Fix SXID_ERASE behavior in setuid programs (BZ #27471)
When parse_tunables tries to erase a tunable marked as SXID_ERASE for
setuid programs, it ends up setting the envvar string iterator
incorrectly, because of which it may parse the next tunable
incorrectly. Given that currently the implementation allows malformed
and unrecognized tunables pass through, it may even allow SXID_ERASE
tunables to go through.
This change revamps the SXID_ERASE implementation so that:
- Only valid tunables are written back to the tunestr string, because
of which children of SXID programs will only inherit a clean list of
identified tunables that are not SXID_ERASE.
- Unrecognized tunables get scrubbed off from the environment and
subsequently from the child environment.
- This has the side-effect that a tunable that is not identified by
the setxid binary, will not be passed on to a non-setxid child even
if the child could have identified that tunable. This may break
applications that expect this behaviour but expecting such tunables
to cross the SXID boundary is wrong. Reviewed-by: Carlos O'Donell <carlos@redhat.com>