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>

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 23c92ad..ba7ced9 100644 (file)
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1526,6 +1526,53 @@ out:
        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;
@@ -2175,7 +2222,7 @@ 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;
 
@@ -2418,7 +2465,7 @@ unlock:
                        /* 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);