[PATCH bpf v3 0/2] bpf, sockmap: fix forward allocation accounting in strparser self-pass path

[Junseo Lim <zirajs7@gmail.com>]

The strparser SK_PASS path can queue cloned skbs back to the same socket.
When one TCP receive skb is split into many strparser messages, repeated
receive-owner assignments for unowned clones can leave sk_forward_alloc
in deficit before the next skb_set_owner_r() charge. Teardown can then
uncharge more memcg pages than were reserved and trigger a page_counter
underflow warning.

Fix by avoiding another receive-owner transition for same-socket skbs
that are already receive-owned by the socket. For unowned strparser
self-pass skbs, settle any existing sk_forward_alloc deficit with
sk_rmem_schedule(sk, skb, 0) before skb_set_owner_r().

Patch 1 also fixes psock backlog retries by restoring the original skb
redirect metadata after skb_bpf_redirect_clear(). If a deferred strparser
self-pass skb needs forward-allocation settlement, that work is done under
the socket lock.

The selftest adds a sockmap_strp case using a one-byte stream parser and
an SK_PASS verdict program. The test checks INET_DIAG_MEMINFO to verify
that sk_forward_alloc does not go negative after exercising the self-pass
delivery path.

Changelog:
v2 -> v3:
- Do not call skb_set_owner_r() again for already receive-owned same-socket
  skbs.
- Preserve the original _sk_redir value across psock backlog retries.
- Add a backlog-specific self-pass path so deferred strparser
  forward-allocation settlement runs under the socket lock.

v1 -> v2:
- Keep skb_set_owner_r() and use sk_rmem_schedule(sk, skb, 0) to settle
  sk_forward_alloc instead of skipping the owner transition.
  (Emil Tsalapatis)
- Apply the same handling to psock backlog retries.
- Add a sockmap_strp selftest based on the reproducer.
- Add a Reported-by tag.
- Change the Fixes tag to point to the commit that introduced the issue.

v1: https://lore.kernel.org/bpf/20260723065244.186916-1-zirajs7@gmail.com/T/
v2: https://lore.kernel.org/bpf/20260801102633.1872012-1-zirajs7@gmail.com/T/

Junseo Lim (2):
  bpf, sockmap: settle sk_forward_alloc for strparser SK_PASS
  selftests/bpf: Cover strparser self-pass forward allocation

 net/core/skmsg.c                              | 125 +++++++++++--
 .../selftests/bpf/prog_tests/sockmap_strp.c   | 171 ++++++++++++++++++
 .../selftests/bpf/progs/test_sockmap_strp.c   |   6 +
 3 files changed, 282 insertions(+), 20 deletions(-)

-- 
2.55.0