Binary package hint: gdm
When adding a user (guest) to the nopasswdlogin group, it is permitted to login via gdm without providing credentials. In my setup, user listing at login screen is enabled and pam_mkhomedir is active every time the guest user logs in. After choosing the user guest, gdm displays a screen lacking the password input box (which is correct) and containing a cancel and a login button.
As pam_mkhomedir takes a moment to copy the full homedir (it is a little big bigger), the user has the opportunity to click on the login button ( though this would not be neccessary to login). This in turn causes gdm to crash, see the attached backtrace (recompiled the original ubuntu gdm package with DEB_BUILD_OPTIONS="nostrip debug").
After debugging I now believe that the following is happening: on clicking on the login button, an answer query dbus message is generated, which is then rejected by the receiver due to no pam authentification request is pending. The latter is correct, as no password has been asked for, so the answer query dbus message should only be generated if an pam input query is pending.
The attached patch fixes this by adding a pending_query boolean which is set on receiving an input request and unset on replying / resetting.
DistroRelease: Ubuntu 10.10
Package: gdm 2.30.5-0ubuntu4
ProcVersionSignature: Ubuntu 2.6.35-25.44-generic 188.8.131.52
Uname: Linux 2.6.35-25-generic x86_64
Date: Thu Feb 24 10:12:40 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)