Before this change it was crucial that nonprivileged users are unable to
create hardlinks to SUID wrappers, lest they be able to provide a
different `.real` file alongside. That was ensured by not providing a
location writable to them in the /run/wrappers tmpfs, (unless
disabled) by the fs.protected_hardlinks=1 sysctl, and by the explicit
own-path check in the wrapper. After this change, ensuring
that property is no longer important, and the check is most likely
redundant.
The simplification of expectations of the wrapper will make it
easier to remove some of the assertions in the wrapper (which currently
cause the wrapper to fail in no_new_privs environments, instead of
executing the target with non-elevated privileges).
Note that wrappers had to be copied (not symlinked) into /run/wrappers
due to the SUID/capability bits, and they couldn't be hard/softlinks of
each other due to those bits potentially differing. Thus, this change
doesn't increase the amount of memory used by /run/wrappers.
This change removes part of the test that is obsoleted by the removal of
`.real` files.
Wrappers generate pieces of apparmor policies for inclusion, which are
used only in a single place in nixpkgs, for `ping`. They are built only
if apparmor is enabled.
This change causes the test to test:
- that the apparmor includes can be generated,
- that `ping` works with apparmor enabled (as the only policy that
references these includes).
Ideally there would be some other NixOS test that verifies that `ping`
specifically works. Sadly, there isn't one.
In user namespaces where an unprivileged user is mapped as root and root
is unmapped, setuid bits have no effect. However setuid root
executables like mount are still usable *in the namespace* as the user
already has the required privileges. This commit detects the situation
where the wrapper gained no privileges that the parent process did not
already have and in this case does less sanity checking. In short there
is no need to be picky since the parent already can execute the foo.real
executable themselves.
Details:
man 7 user_namespaces:
Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID
(set-group-ID) program, the process's effective user (group) ID
inside the namespace is changed to whatever value is mapped for
the user (group) ID of the file. However, if either the user or
the group ID of the file has no mapping inside the namespace, the
set-user-ID (set-group-ID) bit is silently ignored: the new
program is executed, but the process's effective user (group) ID
is left unchanged. (This mirrors the semantics of executing a
set-user-ID or set-group-ID program that resides on a filesystem
that was mounted with the MS_NOSUID flag, as described in
mount(2).)
The effect of the setuid bit is that the real user id is preserved and
the effective and set user ids are changed to the owner of the wrapper.
We detect that no privilege was gained by checking that euid == suid
== ruid. In this case we stop checking that euid == owner of the
wrapper file.
As a reminder here are the values of euid, ruid, suid, stat.st_uid and
stat.st_mode & S_ISUID in various cases when running a setuid 42 executable as user 1000:
Normal case:
ruid=1000 euid=42 suid=42
setuid=2048, st_uid=42
nosuid mount:
ruid=1000 euid=1000 suid=1000
setuid=2048, st_uid=42
inside unshare -rm:
ruid=0 euid=0 suid=0
setuid=2048, st_uid=65534
inside unshare -rm, on a suid mount:
ruid=0 euid=0 suid=0
setuid=2048, st_uid=65534
This is a small smoke test of each piece (setuid, setgid, caps) of
wrappers' functionality. It doesn't try to check for combinations of
functionalities or anything more complicated.
