[Bug 1693] New: ssh prompts for passphrase even when identity file is unreadable

[bugzilla-daemon at bugzilla.mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=1693

           Summary: ssh prompts for passphrase even when identity file is
                    unreadable
           Product: Portable OpenSSH
           Version: 5.3p1
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: minor
          Priority: P2
         Component: ssh
        AssignedTo: unassigned-bugs at mindrot.org
        ReportedBy: tj at castaglia.org


Created an attachment (id=1767)
 --> (https://bugzilla.mindrot.org/attachment.cgi?id=1767)
Uses access(2) to check readability of identity file before loading

When using publickey authentication, the private key is loaded from an
IdentityFile using ssh2connect.c's load_identity_file() function.  A
stat(2) is used to ensure that the file exists, and then the key is
loaded.

If the file exists, but the process does not have read permissions on
that file, then ssh will prompt the user for a passphrase.  This is
particularly confusing if the private key in question is not
passphrase-protected.  The fact that the ssh client prompts for the
passphrase is misleading; the underlying inability to load the private
key stems not from the need for a passphrase, but from the lack of
readability.  It would be better to display a "Permission denied"
message to the user, and/or move on to the next authentication method,
rather than needlessly prompting the user for passphrases in this case.

The attached patch attempts to remedy this situation by using access(2)
after the stat(2) call.  If the file is not readable, then the private
key cannot be loaded, and thus the call to key_load_private_type() is
skipped.

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