Subject: Re: [PATCH net] net: sctp: cache auth_enable per endpoint

From: Joshua Kinard <[email-masked]>
Date: Thu, 17 Apr 2014 21:54:10 -0400

On 04/17/2014 11:26, Daniel Borkmann wrote:
> From: Vlad Yasevich <[email-masked]>
> 
> Currently, it is possible to create an SCTP socket, then switch
> auth_enable via sysctl setting to 1 and crash the system on connect:
> 
> Oops[#1]:
> CPU: 0 PID: 0 Comm: swapper Not tainted 3.14.1-mipsgit-20140415 #1
> task: ffffffff8056ce80 ti: ffffffff8055c000 task.ti: ffffffff8055c000
> [...]
> Call Trace:
> [<ffffffff8043c4e8>] sctp_auth_asoc_set_default_hmac+0x68/0x80
> [<ffffffff8042b300>] sctp_process_init+0x5e0/0x8a4
> [<ffffffff8042188c>] sctp_sf_do_5_1B_init+0x234/0x34c
> [<ffffffff804228c8>] sctp_do_sm+0xb4/0x1e8
> [<ffffffff80425a08>] sctp_endpoint_bh_rcv+0x1c4/0x214
> [<ffffffff8043af68>] sctp_rcv+0x588/0x630
> [<ffffffff8043e8e8>] sctp6_rcv+0x10/0x24
> [<ffffffff803acb50>] ip6_input+0x2c0/0x440
> [<ffffffff8030fc00>] __netif_receive_skb_core+0x4a8/0x564
> [<ffffffff80310650>] process_backlog+0xb4/0x18c
> [<ffffffff80313cbc>] net_rx_action+0x12c/0x210
> [<ffffffff80034254>] __do_softirq+0x17c/0x2ac
> [<ffffffff800345e0>] irq_exit+0x54/0xb0
> [<ffffffff800075a4>] ret_from_irq+0x0/0x4
> [<ffffffff800090ec>] rm7k_wait_irqoff+0x24/0x48
> [<ffffffff8005e388>] cpu_startup_entry+0xc0/0x148
> [<ffffffff805a88b0>] start_kernel+0x37c/0x398
> Code: dd0900b8  000330f8  0126302d <dcc60000> 50c0fff1  0047182a  a48306a0
> 03e00008  00000000
> ---[ end trace b530b0551467f2fd ]---
> Kernel panic - not syncing: Fatal exception in interrupt
> 
> What happens while auth_enable=0 in that case is, that
> ep->auth_hmacs is initialized to NULL in sctp_auth_init_hmacs()
> when endpoint is being created.
> 
> After that point, if an admin switches over to auth_enable=1,
> the machine can crash due to NULL pointer dereference during
> reception of an INIT chunk. When we enter sctp_process_init()
> via sctp_sf_do_5_1B_init() in order to respond to an INIT chunk,
> the INIT verification succeeds and while we walk and process
> all INIT params via sctp_process_param() we find that
> net->sctp.auth_enable is set, therefore do not fall through,
> but invoke sctp_auth_asoc_set_default_hmac() instead, and thus,
> dereference what we have set to NULL during endpoint
> initialization phase.
> 
> The fix is to make auth_enable immutable by caching its value
> during endpoint initialization, so that its original value is
> being carried along until destruction. The bug seems to originate
> from the very first days.
> 
> Fix in joint work with Daniel Borkmann.
> 
> Reported-by: Joshua Kinard <[email-masked]>
> Signed-off-by: Vlad Yasevich <[email-masked]>
> Signed-off-by: Daniel Borkmann <[email-masked]>

This solves the problem for me, thanks!

Tested-by: Joshua Kinard <[email-masked]>