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

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

On 08/07/2026 19:02, Cen Zhang (Microsoft) 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 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),

netlink_range_validation with only .max range doesn't look great, there
is simply NLA_POLICY_MAX() macro.

if you really want to use range check you can technically set .min to
TIPC_MIN_BEARER_MTU, but you still have to have both checks for min mtu
as there is unavoidable encap overhead...

>   	[TIPC_NLA_PROP_BROADCAST]	= { .type = NLA_U32 },
>   	[TIPC_NLA_PROP_BROADCAST_RATIO]	= { .type = NLA_U32 }
>   };