net: unix: fix inflight counting bug in garbage collector

Previously I assumed that the receive queues of candidates don't
change during the GC.  This is only half true, nothing can be received
from the queues (see comment in unix_gc()), but buffers could be added
through the other half of the socket pair, which may still have file
descriptors referring to it.

This can result in inc_inflight_move_tail() erronously increasing the
"inflight" counter for a unix socket for which dec_inflight() wasn't
previously called.  This in turn can trigger the "BUG_ON(total_refs <
inflight_refs)" in a later garbage collection run.

Fix this by only manipulating the "inflight" counter for sockets which
are candidates themselves.  Duplicating the file references in
unix_attach_fds() is also needed to prevent a socket becoming a
candidate for GC while the skb that contains it is not yet queued.

Reported-by: Andrea Bittau <a.bittau@cs.ucl.ac.uk>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/net/af_unix.h

@@ -54,6 +54,7 @@ struct unix_sock {
         atomic_long_t           inflight;
         spinlock_t		lock;
 	unsigned int		gc_candidate : 1;
+	unsigned int		gc_maybe_cycle : 1;
         wait_queue_head_t       peer_wait;
 };
 #define unix_sk(__sk) ((struct unix_sock *)__sk)

net/unix/af_unix.c

@@ -1302,14 +1302,23 @@ static void unix_destruct_fds(struct sk_buff *skb)
 	sock_wfree(skb);
 }
 
-static void unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
+static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 {
 	int i;
+
+	/*
+	 * Need to duplicate file references for the sake of garbage
+	 * collection.  Otherwise a socket in the fps might become a
+	 * candidate for GC while the skb is not yet queued.
+	 */
+	UNIXCB(skb).fp = scm_fp_dup(scm->fp);
+	if (!UNIXCB(skb).fp)
+		return -ENOMEM;
+
 	for (i=scm->fp->count-1; i>=0; i--)
 		unix_inflight(scm->fp->fp[i]);
-	UNIXCB(skb).fp = scm->fp;
 	skb->destructor = unix_destruct_fds;
-	scm->fp = NULL;
+	return 0;
 }
 
 /*
@@ -1368,8 +1377,11 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		goto out;
 
 	memcpy(UNIXCREDS(skb), &siocb->scm->creds, sizeof(struct ucred));
-	if (siocb->scm->fp)
-		unix_attach_fds(siocb->scm, skb);
+	if (siocb->scm->fp) {
+		err = unix_attach_fds(siocb->scm, skb);
+		if (err)
+			goto out_free;
+	}
 	unix_get_secdata(siocb->scm, skb);
 
 	skb_reset_transport_header(skb);
@@ -1538,8 +1550,13 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		size = min_t(int, size, skb_tailroom(skb));
 
 		memcpy(UNIXCREDS(skb), &siocb->scm->creds, sizeof(struct ucred));
-		if (siocb->scm->fp)
-			unix_attach_fds(siocb->scm, skb);
+		if (siocb->scm->fp) {
+			err = unix_attach_fds(siocb->scm, skb);
+			if (err) {
+				kfree_skb(skb);
+				goto out_err;
+			}
+		}
 
 		if ((err = memcpy_fromiovec(skb_put(skb,size), msg->msg_iov, size)) != 0) {
 			kfree_skb(skb);

net/unix/garbage.c

@@ -186,8 +186,17 @@ static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 				 */
 				struct sock *sk = unix_get_socket(*fp++);
 				if (sk) {
+					struct unix_sock *u = unix_sk(sk);
+
+					/*
+					 * Ignore non-candidates, they could
+					 * have been added to the queues after
+					 * starting the garbage collection
+					 */
+					if (u->gc_candidate) {
 						hit = true;
-					func(unix_sk(sk));
+						func(u);
+					}
 				}
 			}
 			if (hit && hitlist != NULL) {
@@ -249,11 +258,11 @@ static void inc_inflight_move_tail(struct unix_sock *u)
 {
 	atomic_long_inc(&u->inflight);
 	/*
-	 * If this is still a candidate, move it to the end of the
-	 * list, so that it's checked even if it was already passed
-	 * over
+	 * If this still might be part of a cycle, move it to the end
+	 * of the list, so that it's checked even if it was already
+	 * passed over
 	 */
-	if (u->gc_candidate)
+	if (u->gc_maybe_cycle)
 		list_move_tail(&u->link, &gc_candidates);
 }
 
@@ -267,6 +276,7 @@ void unix_gc(void)
 	struct unix_sock *next;
 	struct sk_buff_head hitlist;
 	struct list_head cursor;
+	LIST_HEAD(not_cycle_list);
 
 	spin_lock(&unix_gc_lock);
 
@@ -282,10 +292,14 @@ void unix_gc(void)
 	 *
 	 * Holding unix_gc_lock will protect these candidates from
 	 * being detached, and hence from gaining an external
-	 * reference.  This also means, that since there are no
-	 * possible receivers, the receive queues of these sockets are
-	 * static during the GC, even though the dequeue is done
-	 * before the detach without atomicity guarantees.
+	 * reference.  Since there are no possible receivers, all
+	 * buffers currently on the candidates' queues stay there
+	 * during the garbage collection.
+	 *
+	 * We also know that no new candidate can be added onto the
+	 * receive queues.  Other, non candidate sockets _can_ be
+	 * added to queue, so we must make sure only to touch
+	 * candidates.
 	 */
 	list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
 		long total_refs;
@@ -299,6 +313,7 @@ void unix_gc(void)
 		if (total_refs == inflight_refs) {
 			list_move_tail(&u->link, &gc_candidates);
 			u->gc_candidate = 1;
+			u->gc_maybe_cycle = 1;
 		}
 	}
 
@@ -325,13 +340,23 @@ void unix_gc(void)
 		list_move(&cursor, &u->link);
 
 		if (atomic_long_read(&u->inflight) > 0) {
-			list_move_tail(&u->link, &gc_inflight_list);
-			u->gc_candidate = 0;
+			list_move_tail(&u->link, &not_cycle_list);
+			u->gc_maybe_cycle = 0;
 			scan_children(&u->sk, inc_inflight_move_tail, NULL);
 		}
 	}
 	list_del(&cursor);
 
+	/*
+	 * not_cycle_list contains those sockets which do not make up a
+	 * cycle.  Restore these to the inflight list.
+	 */
+	while (!list_empty(&not_cycle_list)) {
+		u = list_entry(not_cycle_list.next, struct unix_sock, link);
+		u->gc_candidate = 0;
+		list_move_tail(&u->link, &gc_inflight_list);
+	}
+
 	/*
 	 * Now gc_candidates contains only garbage.  Restore original
 	 * inflight counters for these as well, and remove the skbuffs