[Bug 1779914] Re: unsquashfs does not preserve sticky bit when run as non-root

[Jamie Strandboge]

** Description changed:

+ [Impact] 
+ unsquashfs does not preserve the stickybit when run as non-root (unlike other archive tools, like tar). While this is a bug in and of itself, it causes snaps with sticky directories to fail automated review because the requashed snap has the bit stripped and the resquashed snap as a result has a different checksum.
+ 
+ The fix is to attempt the chmod with the stickybit and if it fails with
+ EPERM when not root, try again without the stickybit.
+ 
+ [Test Case]
+ 
+ 1. create a squashfs with a sticky dir:
+ 
+ $ mkdir -p /tmp/foo/sticky-dir
+ $ chmod 1777 /tmp/foo/sticky-dir
+ $ mksquashfs /tmp/foo test.squash -all-root
+ 
+ 2. see that the squashfs has the sticky dir in the squash:
+ 
+ $ unsquashfs -lls ./test.squash 
+ ...
+ drwxrwxrwt root/root                 3 2018-07-05 16:03 squashfs-root/sticky-dir
+ 
+ 3. unsquash the squash as non-root:
+ 
+ $ unsquashfs test.squash
+ 
+ 4. verified the stickybit is set:
+ 
+ $ ls -ld squashfs-root/sticky-dir/
+ drwxrwxrwt 2 jamie jamie 4096 Jul  5 16:07 squashfs-root/sticky-dir/
+ 
+ Without the SRU, the directory is 0777 and the file 0775:
+ 
+ $ ls -ld squashfs-root/sticky-dir/
+ drwxrwxrwx 2 jamie jamie 4096 Jul  5 16:07 squashfs-root/sticky-dir/
+ 
+ [Regression Potential]
+ 
+ Due to the fallback behavior, the regression potential is considered
+ low. Furthermore, because the non-root user is still the owner of the
+ resulting unpacked sticky directories, there is no problem with being
+ able to remove the unpacked directories on error, etc.
+ 
+ [ Original description ]
  From https://sourceforge.net/p/squashfs/mailman/message/36343213/:
  
  "This set is an attempt to preserve the sticky bit when running unsquashfs as a non-root user. My main motivation for these changes is to improve
  reproducability when doing a sequence of "unsquashfs -> mksquashfs" as a
  non-root user but I think there's even more value in preserving the sticky bit in the case of a squashfs image containing a world-writable directory filled with files owned by a single user. Dropping the sticky bit could be considered to be a real bug in that scenario."

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https://bugs.launchpad.net/bugs/1779914

Title:
  unsquashfs does not preserve sticky bit when run as non-root

Status in squashfs-tools package in Ubuntu:
  Fix Committed
Status in squashfs-tools source package in Trusty:
  In Progress
Status in squashfs-tools source package in Xenial:
  In Progress
Status in squashfs-tools source package in Bionic:
  In Progress
Status in squashfs-tools source package in Cosmic:
  Fix Committed
Status in squashfs-tools package in Debian:
  Unknown

Bug description:
  [Impact]
  unsquashfs does not preserve the stickybit when run as non-root (unlike other archive tools, like tar). While this is a bug in and of itself, it causes snaps with sticky directories to fail automated review because the requashed snap has the bit stripped and the resquashed snap as a result has a different checksum.

  The fix is to attempt the chmod with the stickybit and if it fails
  with EPERM when not root, try again without the stickybit.

  [Test Case]

  1. create a squashfs with a sticky dir:

  $ mkdir -p /tmp/foo/sticky-dir
  $ chmod 1777 /tmp/foo/sticky-dir
  $ mksquashfs /tmp/foo test.squash -all-root

  2. see that the squashfs has the sticky dir in the squash:

  $ unsquashfs -lls ./test.squash
  ...
  drwxrwxrwt root/root                 3 2018-07-05 16:03 squashfs-root/sticky-dir

  3. unsquash the squash as non-root:

  $ unsquashfs test.squash

  4. verified the stickybit is set:

  $ ls -ld squashfs-root/sticky-dir/
  drwxrwxrwt 2 jamie jamie 4096 Jul  5 16:07 squashfs-root/sticky-dir/

  Without the SRU, the directory is 0777 and the file 0775:

  $ ls -ld squashfs-root/sticky-dir/
  drwxrwxrwx 2 jamie jamie 4096 Jul  5 16:07 squashfs-root/sticky-dir/

  [Regression Potential]

  Due to the fallback behavior, the regression potential is considered
  low. Furthermore, because the non-root user is still the owner of the
  resulting unpacked sticky directories, there is no problem with being
  able to remove the unpacked directories on error, etc.

  [ Other Info ]
  In addition to the above, I've added test-squashfs-tools.py to QRT which verifies type, owner and permissions with mksquashfs and unsquashfs which fail for the sticky bit (but otherwise pass) when unpatched and pass when patched.

  [ Original description ]
  From https://sourceforge.net/p/squashfs/mailman/message/36343213/:

  "This set is an attempt to preserve the sticky bit when running unsquashfs as a non-root user. My main motivation for these changes is to improve
  reproducability when doing a sequence of "unsquashfs -> mksquashfs" as a
  non-root user but I think there's even more value in preserving the sticky bit in the case of a squashfs image containing a world-writable directory filled with files owned by a single user. Dropping the sticky bit could be considered to be a real bug in that scenario."

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