Hi Joshua,
On 04/16/2014 06:52 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:
I presume at this point, it's net->sctp.auth_enable = 1 as we test for
it here before calling sctp_auth_asoc_set_default_hmac():
case SCTP_PARAM_HMAC_ALGO:
if (!net->sctp.auth_enable)
goto fall_through;
Could it be that upon sctp_endpoint_init() time in your case,
sctp.auth_enable was still reset to 0, but you've set it between
sctp_endpoint_init() time and before invocation of
sctp_auth_asoc_set_default_hmac() to 1, so that this code path
will suddenly be taken, causing the NULL ptr deref?
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.
Same symptoms?
Do you have a test pcap though?
OpenSSH SCTP:
https://bugzilla.mindrot.org/show_bug.cgi?id=2016
If anyone's got other ideas to try out, let me know, thanks!
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