[Bug 1846025] [NEW] cron can crash (SIGSEGV) when trying to re-parent orphans

[buck huppmann]

Public bug reported:

We sometimes have people come and go, leaving behind
old crontab files that, when they return, and are given
the same username, no longer have the correct uid.
This can cause cron to crash when it has the old crontab
file on its orphans list. Below's a lightly-edited
backtrace, with username replaced by "joeuser".

Here's the user's (former) crontab:

-rw------- 1 48031 crontab 1216 Jul 26 15:36
/var/spool/cron/crontabs/joeuser

Whereas his new uid is

# id -u joeuser
48529

This is on bionic with cron 3.0pl1-128.1ubuntu1

--buck

Core was generated by `/usr/sbin/cron -f'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  find_user (name=<optimized out>, db=0x0, db at entry=0x55fc9fe93ea0)
    at database.c:316
316	database.c: No such file or directory.
(gdb) bt
#0  find_user (name=<optimized out>, db=0x0, db at entry=0x55fc9fe93ea0)
    at database.c:316
#1  force_rescan_user (old_db=old_db at entry=0x0, 
    new_db=new_db at entry=0x7fffe687cb50, 
    fname=fname at entry=0x55fc9fe93ea0 "joeuser", old_mtime=old_mtime at entry=0)
    at database.c:588
#2  0x000055fc9f2b03ab in process_crontab (uname=0x55fc9fe93df0 "joeuser", 
    fname=<optimized out>, tabname=0x55fc9fe93ec0 "crontabs/joeuser", 
    statbuf=statbuf at entry=0x7fffe687ca80, new_db=new_db at entry=0x7fffe687cb50, 
    old_db=old_db at entry=0x0) at database.c:460
#3  0x000055fc9f2b0ba6 in check_orphans (db=0x7fffe687cb50) at database.c:639
#4  0x000055fc9f2af6ce in main (argc=<optimized out>, argv=<optimized out>)
    at cron.c:178

** Affects: cron (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1846025

Title:
  cron can crash (SIGSEGV) when trying to re-parent orphans

Status in cron package in Ubuntu:
  New

Bug description:
  We sometimes have people come and go, leaving behind
  old crontab files that, when they return, and are given
  the same username, no longer have the correct uid.
  This can cause cron to crash when it has the old crontab
  file on its orphans list. Below's a lightly-edited
  backtrace, with username replaced by "joeuser".

  Here's the user's (former) crontab:

  -rw------- 1 48031 crontab 1216 Jul 26 15:36
  /var/spool/cron/crontabs/joeuser

  Whereas his new uid is

  # id -u joeuser
  48529

  This is on bionic with cron 3.0pl1-128.1ubuntu1

  --buck

  Core was generated by `/usr/sbin/cron -f'.
  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  find_user (name=<optimized out>, db=0x0, db at entry=0x55fc9fe93ea0)
      at database.c:316
  316	database.c: No such file or directory.
  (gdb) bt
  #0  find_user (name=<optimized out>, db=0x0, db at entry=0x55fc9fe93ea0)
      at database.c:316
  #1  force_rescan_user (old_db=old_db at entry=0x0, 
      new_db=new_db at entry=0x7fffe687cb50, 
      fname=fname at entry=0x55fc9fe93ea0 "joeuser", old_mtime=old_mtime at entry=0)
      at database.c:588
  #2  0x000055fc9f2b03ab in process_crontab (uname=0x55fc9fe93df0 "joeuser", 
      fname=<optimized out>, tabname=0x55fc9fe93ec0 "crontabs/joeuser", 
      statbuf=statbuf at entry=0x7fffe687ca80, new_db=new_db at entry=0x7fffe687cb50, 
      old_db=old_db at entry=0x0) at database.c:460
  #3  0x000055fc9f2b0ba6 in check_orphans (db=0x7fffe687cb50) at database.c:639
  #4  0x000055fc9f2af6ce in main (argc=<optimized out>, argv=<optimized out>)
      at cron.c:178

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