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

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 06/07/2026 19:57, Cen Zhang wrote:
> 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):
> 
>    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>
> ---
>   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
Just wonder if this can be done with NLA_POLICY_RANGE/MAX/etc...