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>

3 files changed, 62 insertions, 19 deletions

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 7dd29b7e461d..c29ff1da8a18 100644
--- a/include/net/af_unix.h
+++ b/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)
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 4d3c6071b9a4..eb90f77bb0e2 100644
--- a/net/unix/af_unix.c
+++ b/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);
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 2a27b84f740b..6d4a9a8de5ef 100644
--- a/net/unix/garbage.c
+++ b/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) {
-                                        hit = true;
-                                        func(unix_sk(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(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;
                 }
         }
 
@@ -325,14 +340,24 @@ 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
          * which are creating the cycle(s).