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Commit 686a7e32 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

inetpeer: fix race in unused_list manipulations 

Several crashes in cleanup_once() were reported in recent kernels.

Commit d6cc1d64 (inetpeer: various changes) added a race in
unlink_from_unused().

One way to avoid taking unused_peers.lock before doing the list_empty()
test is to catch 0->1 refcnt transitions, using full barrier atomic
operations variants (atomic_cmpxchg() and atomic_inc_return()) instead
of previous atomic_inc() and atomic_add_unless() variants.

We then call unlink_from_unused() only for the owner of the 0->1
transition.

Add a new atomic_add_unless_return() static helper

With help from Arun Sharma.

Refs: https://bugzilla.kernel.org/show_bug.cgi?id=32772



Reported-by: default avatarArun Sharma <asharma@fb.com>
Reported-by: default avatarMaximilian Engelhardt <maxi@daemonizer.de>
Reported-by: default avatarYann Dupont <Yann.Dupont@univ-nantes.fr>
Reported-by: default avatarDenys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent e7a46b4d

net/ipv4/inetpeer.c

+27 −15
Original line number Diff line number Diff line
@@ -154,12 +154,10 @@ void __init inet_initpeers(void) 
/* Called with or without local BH being disabled. */

static void unlink_from_unused(struct inet_peer *p)

{

	if (!list_empty(&p->unused)) {

	spin_lock_bh(&unused_peers.lock);

	list_del_init(&p->unused);

	spin_unlock_bh(&unused_peers.lock);

}

}



static int addr_compare(const struct inetpeer_addr *a,

			const struct inetpeer_addr *b)
@@ -205,6 +203,20 @@ static int addr_compare(const struct inetpeer_addr *a, 
	u;							\

})



static bool atomic_add_unless_return(atomic_t *ptr, int a, int u, int *newv)

{

	int cur, old = atomic_read(ptr);



	while (old != u) {

		*newv = old + a;

		cur = atomic_cmpxchg(ptr, old, *newv);

		if (cur == old)

			return true;

		old = cur;

	}

	return false;

}



/*

 * Called with rcu_read_lock()

 * Because we hold no lock against a writer, its quite possible we fall
@@ -213,7 +225,8 @@ static int addr_compare(const struct inetpeer_addr *a, 
 * We exit from this function if number of links exceeds PEER_MAXDEPTH

 */

static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,

				    struct inet_peer_base *base)

				    struct inet_peer_base *base,

				    int *newrefcnt)

{

	struct inet_peer *u = rcu_dereference(base->root);

	int count = 0;
@@ -226,7 +239,7 @@ static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr, 
			 * distinction between an unused entry (refcnt=0) and

			 * a freed one.

			 */

			if (unlikely(!atomic_add_unless(&u->refcnt, 1, -1)))

			if (!atomic_add_unless_return(&u->refcnt, 1, -1, newrefcnt))

				u = NULL;

			return u;

		}
@@ -465,21 +478,22 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create) 
	struct inet_peer_base *base = family_to_base(daddr->family);

	struct inet_peer *p;

	unsigned int sequence;

	int invalidated;

	int invalidated, newrefcnt = 0;



	/* Look up for the address quickly, lockless.

	 * Because of a concurrent writer, we might not find an existing entry.

	 */

	rcu_read_lock();

	sequence = read_seqbegin(&base->lock);

	p = lookup_rcu(daddr, base);

	p = lookup_rcu(daddr, base, &newrefcnt);

	invalidated = read_seqretry(&base->lock, sequence);

	rcu_read_unlock();



	if (p) {

		/* The existing node has been found.

found:		/* The existing node has been found.

		 * Remove the entry from unused list if it was there.

		 */

		if (newrefcnt == 1)

			unlink_from_unused(p);

		return p;

	}
@@ -494,11 +508,9 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create) 
	write_seqlock_bh(&base->lock);

	p = lookup(daddr, stack, base);

	if (p != peer_avl_empty) {

		atomic_inc(&p->refcnt);

		newrefcnt = atomic_inc_return(&p->refcnt);

		write_sequnlock_bh(&base->lock);

		/* Remove the entry from unused list if it was there. */

		unlink_from_unused(p);

		return p;

		goto found;

	}

	p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;

	if (p) {