inet: frags: annotate races around fqdir->dead and fqdir->high_thresh

Both fields can be read/written without synchronization,
add proper accessors and documentation.

Fixes: d5dd8879 ("inet: fix various use-after-free in defrags units")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/inet_frag.h

@@ -117,8 +117,15 @@ int fqdir_init(struct fqdir **fqdirp, struct inet_frags *f, struct net *net);
 
 static inline void fqdir_pre_exit(struct fqdir *fqdir)
 {
-	fqdir->high_thresh = 0; /* prevent creation of new frags */
-	fqdir->dead = true;
+	/* Prevent creation of new frags.
+	 * Pairs with READ_ONCE() in inet_frag_find().
+	 */
+	WRITE_ONCE(fqdir->high_thresh, 0);
+
+	/* Pairs with READ_ONCE() in inet_frag_kill(), ip_expire()
+	 * and ip6frag_expire_frag_queue().
+	 */
+	WRITE_ONCE(fqdir->dead, true);
 }
 void fqdir_exit(struct fqdir *fqdir);
 

include/net/ipv6_frag.h

@@ -67,7 +67,8 @@ ip6frag_expire_frag_queue(struct net *net, struct frag_queue *fq)
 	struct sk_buff *head;
 
 	rcu_read_lock();
-	if (fq->q.fqdir->dead)
+	/* Paired with the WRITE_ONCE() in fqdir_pre_exit(). */
+	if (READ_ONCE(fq->q.fqdir->dead))
 		goto out_rcu_unlock;
 	spin_lock(&fq->q.lock);
 

net/ipv4/inet_fragment.c

@@ -235,9 +235,9 @@ void inet_frag_kill(struct inet_frag_queue *fq)
 		/* The RCU read lock provides a memory barrier
 		 * guaranteeing that if fqdir->dead is false then
 		 * the hash table destruction will not start until
-		 * after we unlock.  Paired with inet_frags_exit_net().
+		 * after we unlock.  Paired with fqdir_pre_exit().
 		 */
-		if (!fqdir->dead) {
+		if (!READ_ONCE(fqdir->dead)) {
 			rhashtable_remove_fast(&fqdir->rhashtable, &fq->node,
 					       fqdir->f->rhash_params);
 			refcount_dec(&fq->refcnt);
@@ -352,9 +352,11 @@ static struct inet_frag_queue *inet_frag_create(struct fqdir *fqdir,
 /* TODO : call from rcu_read_lock() and no longer use refcount_inc_not_zero() */
 struct inet_frag_queue *inet_frag_find(struct fqdir *fqdir, void *key)
 {
+	/* This pairs with WRITE_ONCE() in fqdir_pre_exit(). */
+	long high_thresh = READ_ONCE(fqdir->high_thresh);
 	struct inet_frag_queue *fq = NULL, *prev;
 
-	if (!fqdir->high_thresh || frag_mem_limit(fqdir) > fqdir->high_thresh)
+	if (!high_thresh || frag_mem_limit(fqdir) > high_thresh)
 		return NULL;
 
 	rcu_read_lock();

net/ipv4/ip_fragment.c

@@ -144,7 +144,8 @@ static void ip_expire(struct timer_list *t)
 
 	rcu_read_lock();
 
-	if (qp->q.fqdir->dead)
+	/* Paired with WRITE_ONCE() in fqdir_pre_exit(). */
+	if (READ_ONCE(qp->q.fqdir->dead))
 		goto out_rcu_unlock;
 
 	spin_lock(&qp->q.lock);