On Wed, Jun 24, 2026 at 02:39 PM -07, Kuniyuki Iwashima wrote: > On Wed, Jun 24, 2026 at 2:33 PM Kuniyuki Iwashima <[email protected]> wrote: >> >> On Wed, Jun 24, 2026 at 2:26 PM Michal Luczaj <[email protected]> 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 <[email protected]> >> > >>> --- >> > >>> 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/[email protected]/#t >> >> ... and v5 (the final commit) somehow removed the restriction for unconnected >> UDP socket as well. >> https://lore.kernel.org/bpf/[email protected]/ >> >> 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.
