[Bug 2522] New: Key parser should reflect errors from OpenSSL

[bugzilla-daemon at bugzilla.mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=2522

            Bug ID: 2522
           Summary: Key parser should reflect errors from OpenSSL
           Product: Portable OpenSSH
           Version: 7.1p1
          Hardware: Other
                OS: Linux
            Status: NEW
          Keywords: patch
          Severity: enhancement
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: jjelen at redhat.com

Created attachment 2781
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2781&action=edit
proposed patch

TL;DR

    ./ssh-add <(echo "")

asks for passphrase for empty key. The blob is passed to OpenSSL and
every parser failure is reported as "wrong passphrase", which is
broken. Checking the actual error from OpenSSL would solve this issue.


Long story:

Based on Red hat Bugzilla [1], my post on mailing list [2] and SO
question [3] I went down the source code and investigated a bit
communication between OpenSSH and OpenSSL in terms of private key
parsing.

When openSSH can not derive the key type it passes the whole blob to
OpenSSL function and waits if it can parse it. 

    PEM_read_bio_PrivateKey()      # called from sshkey.c @ 3791

Only return value from this function is NULL on failure. The reason can
be obtained using ERR_ functions from OpenSSL

    ERR_get_error();               # actual error code
    ERR_print_errors_fp(stderr);   # prints verbose info

Possible reasons for failure of the above mentioned function are
described in openssl/pem.h [4]. Basically, some of the reasons are
obviously not related to wrong passphrase. I tried with somehow
crippled keys and I ended up with attached patch so far (currently
contains also debug output from openssl on failure). The proposed patch
does not solve only the issue with empty key and "crippled" one, but
also problems with unknown RSA1 keys when built without SSH1 protocol:

    $ ./ssh-add <(echo "")
   
140239613359768:error:0906D06C:lib(9):func(109):reason(108):pem_lib.c:701:Expecting:
ANY PRIVATE KEY
    Error loading key "/dev/fd/63": invalid format
    $ ./ssh-add /tmp/rsa
   
140408665470616:error:0906D066:lib(9):func(109):reason(102):pem_lib.c:809:
    Error loading key "/tmp/rsa": invalid format
    $ ./ssh-add /tmp/rsa1
   
140632696993432:error:0906D06C:lib(9):func(109):reason(108):pem_lib.c:701:Expecting:
ANY PRIVATE KEY
    Error loading key "/tmp/rsa1": invalid format

Let me know if there is something not clear and if it can be included
in the next release.


[1] https://bugzilla.redhat.com/show_bug.cgi?id=1282423
[2]
http://lists.mindrot.org/pipermail/openssh-unix-dev/2015-December/034617.html
[3] http://unix.stackexchange.com/q/251194/121504
[4]
https://github.com/openssl/openssl/blob/master/include/openssl/pem.h#L509

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