netdev - Re: [PATCH v1 net 1/3] udp: Fix multiple wraparounds of sk->sk_rmem

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Message-ID: <CANn89iLu=6jT_2xvOOzkzQJzXVDroJyiMKC2B83dAwycat2Lhg@mail.gmail.com>
Date: Mon, 24 Mar 2025 11:01:15 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v1 net 1/3] udp: Fix multiple wraparounds of sk->sk_rmem_alloc.

On Mon, Mar 24, 2025 at 12:11 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> __udp_enqueue_schedule_skb() has the following condition:
>
>   if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
>           goto drop;
>
> sk->sk_rcvbuf is initialised by net.core.rmem_default and later can
> be configured by SO_RCVBUF, which is limited by net.core.rmem_max,
> or SO_RCVBUFFORCE.
>
> If we set INT_MAX to sk->sk_rcvbuf, the condition is always false
> as sk->sk_rmem_alloc is also signed int.
>
> Then, the size of the incoming skb is added to sk->sk_rmem_alloc
> unconditionally.
>
> This results in integer overflow (possibly multiple times) on
> sk->sk_rmem_alloc and allows a single socket to have skb up to
> net.core.udp_mem[1].
>
> For example, if we set a large value to udp_mem[1] and INT_MAX to
> sk->sk_rcvbuf and flood packets to the socket, we can see multiple
> overflows:
>
>   # cat /proc/net/sockstat | grep UDP:
>   UDP: inuse 3 mem 7956736  <-- (7956736 << 12) bytes > INT_MAX * 15
>                                              ^- PAGE_SHIFT
>   # ss -uam
>   State  Recv-Q      ...
>   UNCONN -1757018048 ...    <-- flipping the sign repeatedly
>          skmem:(r2537949248,rb2147483646,t0,tb212992,f1984,w0,o0,bl0,d0)
>
> Previously, we had a boundary check for INT_MAX, which was removed by
> commit 6a1f12dd85a8 ("udp: relax atomic operation on sk->sk_rmem_alloc").
>
> A complete fix would be to revert it and cap the right operand by
> INT_MAX:
>
>   rmem = atomic_add_return(size, &sk->sk_rmem_alloc);
>   if (rmem > min(size + (unsigned int)sk->sk_rcvbuf, INT_MAX))
>           goto uncharge_drop;
>
> but we do not want to add the expensive atomic_add_return() back just
> for the corner case.
>
> So, let's perform the first check as unsigned int to detect the
> integer overflow.
>
> Note that we still allow a single wraparound, which can be observed
> from userspace, but it's acceptable considering it's unlikely that
> no recv() is called for a long period, and the negative value will
> soon flip back to positive after a few recv() calls.
>
>   # cat /proc/net/sockstat | grep UDP:
>   UDP: inuse 3 mem 524288  <-- (INT_MAX + 1) >> 12
>
>   # ss -uam
>   State  Recv-Q      ...
>   UNCONN -2147482816 ...   <-- INT_MAX + 831 bytes
>          skmem:(r2147484480,rb2147483646,t0,tb212992,f3264,w0,o0,bl0,d14468947)
>
> Fixes: 6a1f12dd85a8 ("udp: relax atomic operation on sk->sk_rmem_alloc")
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  net/ipv4/udp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index a9bb9ce5438e..a1e60aab29b5 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1735,7 +1735,7 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
>          */
>         rmem = atomic_read(&sk->sk_rmem_alloc);
>         rcvbuf = READ_ONCE(sk->sk_rcvbuf);
> -       if (rmem > rcvbuf)
> +       if ((unsigned int)rmem > rcvbuf)

SGTM, but maybe make rmem and rcvbuf  'unsigned int ' to avoid casts ?

BTW piling 2GB worth of skbs in a single UDP receive queue means a
latency spike when
__skb_queue_purge(&sk->sk_receive_queue) is called, say from
inet_sock_destruct(),
which is a problem on its own.


diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index db606f7e4163809d8220be1c1a4adb5662fc914e..575baac391e8af911fc1eff3f2d8e64bb9aa4c70
100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1725,9 +1725,9 @@ 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;
+       int size, err = -ENOMEM;
        spinlock_t *busy = NULL;
-       int size, rcvbuf;

        /* Immediately drop when the receive queue is full.
         * Always allow at least one packet.