Fix for CVE-2015-5600 can sometimes erroneously block logins

[Ethan Rahn]

When testing a fix for CVE-2015-5600 based on the Ubuntu patch in openssh-5.9 ( https://launchpadlibrarian.net/214490716/openssh_1%3A5.9p1-5ubuntu1.4_1%3A5.9p1-5ubuntu1.6.diff.gz ), I noticed that there was an issue with getting permission denied when trying to log in lots of times with what should be valid credentials.

The symptom was when logging in with the command and sshd_config below I would get permission denied sometimes and permission granted other times. Upon investigating the reason for permission being denied was sshd erroneously thinking "pam" had already been used as a login method on the first attempt to use it. This appeared to be related to the kbdinit_alloc function in auth2_chall.c not initializing devices_done. Once I made the following patch the issue went away:

@@ -130,6 +131,7 @@ kbdint_alloc(const char *devs)
        kbdintctxt->ctxt = NULL;
        kbdintctxt->device = NULL;
        kbdintctxt->nreq = 0;
+ kbdintctxt->devices_done = 0;

        return kbdintctxt;
 }

Since openssh uses xmalloc ( i.e. malloc or die ) to initialize data structures, it seems that the issue is the struct not getting zero'ed out at the start. I haven't taken the time to verify this in openssh-6.9 / openssh-7.0, but it seems like since xmalloc / malloc is still in use that it should still fail in the same manner.

These are the ssh command sshd_config that were in use when the issue was happening. I'm not sure if something about them makes the issue more likely to happen:
===
ssh command:
ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o ConnectTimeout=120 -o ServerAliveInterval=15 -m hmac-md5 -c aes256-ctr -e \~ -oKexAlgorithms=diffie-hellman-group-exchange-sha1 <username>@<host>

sshd_config:
Protocol 2
Port 22
SyslogFacility AUTHPRIV
PasswordAuthentication no
ChallengeResponseAuthentication yes
UsePAM yes
MaxStartups 10:30:100
Subsystem sftp /usr/libexec/openssh/sftp-server
PermitEmptyPasswords yes
AllowTcpForwarding no
Banner /etc/issue
StrictModes yes
UsePrivilegeSeparation yes
Compression delayed
GatewayPorts no
GSSAPIAuthentication no
KerberosAuthentication no
LoginGraceTime 120
LogLevel DEBUG2
Ciphers 3des-cbc,aes128-cbc,aes128-ctr,aes192-cbc,aes192-ctr,aes256-cbc,aes256-ctr,arcfour,arcfour128,arcfour256,blowfish-cbc,cast128-cbc
KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
MACs hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96
HostKey <removed rsa keypath>
HostKey <removed dsa keypath>

===

Is anyone else able to see this issue and verify that my fix is correct?

Thanks,

Ethan