[Bug 1927796] Re: [SRU]pam_tally2 can cause accounts to be locked by correct password. pam_faillock use is the recommended fix

[Matthew Ruffell]

Performing verification for Hirsute

I enabled -proposed and installed libpam-modules libpam-modules-bin
libpam-runtime libpam0g version 1.3.1-5ubuntu6.21.04.1

>From there, I set the pam_faillock configuration in:

/etc/security/faillock.conf:
deny = 3
unlock_time = 120

and also:

/etc/pam.d/common-auth:

# here are the per-package modules (the "Primary" block)
auth    requisite                       pam_faillock.so preauth
auth    [success=1 default=ignore]      pam_unix.so nullok_secure
auth    [default=die]                   pam_faillock.so authfail
auth    sufficient                      pam_faillock.so authsucc
# here's the fallback if no module succeeds
auth    requisite                       pam_deny.so
# prime the stack with a positive return value if there isn't one already;
# this avoids us returning an error just because nothing sets a success code
# since the modules above will each just jump around
auth    required                        pam_permit.so
# and here are more per-package modules (the "Additional" block)
auth    optional                        pam_cap.so
# end of pam-auth-update config

>From there, I created a new user "dave", and rebooted the system.

I connected via ssh with the "dave" user and used the wrong password 5 times.
I then tried with the correct password and found the account to be locked.

I waited 2 minutes, and tried again with the correct password, and I was logged
in.

When the account was locked, I logged in as the "ubuntu" user and ran:

$ sudo faillock --user dave
dave:
When                Type  Source                                           Valid
2021-05-19 01:50:25 RHOST 192.168.122.1                                        V
2021-05-19 01:50:28 RHOST 192.168.122.1                                        V
2021-05-19 01:50:31 RHOST 192.168.122.1                                        V

And I could see the times that "dave" was locked.

I also tested resetting via:

$ sudo faillock --user dave --reset

and "dave" was allowed to log in again.

My tests agree with what Richard sees. Marking as verified for Hirsute.

** Tags removed: verification-needed-hirsute
** Tags added: verification-done-hirsute

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https://bugs.launchpad.net/bugs/1927796

Title:
  [SRU]pam_tally2 can cause accounts to be locked by correct password.
  pam_faillock use is the recommended fix

Status in pam package in Ubuntu:
  Fix Committed
Status in pam source package in Bionic:
  Fix Committed
Status in pam source package in Focal:
  Fix Committed
Status in pam source package in Groovy:
  Fix Committed
Status in pam source package in Hirsute:
  Fix Committed
Status in pam source package in Impish:
  Fix Committed

Bug description:
  [IMPACT]
  There is a known issue in pam_tally2 which may cause an account to be lock down even with correct password, in a busy node environment where simultaneous logins takes place (https://github.com/linux-pam/linux-pam/issues/71).

  There are already two customer cases from Canonical clients
  complaining about this behavior (00297697 and 00303806).

  Also, potentially, this will cause further problems in the future,
  since both STIG benchmarks and CIS benchmarks rely on pam_tally2 to
  lock accounts when wrong passwords are used. And both benchmarks - but
  specially STIG - requires use of a lot of audit rules, which can lead
  to the busy node environment.

  The issue impacts all pam_tally2 versions distributed in all currently
  supported Ubuntu versions and also the next unreleased one. Note that,
  according to https://github.com/linux-pam/linux-pam/issues/71, there
  is no plan to fix this issue!

  [FIX]
  This fix proposes to add pam_faillock module to the PAM package, so users of pam_tally2 having issues can migrate to pam_faillock. We also plan to modify the current STIG benchmarks to rely on pam_faillock instead of pam_tally2, but in order to do so, we need the pam_faillock module to be available.

  Note that we don't propose to remove pam_tally2, since not every user
  of this module is affected.

  [TEST]
  Tested on a VM installed with Focal server iso and on another with Bionic server iso. Enabled pam_faillock module as recommeded by its man page. Then tried to log over ssh with an incorrect password, until the account got locked. Waited for the configured grace time to unlock and logged in using the correct password.

  Note that, since the pam_tally2 issue is caused by a racing condition,
  with a hard to recreate environment (we could not even reproduce it
  with pam_tally2), we could not reproduce the conditions to test
  pam_faillock with.

  [REGRESSION POTENTIAL]
  The regression potential for this is small, since we're not removing the old pam_tally2 module, just adding another one. So anyone still using pam_tally2 will be able to do so.

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