Re: [PATCH v4 bpf/net 6/6] sockmap: Fix broken memory accounting for UDP.

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

March 5, 2026 at 18:45, "Jiayuan Chen" <jiayuan.chen@linux.dev mailto:jiayuan.chen@linux.dev?to=%22Jiayuan%20Chen%22%20%3Cjiayuan.chen%40linux.dev%3E > wrote:


> 
> March 5, 2026 at 17:27, "Kuniyuki Iwashima" <kuniyu@google.com mailto:kuniyu@google.com?to=%22Kuniyuki%20Iwashima%22%20%3Ckuniyu%40google.com%3E > wrote:
> 
> > 
> > On Thu, Mar 5, 2026 at 12:30 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> >  
> >  
> >  March 5, 2026 at 15:48, "Kuniyuki Iwashima" <kuniyu@google.com mailto:kuniyu@google.com?to=%22Kuniyuki%20Iwashima%22%20%3Ckuniyu%40google.com%3E > wrote:
> >  
> >  On Wed, Mar 4, 2026 at 10:37 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> >  
> >  >
> >  > On Sat, Feb 21, 2026 at 11:30:53PM +0800, Kuniyuki Iwashima wrote:
> >  > [...]
> >  > </TASK>
> >  >
> >  > Fixes: d7f571188ecf ("udp: Implement ->read_sock() for sockmap")
> >  > Reported-by: syzbot+5b3b7e51dda1be027b7a@syzkaller.appspotmail.com
> >  > Closes: https://lore.kernel.org/netdev/698f84c6.a70a0220.2c38d7.00cb.GAE@google.com/
> >  > Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> >  > ---
> >  > v4: Fix deadlock when requeued
> >  > v2: Fix build failure when CONFIG_INET=n
> >  > ---
> >  > include/net/udp.h | 9 +++++++++
> >  > net/core/skmsg.c | 29 +++++++++++++++++++++++++----
> >  > net/ipv4/udp.c | 9 +++++++++
> >  > 3 files changed, 43 insertions(+), 4 deletions(-)
> >  >
> >  > diff --git a/include/net/udp.h b/include/net/udp.h
> >  > index 700dbedcb15f..ae38a4da9388 100644
> >  > --- a/include/net/udp.h
> >  > +++ b/include/net/udp.h
> >  > @@ -455,6 +455,15 @@ struct sock *__udp6_lib_lookup(const struct net *net,
> >  > struct sk_buff *skb);
> >  > struct sock *udp6_lib_lookup_skb(const struct sk_buff *skb,
> >  > __be16 sport, __be16 dport);
> >  > +
> >  > +#ifdef CONFIG_INET
> >  > +void udp_sock_rfree(struct sk_buff *skb);
> >  > +#else
> >  > +static inline void udp_sock_rfree(struct sk_buff *skb)
> >  > +{
> >  > +}
> >  > +#endif
> >  > +
> >  > int udp_read_skb(struct sock *sk, skb_read_actor_t recv_actor);
> >  >
> >  > /* UDP uses skb->dev_scratch to cache as much information as possible and avoid
> >  > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> >  > index 96f43e0dbb17..dd9134a45663 100644
> >  > --- a/net/core/skmsg.c
> >  > +++ b/net/core/skmsg.c
> >  > @@ -7,6 +7,7 @@
> >  >
> >  > #include <net/sock.h>
> >  > #include <net/tcp.h>
> >  > +#include <net/udp.h>
> >  > #include <net/tls.h>
> >  > #include <trace/events/sock.h>
> >  >
> >  > @@ -576,6 +577,7 @@ static int sk_psock_skb_ingress(struct sk_psock *psock, struct sk_buff *skb,
> >  > u32 off, u32 len, bool take_ref)
> >  > {
> >  > struct sock *sk = psock->sk;
> >  > + bool is_udp = sk_is_udp(sk);
> >  > struct sk_msg *msg;
> >  > int err = -EAGAIN;
> >  >
> >  > @@ -583,13 +585,20 @@ static int sk_psock_skb_ingress(struct sk_psock *psock, struct sk_buff *skb,
> >  > if (!msg)
> >  > goto out;
> >  >
> >  > - if (skb->sk != sk && take_ref) {
> >  > + if (is_udp) {
> >  > + if (unlikely(skb->destructor == udp_sock_rfree))
> >  > + goto enqueue;
> >  > +
> >  > + spin_lock_bh(&sk->sk_receive_queue.lock);
> >  > + }
> >  > +
> >  > + if (is_udp || (skb->sk != sk && take_ref)) {
> >  > if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
> >  > - goto free;
> >  > + goto unlock;
> >  >
> >  > if (!sk_rmem_schedule(sk, skb, skb->truesize))
> >  > - goto free;
> >  > - } else {
> >  > + goto unlock;
> >  > + } else if (skb->sk == sk || !take_ref) {
> >  > /* This is used in tcp_bpf_recvmsg_parser() to determine whether the
> >  > * data originates from the socket's own protocol stack. No need to
> >  > * refcount sk because msg's lifetime is bound to sk via the ingress_msg.
> >  > @@ -604,11 +613,23 @@ static int sk_psock_skb_ingress(struct sk_psock *psock, struct sk_buff *skb,
> >  > * into user buffers.
> >  > */
> >  > skb_set_owner_r(skb, sk);
> >  > +
> >  > + if (is_udp) {
> >  > + spin_unlock_bh(&sk->sk_receive_queue.lock);
> >  > +
> >  > + skb->destructor = udp_sock_rfree;
> >  > + }
> >  > +
> >  > +enqueue:
> >  > err = sk_psock_skb_ingress_enqueue(skb, off, len, psock, sk, msg, take_ref);
