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

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

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