[PATCH bpf v2] bpf: disallow bpf_skb_pull_data() for LWT_SEG6LOCAL

[Weiming Shi <bestswngs@gmail.com>]

An LWT_SEG6LOCAL program can invalidate its cached SRH with
bpf_lwt_seg6_adjust_srh() and then call bpf_skb_pull_data(). The latter
may reallocate skb->head, leaving the per-CPU SRH pointer dangling.
Post-program SRH validation then writes through that pointer.

Disallow bpf_skb_pull_data() for LWT_SEG6LOCAL programs so the verifier
rejects this unsafe helper combination. Other LWT program types continue
to expose the helper through lwt_out_func_proto().

Fixes: 004d4b274e2a ("ipv6: sr: Add seg6local action End.BPF")
Reported-by: co+adfca3e91be95776@bugs.sh
Closes: https://lore.kernel.org/all/GCy0KRM2IcQGoJQTjJEU9D0maBxXzEDHuQpq@bugs.sh/
Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Link: https://lore.kernel.org/bpf/DL9COXZQXX4V.1FN45QO2Q77ZH@gmail.com/
Cc: stable@vger.kernel.org
Assisted-by: Claude:gpt-5
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
Changes in v2:
- Disallow bpf_skb_pull_data() for LWT_SEG6LOCAL instead of adding a
  wrapper to refresh the cached SRH pointer, as suggested by Alexei.

 net/core/filter.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 8513167a858a8..2a84f9d011314 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -9044,6 +9044,8 @@ static const struct bpf_func_proto *
 lwt_seg6local_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
 	switch (func_id) {
+	case BPF_FUNC_skb_pull_data:
+		return NULL;
 #if IS_ENABLED(CONFIG_IPV6_SEG6_BPF)
 	case BPF_FUNC_lwt_seg6_store_bytes:
 		return &bpf_lwt_seg6_store_bytes_proto;
-- 
2.55.0