@ -374,3 +374,50 @@ void unix_gc(void)

  out:

 	spin_unlock(&unix_gc_lock);

 }

+

+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);

+}

-- 

2.26.3