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

On 8/4/26 03:58, Kuniyuki Iwashima wrote:

On Mon, Aug 3, 2026 at 2:01 AM Michal Luczaj [off-list ref] 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.

The same class of issue was reported by listen() + shutdown() + connect().
https://lore.kernel.org/netdev/20260804015349.2353056-1-kuniyu@google.com/ (local)

Can you test the diff in the thread ?

Yeah, it does fix it for sk/sockmap lookups. But bpf_sk_assign() still
leaks; LLM devised a testcase that puts skb into NFQUEUE. Another way of
escaping the RCU section could probably involve bpf_skb_set_tstamp().