Re: kernel BUG at net/unix/garbage.c:149!"

[Miklos Szeredi <mszeredi@redhat.com>]

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On Tue, Aug 30, 2016 at 12:37 AM, Miklos Szeredi <mszeredi@redhat.com> wrote:
> On Sat, Aug 27, 2016 at 11:55 AM, Miklos Szeredi <mszeredi@redhat.com> wrote:

> crash> list -H gc_inflight_list unix_sock.link -s unix_sock.inflight |
> grep counter | cut -d= -f2 | awk '{s+=$1} END {print s}'
> 130
> crash> p unix_tot_inflight
> unix_tot_inflight = $2 = 135
>
> We've lost track of a total of five inflight sockets, so it's not a
> one-off thing.  Really weird...  Now off to sleep, maybe I'll dream of
> the solution.

Okay, found one bug: gc assumes that in-flight sockets that don't have
an external ref can't gain one while unix_gc_lock is held.  That is
true because unix_notinflight() will be called before detaching fds,
which takes unix_gc_lock.  Only MSG_PEEK was somehow overlooked.  That
one also clones the fds, also keeping them in the skb.  But through
MSG_PEEK an external reference can definitely be gained without ever
touching unix_gc_lock.

Not sure whether the reported bug can be explained by this.  Can you
confirm the MSG_PEEK was used in the setup?

Does someone want to write a stress test for SCM_RIGHTS + MSG_PEEK?

Anyway, attaching a fix that works by acquiring unix_gc_lock in case
of MSG_PEEK also.  It is trivially correct, but I haven't tested it.

Thanks,
Miklos

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From: Miklos Szeredi <mszeredi@redhat.com>
Subject: af_unix: fix garbage collect vs. MSG_PEEK

Gc assumes that in-flight sockets that don't have an external ref can't
gain one while unix_gc_lock is held.  That is true because
unix_notinflight() will be called before detaching fds, which takes
unix_gc_lock.

Only MSG_PEEK was somehow overlooked.  That one also clones the fds, also
keeping them in the skb.  But through MSG_PEEK an external reference can
definitely be gained without ever touching unix_gc_lock.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>
---
 include/net/af_unix.h |    1 +
 net/unix/af_unix.c    |   15 +++++++++++++--
 net/unix/garbage.c    |    6 ++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -10,6 +10,7 @@ void unix_inflight(struct user_struct *u
 void unix_notinflight(struct user_struct *user, struct file *fp);
 void unix_gc(void);
 void wait_for_unix_gc(void);
+void unix_gc_barrier(void);
 struct sock *unix_get_socket(struct file *filp);
 struct sock *unix_peer_get(struct sock *);
 
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1563,6 +1563,17 @@ static int unix_attach_fds(struct scm_co
 	return max_level;
 }
 
+static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb)
+{
+	scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+	/*
+	 * During garbage collection it is assumed that in-flight sockets don't
+	 * get a new external reference.  So we need to wait until current run
+	 * finishes.
+	 */
+	unix_gc_barrier();
+}
+
 static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
 {
 	int err = 0;
@@ -2195,7 +2206,7 @@ static int unix_dgram_recvmsg(struct soc
 		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;
 
@@ -2435,7 +2446,7 @@ static int unix_stream_read_generic(stru
 			/* 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);
 
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -266,6 +266,12 @@ void wait_for_unix_gc(void)
 	wait_event(unix_gc_wait, gc_in_progress == false);
 }
 
+void unix_gc_barrier(void)
+{
+	spin_lock(&unix_gc_lock);
+	spin_unlock(&unix_gc_lock);
+}
+
 /* The external entry point: unix_gc() */
 void unix_gc(void)
 {