[PATCH bpf] bpf: refresh seg6local SRH pointer after skb pull

[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.

BUG: KASAN: slab-use-after-free in seg6_bpf_has_valid_srh (net/ipv6/seg6_local.c:1411)
Write of size 1
 seg6_bpf_has_valid_srh (net/ipv6/seg6_local.c:1411)
 input_action_end_bpf (net/ipv6/seg6_local.c:1463)
 seg6_local_input_core (net/ipv6/seg6_local.c:1630)
 seg6_local_input (net/ipv6/seg6_local.c:1639)
 lwtunnel_input (net/core/lwtunnel.c:466)
 ipv6_rcv (net/ipv6/ip6_input.c:351)

Give LWT_SEG6LOCAL its own bpf_skb_pull_data() implementation. Save
the cached SRH offset before the skb operation and rebuild the pointer
from the current skb->data afterwards. Since pulling data can replace
storage but does not change packet layout, this preserves the identity
of the cached SRH even when multiple Routing Headers are present.

Refresh the pointer even on error because __pskb_pull_tail() can replace
the head before a later step fails. Preserve the pending hdrlen and valid
state so SRH validation semantics remain unchanged.

Fixes: 004d4b274e2a ("ipv6: sr: Add seg6local action End.BPF")
Reported-by: co+adfca3e91be95776@bugs.sh
Closes: https://lore.kernel.org/all/GCy0KRM2IcQGoJQTjJEU9D0maBxXzEDHuQpq@bugs.sh/
Cc: stable@vger.kernel.org
Assisted-by: Claude:gpt-5
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 net/core/filter.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 61940e7535523..e61f9e9226b10 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -7162,6 +7162,32 @@ static const struct bpf_func_proto bpf_lwt_seg6_adjust_srh_proto = {
 	.arg2_type	= ARG_ANYTHING,
 	.arg3_type	= ARG_ANYTHING,
 };
+
+BPF_CALL_2(bpf_lwt_seg6_pull_data, struct sk_buff *, skb, u32, len)
+{
+	struct seg6_bpf_srh_state *srh_state =
+		this_cpu_ptr(&seg6_bpf_srh_states);
+	unsigned int srhoff;
+	int ret;
+
+	lockdep_assert_held(&srh_state->bh_lock);
+	if (!srh_state->srh)
+		return ____bpf_skb_pull_data(skb, len);
+
+	srhoff = (unsigned char *)srh_state->srh - skb->data;
+	ret = ____bpf_skb_pull_data(skb, len);
+	srh_state->srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
+
+	return ret;
+}
+
+static const struct bpf_func_proto bpf_lwt_seg6_pull_data_proto = {
+	.func		= bpf_lwt_seg6_pull_data,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_ANYTHING,
+};
 #endif /* CONFIG_IPV6_SEG6_BPF */
 
 #ifdef CONFIG_INET
@@ -9052,6 +9078,8 @@ lwt_seg6local_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_lwt_seg6_action_proto;
 	case BPF_FUNC_lwt_seg6_adjust_srh:
 		return &bpf_lwt_seg6_adjust_srh_proto;
+	case BPF_FUNC_skb_pull_data:
+		return &bpf_lwt_seg6_pull_data_proto;
 #endif
 	default:
 		return lwt_out_func_proto(func_id, prog);
-- 
2.55.0