Re: [PATCH bpf v2 1/4] bpf, sockmap: Reject unhashed UDP sockets on sockmap update

[Jakub Sitnicki <jakub@cloudflare.com>]

On Fri, Jun 26, 2026 at 10:36 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
>
> Instead of special-casing for refcounted sockets, reject unhashed UDP
> sockets during sockmap updates, as there is no benefit to supporting those.
> This effectively reverts the commit under Fixes, with two exceptions:
>
> 1. sock_map_sk_state_allowed() maintains a fall-through `return true`.
> 2. In the spirit of commit b8b8315e39ff ("bpf, sockmap: Remove unhash
>    handler for BPF sockmap usage"), the proto::unhash BPF handler is not
>    reintroduced.
>
> Historical note: this issue is related to commit 67312adc96b5 ("bpf: reject
> unhashed sockets in bpf_sk_assign").
>
> Fixes: 0c48eefae712 ("sock_map: Lift socket state restriction for datagram sockets")
> Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
> Signed-off-by: Michal Luczaj <mhal@rbox.co>
> ---

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>