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

[Kuniyuki Iwashima]

On Wed, Jun 24, 2026 at 2:33 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
>
> On Wed, Jun 24, 2026 at 2:26 PM Michal Luczaj <mhal@xxxxxxx> 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@xxxxxxx>
> > >>> ---
> > >>> 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@xxxxxxxxx/#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@xxxxxxxxx/
>
> 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 :)