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

[Michal Luczaj <mhal@rbox.co>]

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).