Commit 6209344f authored by Miklos Szeredi's avatar Miklos Szeredi Committed by Linus Torvalds
Browse files

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: default avatarAndrea Bittau <a.bittau@cs.ucl.ac.uk>
Signed-off-by: default avatarMiklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 058e3739

include/net/af_unix.h

+1 −0
Original line number Diff line number Diff line
@@ -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

+24 −7
Original line number Diff line number Diff line
@@ -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

+37 −12
Original line number Diff line number Diff line
@@ -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

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