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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 4/16/26 7:23 PM, KaFai Wan wrote:
> 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);
>   }
>   

A simple comment is recommended:

/* TCP_NODELAY triggers tcp_push_pending_frames() and re-enters these 
callbacks. */


Also like Martin pointed before, BPF_SOCK_OPS_HDR_OPT_LEN_CB / 
BPF_SOCK_OPS_WRITE_HDR_OPT_CB

can only be produced under CONFIG_INET so IS_ENABLED(CONFIG_INET) is dead.