From: Kuniyuki Iwashima kuniyu@amazon.com

[ Upstream commit df207de9d9e7a4d92f8567e2c539d9c8c12fd99d ]

Matt Dowling reported a weird UDP memory usage issue.

Under normal operation, the UDP memory usage reported in /proc/net/sockstat remains close to zero. However, it occasionally spiked to 524,288 pages and never dropped. Moreover, the value doubled when the application was terminated. Finally, it caused intermittent packet drops.

We can reproduce the issue with the script below [0]:

1. /proc/net/sockstat reports 0 pages

# cat /proc/net/sockstat | grep UDP: UDP: inuse 1 mem 0

2. Run the script till the report reaches 524,288

# python3 test.py & sleep 5 # cat /proc/net/sockstat | grep UDP: UDP: inuse 3 mem 524288 <-- (INT_MAX + 1) >> PAGE_SHIFT

3. Kill the socket and confirm the number never drops

# pkill python3 && sleep 5 # cat /proc/net/sockstat | grep UDP: UDP: inuse 1 mem 524288

4. (necessary since v6.0) Trigger proto_memory_pcpu_drain()

# python3 test.py & sleep 1 && pkill python3

5. The number doubles

# cat /proc/net/sockstat | grep UDP: UDP: inuse 1 mem 1048577

The application set INT_MAX to SO_RCVBUF, which triggered an integer overflow in udp_rmem_release().

When a socket is close()d, udp_destruct_common() purges its receive queue and sums up skb->truesize in the queue. This total is calculated and stored in a local unsigned integer variable.

The total size is then passed to udp_rmem_release() to adjust memory accounting. However, because the function takes a signed integer argument, the total size can wrap around, causing an overflow.

Then, the released amount is calculated as follows:

1) Add size to sk->sk_forward_alloc. 2) Round down sk->sk_forward_alloc to the nearest lower multiple of PAGE_SIZE and assign it to amount. 3) Subtract amount from sk->sk_forward_alloc. 4) Pass amount >> PAGE_SHIFT to __sk_mem_reduce_allocated().

When the issue occurred, the total in udp_destruct_common() was 2147484480 (INT_MAX + 833), which was cast to -2147482816 in udp_rmem_release().

At 1) sk->sk_forward_alloc is changed from 3264 to -2147479552, and 2) sets -2147479552 to amount. 3) reverts the wraparound, so we don't see a warning in inet_sock_destruct(). However, udp_memory_allocated ends up doubling at 4).

Since commit 3cd3399dd7a8 ("net: implement per-cpu reserves for memory_allocated"), memory usage no longer doubles immediately after a socket is close()d because __sk_mem_reduce_allocated() caches the amount in udp_memory_per_cpu_fw_alloc. However, the next time a UDP socket receives a packet, the subtraction takes effect, causing UDP memory usage to double.

This issue makes further memory allocation fail once the socket's sk->sk_rmem_alloc exceeds net.ipv4.udp_rmem_min, resulting in packet drops.

To prevent this issue, let's use unsigned int for the calculation and call sk_forward_alloc_add() only once for the small delta.

Note that first_packet_length() also potentially has the same problem.

[0]: from socket import *

SO_RCVBUFFORCE = 33 INT_MAX = (2 ** 31) - 1

s = socket(AF_INET, SOCK_DGRAM) s.bind(('', 0)) s.setsockopt(SOL_SOCKET, SO_RCVBUFFORCE, INT_MAX)

c = socket(AF_INET, SOCK_DGRAM) c.connect(s.getsockname())

data = b'a' * 100

while True: c.send(data)

Fixes: f970bd9e3a06 ("udp: implement memory accounting helpers") Reported-by: Matt Dowling madowlin@amazon.com Signed-off-by: Kuniyuki Iwashima kuniyu@amazon.com Reviewed-by: Willem de Bruijn willemb@google.com Link: https://patch.msgid.link/20250401184501.67377-3-kuniyu@amazon.com Signed-off-by: Jakub Kicinski kuba@kernel.org (cherry picked from commit df207de9d9e7a4d92f8567e2c539d9c8c12fd99d) [Yifei: resolve minor conflicts and fix CVE-2025-22058] Signed-off-by: Yifei Liu yifei.l.liu@oracle.com --- net/ipv4/udp.c | 16 +++++++--------- 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index 51a12fa486b6..3ebd5765fb9f 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -1459,12 +1459,12 @@ 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; @@ -1484,10 +1484,8 @@ static void udp_rmem_release(struct sock *sk, int size, int partial, if (!rx_queue_lock_held) spin_lock(&sk_queue->lock);

- - sk->sk_forward_alloc += size; - amt = (sk->sk_forward_alloc - partial) & ~(SK_MEM_QUANTUM - 1); - sk->sk_forward_alloc -= amt; + amt = (size + sk->sk_forward_alloc - partial) & ~(PAGE_SIZE - 1); + sk->sk_forward_alloc += size - amt;

if (amt) __sk_mem_reduce_allocated(sk, amt >> SK_MEM_QUANTUM_SHIFT); @@ -1671,7 +1669,7 @@ EXPORT_SYMBOL_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;

@@ -1704,8 +1702,8 @@ 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);