Merge branch 'udp-fix-two-integer-overflows-when-sk-sk_rcvbuf-is-close-to-int_max'

Kuniyuki Iwashima says:

====================
udp: Fix two integer overflows when sk->sk_rcvbuf is close to INT_MAX.

I got a report that UDP mem usage in /proc/net/sockstat did not
drop even after an application was terminated.

The issue could happen if sk->sk_rmem_alloc wraps around due
to a large sk->sk_rcvbuf, which was INT_MAX in our case.

The patch 2 fixes the issue, and the patch 1 fixes yet another
overflow I found while investigating the issue.

v3: https://lore.kernel.org/20250327202722.63756-1-kuniyu@amazon.com
v2: https://lore.kernel.org/20250325195826.52385-1-kuniyu@amazon.com
v1: https://lore.kernel.org/20250323231016.74813-1-kuniyu@amazon.com
====================

Link: https://patch.msgid.link/20250401184501.67377-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

net/ipv4/udp.c

@@ -1625,12 +1625,12 @@ static bool udp_skb_has_head_state(struct sk_buff *skb)
 }
 
 /* fully reclaim rmem/fwd memory allocated for skb */
-static void udp_rmem_release(struct sock *sk, int size, int partial,
-			     bool rx_queue_lock_held)
+static void udp_rmem_release(struct sock *sk, unsigned int size,
+			     int partial, bool rx_queue_lock_held)
 {
 	struct udp_sock *up = udp_sk(sk);
 	struct sk_buff_head *sk_queue;
-	int amt;
+	unsigned int amt;
 
 	if (likely(partial)) {
 		up->forward_deficit += size;
@@ -1650,10 +1650,8 @@ static void udp_rmem_release(struct sock *sk, int size, int partial,
 	if (!rx_queue_lock_held)
 		spin_lock(&sk_queue->lock);
 
-
-	sk_forward_alloc_add(sk, size);
-	amt = (sk->sk_forward_alloc - partial) & ~(PAGE_SIZE - 1);
-	sk_forward_alloc_add(sk, -amt);
+	amt = (size + sk->sk_forward_alloc - partial) & ~(PAGE_SIZE - 1);
+	sk_forward_alloc_add(sk, size - amt);
 
 	if (amt)
 		__sk_mem_reduce_allocated(sk, amt >> PAGE_SHIFT);
@@ -1725,17 +1723,25 @@ static int udp_rmem_schedule(struct sock *sk, int size)
 int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 {
 	struct sk_buff_head *list = &sk->sk_receive_queue;
-	int rmem, err = -ENOMEM;
+	unsigned int rmem, rcvbuf;
 	spinlock_t *busy = NULL;
-	int size, rcvbuf;
+	int size, err = -ENOMEM;
 
-	/* Immediately drop when the receive queue is full.
-	 * Always allow at least one packet.
-	 */
 	rmem = atomic_read(&sk->sk_rmem_alloc);
 	rcvbuf = READ_ONCE(sk->sk_rcvbuf);
-	if (rmem > rcvbuf)
-		goto drop;
+	size = skb->truesize;
+
+	/* Immediately drop when the receive queue is full.
+	 * Cast to unsigned int performs the boundary check for INT_MAX.
+	 */
+	if (rmem + size > rcvbuf) {
+		if (rcvbuf > INT_MAX >> 1)
+			goto drop;
+
+		/* Always allow at least one packet for small buffer. */
+		if (rmem > rcvbuf)
+			goto drop;
+	}
 
 	/* Under mem pressure, it might be helpful to help udp_recvmsg()
 	 * having linear skbs :
@@ -1745,10 +1751,10 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
 	 */
 	if (rmem > (rcvbuf >> 1)) {
 		skb_condense(skb);
-
+		size = skb->truesize;
 		busy = busylock_acquire(sk);
 	}
-	size = skb->truesize;
+
 	udp_set_dev_scratch(skb);
 
 	atomic_add(size, &sk->sk_rmem_alloc);
@@ -1835,7 +1841,7 @@ EXPORT_IPV6_MOD_GPL(skb_consume_udp);
 
 static struct sk_buff *__first_packet_length(struct sock *sk,
 					     struct sk_buff_head *rcvq,
-					     int *total)
+					     unsigned int *total)
 {
 	struct sk_buff *skb;
 
@@ -1868,8 +1874,8 @@ static int first_packet_length(struct sock *sk)
 {
 	struct sk_buff_head *rcvq = &udp_sk(sk)->reader_queue;
 	struct sk_buff_head *sk_queue = &sk->sk_receive_queue;
+	unsigned int total = 0;
 	struct sk_buff *skb;
-	int total = 0;
 	int res;
 
 	spin_lock_bh(&rcvq->lock);