[PATCH bpf v2 1/2] bpf: Reject TCP_NODELAY in TCP header option callbacks

KaFai Wan
2026-04-16T11:24:24Z

A BPF_SOCK_OPS program can enable
BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG and then call
bpf_setsockopt(TCP_NODELAY) from BPF_SOCK_OPS_HDR_OPT_LEN_CB or
BPF_SOCK_OPS_WRITE_HDR_OPT_CB.

In these callbacks, bpf_setsockopt(TCP_NODELAY) can reach
__tcp_sock_set_nodelay(), which can call tcp_push_pending_frames().

From BPF_SOCK_OPS_HDR_OPT_LEN_CB, tcp_push_pending_frames() can call
tcp_current_mss(), which calls tcp_established_options() and re-enters
bpf_skops_hdr_opt_len().

BPF_SOCK_OPS_HDR_OPT_LEN_CB
-> bpf_setsockopt(TCP_NODELAY)
-> tcp_push_pending_frames()
-> tcp_current_mss()
-> tcp_established_options()
-> bpf_skops_hdr_opt_len()
-> BPF_SOCK_OPS_HDR_OPT_LEN_CB

From BPF_SOCK_OPS_WRITE_HDR_OPT_CB, tcp_push_pending_frames() can call
tcp_write_xmit(), which calls tcp_transmit_skb(). That path recomputes
header option length through tcp_established_options() and
bpf_skops_hdr_opt_len() before re-entering bpf_skops_write_hdr_opt().

BPF_SOCK_OPS_WRITE_HDR_OPT_CB
-> bpf_setsockopt(TCP_NODELAY)
-> tcp_push_pending_frames()
-> tcp_write_xmit()
-> tcp_transmit_skb()
-> tcp_established_options()
-> bpf_skops_hdr_opt_len()
-> bpf_skops_write_hdr_opt()
-> BPF_SOCK_OPS_WRITE_HDR_OPT_CB

This leads to unbounded recursion and can overflow the kernel stack.

Reject TCP_NODELAY with -EOPNOTSUPP in bpf_sock_ops_setsockopt()
when bpf_setsockopt() is called from
BPF_SOCK_OPS_HDR_OPT_LEN_CB or BPF_SOCK_OPS_WRITE_HDR_OPT_CB.

Reported-by: Quan Sun <2022090917019@std.uestc.edu.cn>
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Closes: https://lore.kernel.org/bpf/d1d523c9-6901-4454-a183-94462b8f3e4e@std.uestc.edu.cn/
Fixes: 7e41df5dbba2 ("bpf: Add a few optnames to bpf_setsockopt")
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
net/core/filter.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index fcfcb72663ca..911ff04bca5a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5833,6 +5833,11 @@ BPF_CALL_5(bpf_sock_ops_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
if (!is_locked_tcp_sock_ops(bpf_sock))
return -EOPNOTSUPP;

+ if ((bpf_sock->op == BPF_SOCK_OPS_HDR_OPT_LEN_CB ||
+ bpf_sock->op == BPF_SOCK_OPS_WRITE_HDR_OPT_CB) &&
+ IS_ENABLED(CONFIG_INET) && level == SOL_TCP && optname == TCP_NODELAY)
+ return -EOPNOTSUPP;
+
return _bpf_setsockopt(bpf_sock->sk, level, optname, optval, optlen);
}

--
2.43.0