[PATCH net v3] tipc: fix u16 MTU truncation in media and bearer MTU validation

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

Both TIPC_NL_MEDIA_SET and TIPC_NL_BEARER_SET accept user-supplied
MTU values but only enforce a minimum bound, not a maximum. When a user
sets the MTU to a value exceeding U16_MAX (65535), it passes validation
but is silently truncated when assigned to u16 fields l->mtu and
l->advertised_mtu in tipc_link_create(). Values like 65536 (0x10000)
truncate to 0, causing a division by zero in tipc_link_set_queue_limits()
which computes TIPC_MAX_PUBL / (l->mtu / ITEM_SIZE). Other overflowing
values (e.g. 65537-131071) produce small incorrect MTU values, resulting
in link malfunction behaviors.

Crash stack (triggered as unprivileged user via user namespace):

  tipc_link_set_queue_limits  net/tipc/link.c:2531
  tipc_link_create            net/tipc/link.c:520
  tipc_node_check_dest        net/tipc/node.c:1279
  tipc_disc_rcv               net/tipc/discover.c:252
  tipc_rcv                    net/tipc/node.c:2129
  tipc_udp_recv               net/tipc/udp_media.c:392

Two independent paths lack the upper bound check:
1. tipc_udp_mtu_bad() -- called from __tipc_nl_media_set() (MEDIA_SET)
2. inline check in __tipc_nl_bearer_set() at bearer.c:1160 (BEARER_SET)

Fix both by rejecting MTU values above U16_MAX.

Fixes: 901271e0403a ("tipc: implement configuration of UDP media MTU")
Reported-by: AutonomousCodeSecurity@microsoft.com
Closes: https://lore.kernel.org/all/CAB8m9WgETt0AjmFwE=F-CKjGXsK6_WDv0=kbYRcC8-noo+amnA@mail.gmail.com
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
v3: Use nla_policy check to limit MTU max value as suggested by Vadim
v2: Solved format issue
Link: https://lore.kernel.org/all/CAB8m9WgETt0AjmFwE=F-CKjGXsK6_WDv0=kbYRcC8-noo+amnA@mail.gmail.com

 net/tipc/netlink.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/tipc/netlink.c b/net/tipc/netlink.c
index 8336a9664703..1307dd1a9613 100644
--- a/net/tipc/netlink.c
+++ b/net/tipc/netlink.c
@@ -113,12 +113,16 @@ const struct nla_policy tipc_nl_node_policy[TIPC_NLA_NODE_MAX + 1] = {
 };
 
 /* Properties valid for media, bearer and link */
+static const struct netlink_range_validation tipc_nl_mtu_range = {
+	.max = U16_MAX,
+};
+
 const struct nla_policy tipc_nl_prop_policy[TIPC_NLA_PROP_MAX + 1] = {
 	[TIPC_NLA_PROP_UNSPEC]		= { .type = NLA_UNSPEC },
 	[TIPC_NLA_PROP_PRIO]		= { .type = NLA_U32 },
 	[TIPC_NLA_PROP_TOL]		= { .type = NLA_U32 },
 	[TIPC_NLA_PROP_WIN]		= { .type = NLA_U32 },
-	[TIPC_NLA_PROP_MTU]		= { .type = NLA_U32 },
+	[TIPC_NLA_PROP_MTU]		= NLA_POLICY_FULL_RANGE(NLA_U32, &tipc_nl_mtu_range),
 	[TIPC_NLA_PROP_BROADCAST]	= { .type = NLA_U32 },
 	[TIPC_NLA_PROP_BROADCAST_RATIO]	= { .type = NLA_U32 }
 };
-- 
2.53.0