Re: [tipc-discussion] [PATCH net 2/2] tipc: do not update mtu if msg_max is too small

[Tung Q. N. <tun...@de...>]

>-----Original Message-----
>From: Xin Long <luc...@gm...>
>Sent: Sunday, April 30, 2023 5:41 AM
>To: network dev <ne...@vg...>; tip...@li...
>Cc: da...@da...; ku...@ke...; Eric Dumazet <edu...@go...>; Paolo Abeni <pa...@re...>; Jon Maloy
><jm...@re...>
>Subject: [PATCH net 2/2] tipc: do not update mtu if msg_max is too small
>
>When doing link mtu negotiation, a malicious peer may send Activate msg
>with a very small mtu, e.g. 4 in Shuang's testing, without checking for
>the minimum mtu, l->mtu will be set to 4 in tipc_link_proto_rcv(), then
>n->links[bearer_id].mtu is set to 4294967228, which is a overflow of
>'4 - INT_H_SIZE - EMSG_OVERHEAD' in tipc_link_mss().
>
>With tipc_link.mtu = 4, tipc_link_xmit() kept printing the warning:
>
> tipc: Too large msg, purging xmit list 1 5 0 40 4!
> tipc: Too large msg, purging xmit list 1 15 0 60 4!
>
>And with tipc_link_entry.mtu 4294967228, a huge skb was allocated in
>named_distribute(), and when purging it in tipc_link_xmit(), a crash
>was even caused:
>
>  general protection fault, probably for non-canonical address 0x2100001011000dd: 0000 [#1] PREEMPT SMP PTI
>  CPU: 0 PID: 0 Comm: swapper/0 Kdump: loaded Not tainted 6.3.0.neta #19
>  RIP: 0010:kfree_skb_list_reason+0x7e/0x1f0
>  Call Trace:
>   <IRQ>
>   skb_release_data+0xf9/0x1d0
>   kfree_skb_reason+0x40/0x100
>   tipc_link_xmit+0x57a/0x740 [tipc]
>   tipc_node_xmit+0x16c/0x5c0 [tipc]
>   tipc_named_node_up+0x27f/0x2c0 [tipc]
>   tipc_node_write_unlock+0x149/0x170 [tipc]
>   tipc_rcv+0x608/0x740 [tipc]
>   tipc_udp_recv+0xdc/0x1f0 [tipc]
>   udp_queue_rcv_one_skb+0x33e/0x620
>   udp_unicast_rcv_skb.isra.72+0x75/0x90
>   __udp4_lib_rcv+0x56d/0xc20
>   ip_protocol_deliver_rcu+0x100/0x2d0
>
>This patch fixes it by checking the new mtu against tipc_bearer_min_mtu(),
>and not updating mtu if it is too small.
>
>Fixes: ed193ece2649 ("tipc: simplify link mtu negotiation")
>Reported-by: Shuang Li <sh...@re...>
>Signed-off-by: Xin Long <luc...@gm...>
>---
> net/tipc/link.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
>diff --git a/net/tipc/link.c b/net/tipc/link.c
>index b3ce24823f50..a9e46c58b28a 100644
>--- a/net/tipc/link.c
>+++ b/net/tipc/link.c
>@@ -2200,7 +2200,7 @@ static int tipc_link_proto_rcv(struct tipc_link *l, struct sk_buff *skb,
> 	struct tipc_msg *hdr = buf_msg(skb);
> 	struct tipc_gap_ack_blks *ga = NULL;
> 	bool reply = msg_probe(hdr), retransmitted = false;
>-	u32 dlen = msg_data_sz(hdr), glen = 0;
>+	u32 dlen = msg_data_sz(hdr), glen = 0, msg_max;
> 	u16 peers_snd_nxt =  msg_next_sent(hdr);
> 	u16 peers_tol = msg_link_tolerance(hdr);
> 	u16 peers_prio = msg_linkprio(hdr);
>@@ -2283,8 +2283,9 @@ static int tipc_link_proto_rcv(struct tipc_link *l, struct sk_buff *skb,
> 		l->peer_session = msg_session(hdr);
> 		l->in_session = true;
> 		l->peer_bearer_id = msg_bearer_id(hdr);
>-		if (l->mtu > msg_max_pkt(hdr))
>-			l->mtu = msg_max_pkt(hdr);
>+		msg_max = msg_max_pkt(hdr);
>+		if (msg_max >= tipc_bearer_min_mtu(l->net, l->bearer_id) && l->mtu > msg_max)
>+			l->mtu = msg_max;
If this link receives a malicious ACTIVATE_MSG from a peer, this message should be dropped. It is better if the check " msg_max < tipc_bearer_min_mtu()" is put at the beginning of this ACTIVATE_MSG handling and we break immediately.
> 		break;
>
> 	case STATE_MSG:
>--
>2.39.1