Re: [PATCH]: NULL pointer dereference in sctp_auth_asoc_set_default_hmac

Vlad Yasevich
2014-04-16T19:12:15Z

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