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

[Jakub Sitnicki <jakub@cloudflare.com>]

On Wed, Jun 24, 2026 at 02:39 PM -07, Kuniyuki Iwashima wrote:
> On Wed, Jun 24, 2026 at 2:33 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>>
>> On Wed, Jun 24, 2026 at 2:26 PM Michal Luczaj <mhal@rbox.co> wrote:
>> >
>> > On 6/24/26 22:01, Willem de Bruijn wrote:
>> > > Jakub Sitnicki wrote:
>> > >> 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.
>> > >
>> > > The commit does mention a related fix, beneath the ---, commit
>> > > 67312adc96b5 ("bpf: reject unhashed sockets in bpf_sk_assign").
>> > > That fixes a similar issue by exactly disallowing this:
>> > >
>> > >     Fix the problem by rejecting unhashed sockets in bpf_sk_assign().
>> > >     This matches the behaviour of __inet_lookup_skb which is ultimately
>> > >     the goal of bpf_sk_assign().
>> > >
>> > > So ..
>> > >
>> > >> 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.
>> > >
>> > > .. there are two conflicting types of consistency here? Consistent with
>> > > __inet_lookup_skb or the TC bpf hook. Of those the first is the more
>> > > canonical.
>> > >
>> > >> 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?
>> > >
>> > > I don't immediately see a problem, but I'm not an expert on SOCK_RCU_FREE.
>> >
>> > Perhaps it's worth mentioning that unhashed non-refcounted UDP socket is
>> > already possible: first auto-bind via connect(AF_INET) (which also sets
>> > SOCK_RCU_FREE), then unhash via connect(AF_UNSPEC).
>>
>> Setting SOCK_RCU_FREE itself should not cause a problem, but I think
>> we should take a step back.
>>
>> AFAIU, 0c48eefae712 was to allow putting AF_UNIX SOCK_DGRAM sockets
>> into sockmap, not to allow using unconnected UDP sockets in sk_lookup etc.
>>
>> Actually, v4 of the patch was implemented as such but did not get any feedback,
>> https://lore.kernel.org/bpf/20210508220835.53801-9-xiyou.wangcong@gmail.com/#t
>>
>> ... and v5 (the final commit) somehow removed the restriction for unconnected
>> UDP socket as well.
>> https://lore.kernel.org/bpf/20210704190252.11866-3-xiyou.wangcong@gmail.com/
>>
>> Given the initial use case, sockmap redirect, is still blocked by
>> TCP_ESTABLISHED
>> check in sock_map_redirect_allowed(), I feel there is no point in supporting
>> unconnected UDP sockets in sockmap.  It cannot get any skb from anywhere
>> (without buggy sk_lookup).
>
> s/unconnected/unhashed/g :)

Rejecting unhashed UDP sockets on insert to sockmap SGTM.
It is also in line with disable-problematic-cases strategy.