All commands in my crontab fail with "Permission denied"

Update: This issue will not be answered conclusively; I have moved to another distro and have not observed this problem since. I was never able to fix it with the insightful answers available at the time, but your fuel efficiency may vary (YMMV).

crontab -e and crontab -l work just fine:

$ crontab -l | grep -v '^#'
* * * * * /usr/bin/env
* * * * * echo 'Hello from crontab'

However, I see two messages like this every minute in /var/log/syslog:

Mon DD hh:mm:01 username CRON[PID]: Permission denied

So the crontab is being read, but somehow it can't execute anything at all (of course I verified the commands when logged in as the same user). Any idea why?

/etc/cron.allow and /etc/cron.deny do not exist.

crontab is set group setuid:

$ stat --format '%A %U %G' /usr/bin/crontab
-rwxr-sr-x root crontab

The crontabs directory seems to have the right permissions:

$ stat --format '%A %U %G' /var/spool/cron/crontabs
drwx-wx--T root crontab

The crontab itself is owned by me (not surprisingly, since I'm able to edit it):

$ sudo stat --format '%A %U %G' /var/spool/cron/crontabs/$USER
-rw------- username crontab

I am not a member of the crontab group.

These lines appear in /var/log/auth.log every minute (thanks @Alaa):

Mon DD hh:mm:01 username CRON[1752]: pam_unix(cron:session): session opened for user username by (uid=0)
Mon DD hh:mm:01 username CRON[1752]: PAM bad jump in stack

Maybe PAM is broken? pam-auth-update (thanks @coteyr) lists all of these, and all of them are enabled:

• Unix authentication
• GNOME Keyring Daemon - Login keyring management
• eCryptfs Key/Mount Management
• ConsoleKit Session Management
• Inheritable Capabilities Management

Can any of them be safely disabled? I'm not using any encrypted filesystems.

Based on a Debian bug entry I tried running debconf-show libpam-runtime, and I got the following error message:

debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied

The contents of /etc/pam.d/cron:

# The PAM configuration file for the cron daemon

@include common-auth

# Read environment variables from pam_env's default files, /etc/environment
# and /etc/security/pam_env.conf.
session       required   pam_env.so

# In addition, read system locale information
session       required   pam_env.so envfile=/etc/default/locale

@include common-account
@include common-session-noninteractive 

# Sets up user limits, please define limits for cron tasks
# through /etc/security/limits.conf
session    required   pam_limits.so

session [success=1 default=ignore] pam_succeed_if.so service in cron quiet use_uid

The files mentioned (/etc/environment, pam_env.so, /etc/default/locale, pam_limits.so, pam_succeed_if.so) are all readable by my user.

On another host with Ubuntu 13.04, with the same user crontab, no /etc/cron.{allow,deny}, same permissions as above, and not being a member of the crontab group, it works just fine (logs the commands but not the output in /var/log/syslog).

By changing the first crontab line:

* * * * * /usr/bin/env >/tmp/env.log 2>&1

and checking that /tmp is world writeable:

$ sudo -u nobody touch /tmp/test
$ ls /tmp/test
/tmp/test
$ ls -ld /tmp
drwxrwxrwt 15 root root 12288 May 27 10:18 /tmp

I've verified that the crontab commands are not run at all: The Permission denied messages still show up in /var/log/syslog, but /tmp/env.log is not created.

Based on a random listing of /etc/pam.d settings I found the following discrepancies:

$ grep '^[^#]' /etc/pam.d/sshd 
@include common-auth
account    required     pam_nologin.so
@include common-account
@include common-session
session    optional     pam_motd.so # [1]
session    optional     pam_mail.so standard noenv # [1]
session    required     pam_limits.so
session    required     pam_env.so # [1]
session    required     pam_env.so user_readenv=1 envfile=/etc/default/locale
@include common-password
$ grep '^[^#]' /etc/pam.d/common-session
session [default=1]         pam_permit.so
session requisite           pam_deny.so
session required            pam_permit.so
session optional            pam_umask.so
session required    pam_unix.so 
session optional    pam_ecryptfs.so unwrap
session optional            pam_ck_connector.so nox11
$ grep '^[^#]' /etc/pam.d/common-account
account [success=1 new_authtok_reqd=done default=ignore]    pam_unix.so 
account requisite           pam_deny.so
account required            pam_permit.so
$ grep '^[^#]' /etc/pam.d/common-session-noninteractive 
session [default=1]         pam_permit.so
session requisite           pam_deny.so
session required            pam_permit.so
session optional            pam_umask.so
session required    pam_unix.so 
session optional    pam_ecryptfs.so unwrap

PAM packages installed:

$ dpkg --get-selections | grep --invert-match deinstall | cut --fields 1 | grep pam
libpam-cap
libpam-ck-connector
libpam-gnome-keyring
libpam-modules
libpam-modules-bin
libpam-runtime
libpam0g
python-pam

I tried reinstalling these - didn't help:

$ sudo apt-get install --reinstall $(dpkg --get-selections | grep --invert-match deinstall | cut --fields 1 | grep pam)

I can't purge and then reinstall these because of unmet dependencies.