> >  > if (err < 0)
> >  > goto free;
> >  > out:
> >  > return err;
> >  > +
> >  > +unlock:
> >  > + if (is_udp)
> >  > + spin_unlock_bh(&sk->sk_receive_queue.lock);
> >  > free:
> >  > kfree(msg);
> >  > goto out;
> >  > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> >  > index 422c96fea249..831d26748a90 100644
> >  > --- a/net/ipv4/udp.c
> >  > +++ b/net/ipv4/udp.c
> >  > @@ -2039,6 +2039,15 @@ struct sk_buff *__skb_recv_udp(struct sock *sk, unsigned int flags,
> >  > }
> >  > EXPORT_SYMBOL(__skb_recv_udp);
> >  >
> >  > +void udp_sock_rfree(struct sk_buff *skb)
> >  > +{
> >  > + struct sock *sk = skb->sk;
> >  > +
> >  > + spin_lock_bh(&sk->sk_receive_queue.lock);
> >  > + sock_rfree(skb);
> >  > + spin_unlock_bh(&sk->sk_receive_queue.lock);
> >  > +}
> >  > +
> >  > int udp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
> >  > {
> >  > struct sk_buff *skb;
> >  > --
> >  > 2.53.0.371.g1d285c8824-goog
> >  >
> >  > There are too many protocol-specific checks in the generic skmsg code
> >  > here. This should be abstracted out.
> >  >
> >  > Also TCP also has a similar issue with sk_forward_alloc concurrency
> >  > between the backlog charge and recvmsg uncharge paths so the abstraction
> >  > is necessary.
> >  >
> >  > I've put together a simple diff based on your patch for reference.
> >  > I haven't tested it thoroughly, but it at least handles TCP and UDP
> >  > separately through a callback, keeping the generic code clean.
> >  >
> >  I can follow up on TCP, but TCP needs another fix, which
> >  cannot be factored out.
> >  
> >  Some wrong assumptions in your patch:
> >  
> >  1) tcp_psock_ingress_charge() uses psock->ingress_lock,
> >  but it does not protect any tcp_sock fields, especially
> >  sk->sk_forwad_alloc.
> >  
> >  2) TCP memory accounting happens under bh_lock_sock()
> >  _or_ lock_sock(). sendmsg()/recvmsg() works under
> >  lock_sock() and TCP fast path puts skb to backlog if
> >  lock_sock() is held by userspace. This means even
> >  a simple bh_lock_sock() in tcp_psock_ingress_charge()
> >  race with memory accounting happening under lock_sock().
> >  
> >  Since sk_psock_skb_ingress() is called from both GFP_KERNEL
> >  and GFP_ATOMIC context, the simplest fix would be to put
> >  lock_sock() in sk_psock_backlog() for TCP, which is ugly though.
> >  
> >  OK, thanks.
> >  
> >  Now my only concern is keeping UDP-specific logic
> >  out of skmsg.c.
> >  
> >  We already have TCP-only msg->sk logic there that
> >  you added. It just does not look like TCP code.
> > 
> The msg->sk field marks ingress-self vs ingress-other, which is
> protocol-agnostic metadata. TCP happens to consume it for seq
> tracking, but the assignment itself has no TCP-specific check
> (no is_tcp_sk() or similar).
> 
> > 
> > If really needed, we can do such cleanup in bpf-next
> >  for TCP and UDP altogether (probably after another
> >  TCP fix ?).
> >  
> >  
> >  We could use a function pointer so that UDP
> >  implements its own ingress charge in udp_bpf.c, while other
> >  protocols just use a default no-op or even NULL.
> >  
> >  If TCP requires locking outside of the scope, there's
> >  no point adding one more extra layer/complexity just
> >  for UDP.
> >  
> >  Even if we go that way, most likely we cannot get rid
> >  of protocol-specific code (UDP/TCP-only hook) and
> >  end up with the indirect call helpers, which will be
> >  expensive than simple if logic.
> > 
> An alternative would be to add such sk_psock_udp_skb_ingress() in
> udp_bpf.c, without adding any indirection layer.
> 
> This way we don't need to design a new abstraction before the
> approach is settled. Just a UDP-specific ingress helper, which
> keeps things clean enough for now, and avoid scattering is_udp
> checks all over sk_psock_skb_ingress, just need one if (is_udp).
> 
> Thanks.
>

After thinking a while, since the UDP-specific behaviour only appears
in the redirect-to-self path, how about implementing
sk_psock_udp_skb_ingress_self() in udp_bpf.c and calling it
from sk_psock_skb_ingress_self()? That gives us a single,
contained check point rather than scattering is_udp all over
skmsg.c, which seems more aligned with the "simple" you described before.