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

[Jakub Sitnicki <jakub@cloudflare.com>]

On Mon, Aug 03, 2026 at 11:00 AM +02, Michal Luczaj wrote:
> 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/
> Signed-off-by: Michal Luczaj <mhal@rbox.co>
> 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?