Re: [PATCH bpf v2 2/2] bpf: Unconditionally take socket references in lookup helpers

On Tue, Aug 4, 2026 at 3:14 AM Jakub Sitnicki [off-list ref] wrote:

On Mon, Aug 03, 2026 at 11:00 AM +02, Michal Luczaj wrote:

quoted

Lookup helpers gate whether to acquire a socket reference on
sk_is_refcounted(), a check re-evaluated at release. An established socket
refcounted at acquire time can gain SOCK_RCU_FREE via
connect(AF_UNSPEC)+listen() before release runs; the release-side re-check
then reads sk_is_refcounted() == false and skips the put. The reference
leaks.

Make acquire and release unconditional and symmetric: always take a
reference, always put it. Adapt sk_select_reuseport().

Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
Fixes: 64d85290d79c ("bpf: Allow bpf_map_lookup_elem for SOCKMAP and SOCKHASH")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/bpf/20260701235552.2B0AA1F00A3F@smtp.kernel.org/ (local)
Signed-off-by: Michal Luczaj <redacted>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
---
TC bpf_sk_assign() has the same issue; it takes a reference only when
sk_is_refcounted() is true at assign time, but sock_pfree() (the skb
destructor it installs) re-checks sk_is_refcounted() independently at
release time. The same connect(AF_UNSPEC)+listen() transition leaks the
socket here too. I'd welcome suggestions on the right way to handle this.

Can we make this scenario unsupported?

listen() could return EBUSY if called on a socket that is refcounted.

WDYT?

I discussed this kind of buggy rehash with Eric today.

We can't make it unsupported although it's super unlikely
that this is used by a real application.