Re: [PATCH]: NULL pointer dereference in sctp_auth_asoc_set_default_hmac

Joshua Kinard
2014-04-16T21:49:21Z

On 04/16/2014 16:30, Daniel Borkmann wrote:
> On 04/16/2014 09:56 PM, Joshua Kinard wrote:
>> On 04/16/2014 15:12, Vlad Yasevich wrote:
>>> On 04/16/2014 12:52 AM, Joshua Kinard wrote:
>>>> Hi linux-sctp,
>>>>
>>>> I stumbled into a NULL pointer dereference on amd64 and mips when receiving
>>>> an INIT chunk containing the HMAC Algorithm Parameter (0x8004) when
>>>> net.sctp.auth_enable = 1.
>>>>
>>>> From some quick debugging I did, even if net.sctp.auth_enable = 1, the if
>>>> statement on line 448 in net/sctp/auth.c::sctp_auth_init_hmacs() checks
>>>> net->sctp.auth_enable and gets '0' back, which causes ep->auth_hmacs to get
>>>> set to NULL:
>>>>
>>>> 448 if (!net->sctp.auth_enable) {
>>>> 449 ep->auth_hmacs = NULL;
>>>> 450 return 0;
>>>> 451 }
>>>>
>>>>
>>>> Later, the if statement on line 621 in
>>>> net/sctp/auth.c::sctp_auth_asoc_set_default_hmac() attempts to access
>>>> ep->auth_hmacs without first checking for NULL, which triggers the oops:
>>>>
>>>> 620 /* If this TFM has been allocated, use this id */
>>>> 621 if (ep->auth_hmacs[id]) {
>>>> 622 asoc->default_hmac_id = id;
>>>> 623 break;
>>>> 624 }
>>>>
>>>>
>>>> I am not sure why net->sctp.auth_enable is initially returning '0' when
>>>> it's
>>>> set in sysctl, and verified in /proc/sys/net/sctp/auth_enable. Adding a
>>>> check for NULL on ep->auth_hmacs in the if statement stops the oops from
>>>> happening, though I am not sure if this is the correct fix.
>>>>
>>>> Another thing I noticed, is that I cannot trigger the Oops from the
>>>> SCTP/DTLS samples on this page:
>>>> http://sctp.fh-muenster.de/dtls-samples.html
>>>>
>>>> But if I patch OpenSSH with the SCTP patch below, that does trigger it on
>>>> the sshd server machine as soon as I issue 'ssh -z user@host ...'. I've
>>>> looked at both INIT chunks sent out by the respective programs in
>>>> Wireshark,
>>>> but nothing stands out.
>>>>
>>>> OpenSSH SCTP:
>>>> https://bugzilla.mindrot.org/show_bug.cgi?id 16
>>>>
>>>> If anyone's got other ideas to try out, let me know, thanks!
>>>>
>>>
>>>
>>> Which kernel are you running? Does it have this commit in it?
>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?idïb842c45e667332774196bd5a1d539d048159cc
>>>
>>>
>>> Without that, I could see how a bug might happen...
>>>
>>> -vlad
>>
>> For my MIPS machine (SGI O2), 3.14.1, built off of a linux-mips git pull.
>> For the amd64 VM, 3.14 from Gentoo's sources. I tested reversing that patch
>> on one of my kernel trees, and it's reversible, so it's definitely included.
>>
>> I haven't tried stepping through the kernel w/ kgdb yet. The MIPS machine
>> runs headless, and breaking into kdb to set a breakpoint, then resuming
>> kills ssh some how (still responds to pings, but won't connect), and ttyS0
>> gets stuck trying to talk to a remote gdb session. The amd64 VM spikes my
>> host system's CPU when it enters the debugger, so I can't run that for too
>> long of a time. I suppose I could enable telnet on the mips box and see if
>> that's still usable after setting the breakpoint.
>>
>> If someone wants to try and reproduce it:
>> 1. Compile OpenSSH w/ the patch from their bugzilla #2016 (use the latest
>> copy I uploaded for 6.6p1), making sure to enable SCTP via --with-sctp on
>> both a client and server machine.
>> 2. Start sshd w/ at least "Transport all" in sshd_config (the SCTP patch
>> adds this config directive) or via the command line.
>> 3. On sshd machine, sysctl -w net.sctp.auth_enable=1.
>
> Could you give it a try and do *first* the sysctl -w net.sctp.auth_enable=1
> on server side and after that start the sshd instance? We definitely need
> to fix that, but it seems ep is created on server side at time of auth_enable=0
> and you're setting if *afterwards* to 1. Looks this needs to be made immutable
> during ep life-time.

Confirmed, this fixes it. I shut sshd down, set the sysctl flag, and then
restarted it. No oops. Also, no oops if I do it in reverse. I.e.,
auth_enable=1, set to 0, without restarting sshd. So I guess making it
immutable at some point after the after the system is booted looks like the
way to go (I assume so it can be set in sysctl.conf, before the network is
brought up).

>> 4. On client, ssh -z <server>, and watch the bada boom.
>>
>> If there's a specific function I should go look at or pepper w/ printk()
>> statements, let me know. Not fully familiar w/ the call chain that the SCTP
>> stack uses when it receives an INIT chunk. If it's something bad in the
>> INIT chunk sent by the ssh client machine, shouldn't be too hard to manually
>> walk the functions and deduce how the code is interpreting things.
>>
>> --J