RE: [PATCH net v4] tipc: fix u16 MTU truncation in media and bearer MTU validation

[Tung Quang Nguyen]

>Subject: Re: [PATCH net v4] tipc: fix u16 MTU truncation in media and bearer
>MTU validation
>
>On 10/07/2026 13:57, Tung Quang Nguyen wrote:
>>> Subject: [PATCH net v4] tipc: fix u16 MTU truncation in media and
>>> bearer MTU validation
>>>
>>> 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.
>>
>> Yes, this is a bug. I can reproduce the same div-by-zero.
>>
>>>
>>> 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@xxxxxxxxxxxxx
>>> Closes: https://lore.kernel.org/all/CAB8m9WgETt0AjmFwE=F-
>>> CKjGXsK6_WDv0=kbYRcC8-noo+amnA@xxxxxxxxxxxxxx
>>> Signed-off-by: Cen Zhang (Microsoft) <blbllhy@xxxxxxxxx>
>>> ---
>>> v4: Add .min check value
>>> 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@xxxxxxxxxxxxxx
>>>
>>> net/tipc/netlink.c | 7 ++++++-
>>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/net/tipc/netlink.c b/net/tipc/netlink.c index
>>> 8336a9664703..811438c44542 100644
>>> --- a/net/tipc/netlink.c
>>> +++ b/net/tipc/netlink.c
>>> @@ -113,12 +113,17 @@ 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 = {
>>> +	.min = TIPC_MIN_BEARER_MTU,
>>
>> Adding lower threshold checking introduces two issues:
>> 1. Existing function tipc_udp_mtu_bad() can never return "true".
>
>it can. if MTU is in the range [TIPC_MIN_BEARER_MTU,
>TIPC_MIN_BEARER_MTU
>+ sizeof(iphdr) + sizeof(udphdr)]

The context I was talking about is error path (mtu < min) as shown in examples. This makes adding .min check redundant.
                
>
>> 2. Breaking user-space applications looking for error message " Error: MTU
>value is out-of-range". Look at the error messages before and after your patch:
>> [Before patch]
>> node1 ~ # tipc bearer set mtu 15 media udp name UDP0
>> Error: MTU value is out-of-range.
>> kernel answers: Invalid argument
>>
>> [After  patch]
>> node1 ~ # tipc bearer set mtu 15 media udp name UDP0 kernel answers:
>> Numerical result out of range
>
>that's interesting, because this error message belongs to 9p scope...
>
>anyways, we can keep v3 of this patch with .max value set only
>
>
>>
>>> +	.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
>>