Bug 14913

Created attachment 24325 [details]
Simple open() test

Posix 1003.1e draft and capabilities(7) tells that CAP_DAC_READ_SEARCH
should bypass read permissions:

  This capability shall override file mode read and search access
  restrictions when accessing an object...

In reality it bypasses directory modes, but not file's.


Following is the results of open(2) from a process with the capability in question enabled (test.c source is attached).
Tests are against non-SELinux-enabled (or any other LSM-enabled) kernel.
Filesystem is ext4, but the same behavior was observed with xfs and reiserfs.


  # gcc -lcap test.c -o /test
  # setcap 'cap_dac_read_search=ep' /test

  # touch /root/test{1,2}
  # chmod 600 /root/test1
  # chmod 644 /root/test2
  
  # ls -ld /root/{,test{1,2}}
  drwx------ 21 root root 4.0K Dec 28 09:21 /root/
  -rw-------  1 root root    0 Dec 28 09:21 /root/test1
  -rw-r--r--  1 root root    0 Dec 28 09:21 /root/test2

  # su -s /bin/zsh nobody

  % ls -ld /root/{,test{1,2}}
  ls: cannot access /root/test1: Permission denied
  ls: cannot access /root/test2: Permission denied
  drwx------ 21 root root 4.0K Dec 28 09:21 /root/
  
  % /sbin/getcap /test
  /test = cap_dac_read_search+ep

  % /test /root/test1
  Process capabilities: = cap_dac_read_search+ep; Access: error (13) - Permission denied
  % /test /root/test2
  Process capabilities: = cap_dac_read_search+ep; Access: success!


Naturally, expected result is a success to open both files, since cap_dac_read_search capability is effective.

In fact, sources from the tip of master branch of Serge E. Hallyn's tree (at least true for ea21e0baaa972aa0b4) doesn't suffer from this problem.

Related thread: http://thread.gmane.org/gmane.linux.file-systems/37470