Re: [PATCH net 4/5] ipv4: restrict igmp_qrv to 255

[Kuniyuki Iwashima <kuniyu@google.com>]

On Fri, 23 Oct 2026 17:13:30 +0800, Jiayuan Chen wrote:
> net.ipv4.igmp_qrv only has a lower bound, but ip_mc_reset() stores it
> into the u8 in_device::mr_qrv, and the IGMPv3 retransmit counters filled
> from it (crcount, sf_crcount) are u8 as well. Anything above 255
> silently wraps. With 256, mr_qrv becomes 0, every
> "in_dev->mr_qrv ?: sysctl_igmp_qrv" falls back to 256, which truncates
> to 0 again in crcount, and igmpv3_send_cr() never puts the state change
> records for the group into a report. The older version querier present
> timeout (mr_qrv * mr_qi + mr_qri) also collapses to just mr_qri.
> 
> Unlike MLD there is no WARN_ON() here, the reports just go missing.
> 
> Cap the sysctl at U8_MAX, the largest value mr_qrv can actually hold.
> 
> Fixes: a9fe8e29945d ("ipv4: implement igmp_qrv sysctl to tune igmp robustness variable")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  net/ipv4/sysctl_net_ipv4.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index e3760daa3470..ef631a2759b0 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -33,6 +33,7 @@ static int ip_privileged_port_min;
>  static int ip_privileged_port_max = 65535;
>  static int ip_ttl_min = 1;
>  static int ip_ttl_max = 255;
> +static int igmp_qrv_max __maybe_unused = U8_MAX;
>  static int tcp_syn_retries_min = 1;
>  static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
>  static int tcp_syn_linear_timeouts_max = MAX_TCP_SYNCNT;
> @@ -978,7 +979,8 @@ static const struct ctl_table ipv4_net_table[] = {
>  		.maxlen		= sizeof(int),
>  		.mode		= 0644,
>  		.proc_handler	= proc_dointvec_minmax,

Same, please use proc_dou8vec_minmax().


> -		.extra1		= SYSCTL_ONE
> +		.extra1		= SYSCTL_ONE,
> +		.extra2		= &igmp_qrv_max,
>  	},
>  #endif
>  	{
> -- 
> 2.43.0
>