inet: frags: avoid theoretical race in ip_frag_reinit()

In ip_frag_reinit() we want to move the frag timeout timer into
the future. If the timer fires in the meantime we inadvertently
scheduled it again, and since the timer assumes a ref on frag_queue
we need to acquire one to balance things out.

This is technically racy, we should have acquired the reference
_before_ we touch the timer, it may fire again before we take the ref.
Avoid this entire dance by using mod_timer_pending() which only modifies
the timer if its pending (and which exists since Linux v2.6.30)

Note that this was the only place we ever took a ref on frag_queue
since Eric's conversion to RCU. So we could potentially replace
the whole refcnt field with an atomic flag and a bit more RCU.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251207010942.1672972-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

net/ipv4/inet_fragment.c

@@ -327,7 +327,9 @@ static struct inet_frag_queue *inet_frag_alloc(struct fqdir *fqdir,
 
 	timer_setup(&q->timer, f->frag_expire, 0);
 	spin_lock_init(&q->lock);
-	/* One reference for the timer, one for the hash table. */
+	/* One reference for the timer, one for the hash table.
+	 * We never take any extra references, only decrement this field.
+	 */
 	refcount_set(&q->refcnt, 2);
 
 	return q;

net/ipv4/ip_fragment.c

@@ -242,10 +242,8 @@ static int ip_frag_reinit(struct ipq *qp)
 {
 	unsigned int sum_truesize = 0;
 
-	if (!mod_timer(&qp->q.timer, jiffies + qp->q.fqdir->timeout)) {
-		refcount_inc(&qp->q.refcnt);
+	if (!mod_timer_pending(&qp->q.timer, jiffies + qp->q.fqdir->timeout))
 		return -ETIMEDOUT;
-	}
 
 	sum_truesize = inet_frag_rbtree_purge(&qp->q.rb_fragments,
 					      SKB_DROP_REASON_FRAG_TOO_FAR);