[PATCH net v2] 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
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
v2: Solved format issue

 net/tipc/bearer.c    | 6 ++++++
 net/tipc/udp_media.h | 5 +++++
 2 files changed, 11 insertions(+)

diff --git a/net/tipc/bearer.c b/net/tipc/bearer.c
index 05dcd2f9e..0926692dd 100644
--- a/net/tipc/bearer.c
+++ b/net/tipc/bearer.c
@@ -1163,6 +1163,12 @@ int __tipc_nl_bearer_set(struct sk_buff *skb, struct genl_info *info)
 					       "MTU value is out-of-range");
 				return -EINVAL;
 			}
+			if (nla_get_u32(props[TIPC_NLA_PROP_MTU]) >
+			    U16_MAX) {
+				NL_SET_ERR_MSG(info->extack,
+					       "MTU value is out-of-range");
+				return -EINVAL;
+			}
 			b->mtu = nla_get_u32(props[TIPC_NLA_PROP_MTU]);
 			tipc_node_apply_property(net, b, TIPC_NLA_PROP_MTU);
 #endif
diff --git a/net/tipc/udp_media.h b/net/tipc/udp_media.h
index e7455cc73..0d62a57c8 100644
--- a/net/tipc/udp_media.h
+++ b/net/tipc/udp_media.h
@@ -48,6 +48,11 @@ int tipc_udp_nl_dump_remoteip(struct sk_buff *skb, struct netlink_callback *cb);
 /* check if configured MTU is too low for tipc headers */
 static inline bool tipc_udp_mtu_bad(u32 mtu)
 {
+	if (mtu > U16_MAX) {
+		pr_warn("MTU too large for tipc bearer\n");
+		return true;
+	}
+
 	if (mtu >= (TIPC_MIN_BEARER_MTU + sizeof(struct iphdr) +
 	    sizeof(struct udphdr)))
 		return false;
-- 
2.53.0