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Bug#726818: openssh-server: fatal xfree error with sftp when RekeyLimit hit



Package: openssh-server
Version: 1:6.2p2-6
Severity: normal

I am attempting to transfer a large (1.6 GiB) file over sftp.  I have
set my RekeyLimit to 1G in my ~/.ssh/config file.  When I get to the
rekey limit, the connection is reset:

  vauxhall ok % sftp bmc@castro:/media/data/bmc/finished/upload/
  Connected to castro.
  Changing to: /media/data/bmc/finished/upload/
  sftp> put large-file 
  Uploading large-file to /media/data/bmc/finished/upload/large-file
  large-file                                                                                                                                                     67% 1021MB   8.0MB/s   01:02 ETAConnection closed by 173.11.243.49
  Couldn't read packet: Connection reset by peer

The auth.log file on the server says:

  Oct 19 14:01:12 castro sshd[649315]: Authorized to bmc, krb5 principal bmc@CRUSTYTOOTHPASTE.NET (krb5_kuserok)
  Oct 19 14:01:12 castro sshd[649315]: Accepted gssapi-with-mic for bmc from 172.16.2.247 port 44985 ssh2
  Oct 19 14:01:12 castro sshd[649315]: pam_unix(sshd:session): session opened for user bmc by (uid=0)
  Oct 19 14:01:12 castro sshd[649317]: subsystem request for sftp by user bmc
  Oct 19 14:03:44 castro sshd[649317]: fatal: xfree: NULL pointer given as argument
  Oct 19 14:03:44 castro sshd[649315]: pam_unix(sshd:session): session closed for user bmc

Note the "fatal: xfree: NULL pointer given as argument" error.  This
terminates the session.  If I set the RekeyLimit value to 2G (larger
than my file), it works just fine.  If you don't feel like transferring
gigabytes of data, I can also reproduce this problem with a 200M
RekeyLimit (I haven't tried smaller).  It also happens with publickey
authentication as well as GSSAPI, although the line starting with
"fatal" is not logged in that case.

I originally saw this error with sshfs-fuse, but as you can see, it also
happens with the plain sftp client.  Please let me know if you need more
information, as this is easily reproducible for me.

-- System Information:
Debian Release: jessie/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.11-trunk-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages openssh-server depends on:
ii  adduser           3.113+nmu3
ii  dpkg              1.17.1
ii  libc6             2.17-93
ii  libcomerr2        1.42.8-1
ii  libgssapi-krb5-2  1.11.3+dfsg-3
ii  libkrb5-3         1.11.3+dfsg-3
ii  libpam-modules    1.1.3-9
ii  libpam-runtime    1.1.3-9
ii  libpam0g          1.1.3-9
ii  libselinux1       2.1.13-3
ii  libssl1.0.0       1.0.1e-3
ii  libwrap0          7.6.q-24
ii  lsb-base          4.1+Debian12
ii  openssh-client    1:6.2p2-6
ii  procps            1:3.3.8-2
ii  sysv-rc           2.88dsf-43
ii  zlib1g            1:1.2.8.dfsg-1

Versions of packages openssh-server recommends:
ii  ncurses-term  5.9+20130608-1
ii  xauth         1:1.0.7-1

Versions of packages openssh-server suggests:
pn  molly-guard              <none>
pn  monkeysphere             <none>
ii  openssh-blacklist        0.4.1+nmu1
ii  openssh-blacklist-extra  0.4.1+nmu1
pn  rssh                     <none>
pn  ssh-askpass              <none>
pn  ufw                      <none>

-- debconf information:
  ssh/disable_cr_auth: false
  ssh/encrypted_host_key_but_no_keygen:
  ssh/vulnerable_host_keys:
* ssh/use_old_init_script: true

-- 
brian m. carlson / brian with sandals: Houston, Texas, US
+1 832 623 2791 | http://www.crustytoothpaste.net/~bmc | My opinion only
OpenPGP: RSA v4 4096b: 88AC E9B2 9196 305B A994 7552 F1BA 225C 0223 B187

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