Config file support

[dacav]

Fixes #265

[dacav]

• Manpages update

[LDVG]

This looks like a pretty good start to me. I've left mostly nitpicking comments in-line. 🙂

[dacav]

Current situation:

What is left to do for the current PR (soon follow-up):

• A proper MAX_PATH (currently using hard-wired 4096 - Linux).
• A temporary --with-security-confdir solution, mimicking --with-pam-dir (future refinement)
• Trailing comments and spaces around '='
• Only file owner can write (hopefully that's root)
• Only root can own the file (requires fakeroot-ish behaviour)
• Comprehensive config unit test
• Inject the security-confdir into manpages (currently "/etc/security" is a fixed string

[dacav]

Oh no. I figured out this even more work!

Autoconf BUG (?)

1. ./configure --with-sconf-dir=/a
2. make && sudo make install
3. ./configure --with-sconf-dir=/b
4. make # WILL NOT RECOMPILE (well, only manpage)
5. sudo make install # not what expected: old binary is installed again

This is a little pathological (maybe low prio?) but actually a defect.

Vulnerable for untidy sysadmin:

Currently my code is checking that the configuration file (1) belongs to root, (2) can only be written by root, (3) is not a symlink. But I'm not checking the status of the above directories... Therefore, assuming that pam_u2f.so is configured to use /my/hopefully/notalink/pam_u2f.conf as a configuration file, in the (also pathological?) situation the unprivileged user might be able to hijack the configuration:

  user@localhost:/my$ ls -lR .
  .:
  total 4
  drwxrwxrwx 4 root root 4096 Jan  2 11:23 hopefully [1]

  ./hopefully:
  total 8
  drwxr-xr-x 2 root root 4096 Jan  2 11:15 notalink
  drwxr-xr-x 2 root root 4096 Jan  2 11:23 oops

  ./hopefully/notalink:
  total 4
  -rw-r--r-- 1 root root 13 Jan  2 11:05 pam_u2f.conf [2]

  ./hopefully/oops:
  total 4
  -rw-r--r-- 1 root root 13 Jan  2 11:23 pam_u2f.conf [2]

[1] world-writable directory
[2] a root-owned-not-writable copy exists

Attack:

  user@localhost:/my$ cd hopefully
  user@localhost:/my/hopefully$ mv notalink/ goodbye
  user@localhost:/my/hopefully$ ln -s oops notalink
  user@localhost:/my/hopefully$ cd ..
  user@localhost:/my$ ls -lR
  .:
  total 4
  drwxrwxrwx 4 root root 4096 Jan  2 11:25 hopefully

  ./hopefully:
  total 8
  drwxr-xr-x 2 root root 4096 Jan  2 11:15 goodbye
  lrwxrwxrwx 1 user user    4 Jan  2 11:25 notalink -> oops
  drwxr-xr-x 2 root root 4096 Jan  2 11:23 oops

  ./hopefully/goodbye:
  total 4
  -rw-r--r-- 1 root root 13 Jan  2 11:05 pam_u2f.conf

  ./hopefully/oops:
  total 4
  -rw-r--r-- 1 root root 15 Jan  2 11:23 pam_u2f.conf

[dacav]

Rebased on current main.

[LDVG]

last iteration of nit picking, this looks good to go otherwise. nice work! :)

[dacav]

Applied requested changes, and rebased on current main branch.