bpf, sockmap: Reject unhashed UDP sockets on sockmap update

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: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20260707-sockmap-lookup-udp-leak-v4-2-f878346f27ab@rbox.co
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index c60ba6d292f9ae9f1526880ee933601cdf4aabe3..9efbd8ca7db832e31fc64ae2e9f04a7b9179d28a 100644 (file)
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -542,6 +542,8 @@ static bool sock_map_sk_state_allowed(const struct sock *sk)
 {
        if (sk_is_tcp(sk))
                return (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_LISTEN);
+       if (sk_is_udp(sk))
+               return sk_hashed(sk);
        if (sk_is_stream_unix(sk))
                return (1 << READ_ONCE(sk->sk_state)) & TCPF_ESTABLISHED;
        if (sk_is_vsock(sk) &&