Re: [PATCH net v2] tcp: limit tcp_tso_rtt_log to a valid shift count

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

On 10/23/26 10:40 PM, Yingjie Wang wrote:
> tcp_tso_autosize() right-shifts the u32 returned by tcp_min_rtt() by
> sysctl_tcp_tso_rtt_log. The sysctl uses proc_dou8vec_minmax() without
> an upper bound, so it accepts values through 255. Values above 31 make
> the shift undefined.
>
> A process with CAP_NET_ADMIN in the target network namespace can set the
> per-netns sysctl. With CONFIG_UBSAN_SHIFT=y, setting it to 32 and sending
> TCP data over loopback triggers:
>
>    UBSAN: shift-out-of-bounds in net/ipv4/tcp_output.c:2264:30
>    shift exponent 32 is too large for 32-bit type 'unsigned int'
>    Call Trace:
>     __ubsan_handle_shift_out_of_bounds
>     tcp_tso_segs
>     tcp_write_xmit
>     __tcp_push_pending_frames
>     tcp_push
>     tcp_sendmsg_locked
>
> Limit the sysctl to 31, the largest valid shift count for a u32, and
> document the accepted range. Writes of larger values now fail with
> -EINVAL.
>
> On a UBSAN/KASAN build of the net tree, values 0 and 31 remained
> accepted, values 32 and 255 returned -EINVAL, and TCP traffic with the
> valid boundary values produced no sanitizer diagnostics.
>
> Fixes: 65466904b015 ("tcp: adjust TSO packet sizes based on min_rtt")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yingjie Wang <1075151112@qq.com>


Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


Thanks !

> ---
> v2:
> - Attach the upper bound to tcp_tso_rtt_log instead of ip_dynaddr, as
>    pointed out by Jiayuan Chen and Neal Cardwell.
> - Re-run checkpatch and boundary tests on the corrected patch.
> - Do not carry Eric Dumazet's Reviewed-by tag because the code changed.
>
> v1: https://lore.kernel.org/netdev/tencent_53CDCF8AA0022B75CF72A294CB9993D2FF07@qq.com/
>
>   Documentation/networking/ip-sysctl.rst | 2 ++
>   net/ipv4/sysctl_net_ipv4.c             | 2 ++
>   2 files changed, 4 insertions(+)
>
> diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
> index 8edd2eb2a..66a54c510 100644
> --- a/Documentation/networking/ip-sysctl.rst
> +++ b/Documentation/networking/ip-sysctl.rst
> @@ -1128,6 +1128,8 @@ tcp_tso_rtt_log - INTEGER
>   
>   	If you want to use the old autosizing, set this sysctl to 0.
>   
> +	Possible values are [0, 31], inclusive.
> +
>   	Default: 9  (2^9 = 512 usec)
>   
>   tcp_pacing_ss_ratio - INTEGER
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index e3760daa3..bf193e7ec 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -26,6 +26,7 @@ static int ip_local_port_range_max[] = { 65535, 65535 };
>   static int tcp_adv_win_scale_min = -31;
>   static int tcp_adv_win_scale_max = 31;
>   static int tcp_app_win_max = 31;
> +static int tcp_tso_rtt_log_max = 31;
>   static int tcp_min_snd_mss_min = TCP_MIN_SND_MSS;
>   static int tcp_min_snd_mss_max = 65535;
>   static int tcp_rto_max_max = TCP_RTO_MAX_SEC * MSEC_PER_SEC;
> @@ -1406,6 +1407,7 @@ static const struct ctl_table ipv4_net_table[] = {
>   		.maxlen		= sizeof(u8),
>   		.mode		= 0644,
>   		.proc_handler	= proc_dou8vec_minmax,
> +		.extra2		= &tcp_tso_rtt_log_max,
>   	},
>   	{
>   		.procname	= "tcp_min_rtt_wlen",
>
> base-commit: f4377bf85887f57b59abb5479f7660a81ffe4449