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

[Tung Quang Nguyen]

>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".
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

>+	.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