Re: [PATCH bpf 1/2] bpf, sockmap: Don't leak UDP socks on lookup-bind-release

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Jun 23, 2026 at 08:03 PM +02, Michal Luczaj wrote:
> UDP sockets get SOCK_RCU_FREE set when (auto-)bound. This means
> sk_is_refcounted(unbound) = true, while sk_is_refcounted(bound) = false.
>
> Because sockmap accepts unbound UDP sockets, a BPF program can increment a
> socket's refcount via lookup. If the socket is subsequently bound, the
> transition from unbound to bound causes bpf_sk_release() to skip the
> decrement of the refcount, causing a memory leak.
>
> unreferenced object 0xffff88810bc2eb40 (size 1984):
>   comm "test_progs", pid 2451, jiffies 4295320596
>   hex dump (first 32 bytes):
>     7f 00 00 01 7f 00 00 01 d2 04 1b b7 04 d2 00 00  ................
>     02 00 01 40 00 00 00 00 00 00 00 00 00 00 00 00  ...@............
>   backtrace (crc bdee079d):
>     kmem_cache_alloc_noprof+0x557/0x660
>     sk_prot_alloc+0x69/0x240
>     sk_alloc+0x30/0x460
>     inet_create+0x2ce/0xf80
>     __sock_create+0x25b/0x5c0
>     __sys_socket+0x119/0x1d0
>     __x64_sys_socket+0x72/0xd0
>     do_syscall_64+0xa1/0x5f0
>     entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Maintain balanced refcounts across sk lookup/release: (re-)set
> SOCK_RCU_FREE on proto update to treat the socket (whether bound or
> unbound) as not requiring a refcount increment on (a RCU protected) lookup.
>
> Fixes: 0c48eefae712 ("sock_map: Lift socket state restriction for datagram sockets")
> Signed-off-by: Michal Luczaj <mhal@rbox.co>
> ---
> Note: this issue is related to commit 67312adc96b5 ("bpf: reject unhashed
> sockets in bpf_sk_assign").
> ---
>  net/ipv4/udp_bpf.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
> index ad57c4c9eaab..970327b59582 100644
> --- a/net/ipv4/udp_bpf.c
> +++ b/net/ipv4/udp_bpf.c
> @@ -173,6 +173,9 @@ int udp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
>  	if (sk->sk_family == AF_INET6)
>  		udp_bpf_check_v6_needs_rebuild(psock->sk_proto);
>  
> +	/* Treat all sockets as non-refcounted, regardless of binding state. */
> +	sock_set_flag(sk, SOCK_RCU_FREE);
> +
>  	sock_replace_proto(sk, &udp_bpf_prots[family]);
>  	return 0;
>  }

There is a side effect that an unhashed (unbound) UDP socket can now be
selected in sk_lookup with bpf_sk_assign. Though perhaps that's for the
better because TC bpf_sk_assign doesn't reject non-refcounted UDP
sockets either, so we would have both socket dispatch sites behave the
same way.

Also, with this patch, if we insert & remove an unhashed UDP socket
into/from a sockmap, we end up with an unhashed non-refcounted UDP
socket. Not entirely sure if that is actually a problem or not.

Willem, what is your take on having unhashed non-refcoted UDP sockets?