On Tue, May 12, 2020 at 6:33 PM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Since the meaning of combining the KEY_NEED_* constants is undefined, make
> it so that you can't do that by turning them into an enum.
>
> The enum is also given some extra values to represent special
> circumstances, such as:
>
> (1) The '0' value is reserved and causes a warning to trap the parameter
> being unset.
>
> (2) The key is to be unlinked and we require no permissions on it, only
> the keyring, (this replaces the KEY_LOOKUP_FOR_UNLINK flag).
>
> (3) An override due to CAP_SYS_ADMIN.
CAP_SYS_ADMIN should never skip SELinux checking. Even for Smack,
there is a separate capability (CAP_MAC_ADMIN) for that purpose.
> (4) An override due to an instantiation token being present.
Not sure what this means but again we shouldn't skip SELinux checking
based on mere possession of an object capability (not a POSIX
capability).
>
> (5) The permissions check is being deferred to later key_permission()
> calls.
>
> The extra values give the opportunity for LSMs to audit these situations.
> ---
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index 7d8de1c9a478..6763ee45e04d 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -434,7 +434,7 @@ long keyctl_invalidate_key(key_serial_t id)
>
> /* Root is permitted to invalidate certain special keys */
> if (capable(CAP_SYS_ADMIN)) {
> - key_ref = lookup_user_key(id, 0, 0);
> + key_ref = lookup_user_key(id, 0, KEY_SYSADMIN_OVERRIDE);
It would be better if the permission indicated the actual operation
(e.g. KEY_NEED_INVALIDATE_SPECIAL), and the decision whether to permit
CAP_SYS_ADMIN processes to override was left to the security modules.
SELinux doesn't automatically allow CAP_SYS_ADMIN processes to do
everything.
> @@ -479,7 +479,8 @@ long keyctl_keyring_clear(key_serial_t ringid)
>
> /* Root is permitted to invalidate certain special keyrings */
> if (capable(CAP_SYS_ADMIN)) {
> - keyring_ref = lookup_user_key(ringid, 0, 0);
> + keyring_ref = lookup_user_key(ringid, 0,
> + KEY_SYSADMIN_OVERRIDE);
Ditto.
> @@ -663,7 +664,7 @@ long keyctl_describe_key(key_serial_t keyid,
> key_put(instkey);
> key_ref = lookup_user_key(keyid,
> KEY_LOOKUP_PARTIAL,
> - 0);
> + KEY_AUTHTOKEN_OVERRIDE);
Similarly, it would be better if the permission indicated the
operation (e.g. KEY_NEED_DESCRIBE) rather than the means by which it
is being authorized. A MAC scheme won't allow mere knowledge of a
token/password-capability to permit violation of its policy.
> @@ -1471,7 +1472,7 @@ long keyctl_set_timeout(key_serial_t id, unsigned timeout)
> key_put(instkey);
> key_ref = lookup_user_key(id,
> KEY_LOOKUP_PARTIAL,
> - 0);
> + KEY_AUTHTOKEN_OVERRIDE);
Ditto.
> @@ -1579,7 +1580,8 @@ long keyctl_get_security(key_serial_t keyid,
> return PTR_ERR(instkey);
> key_put(instkey);
>
> - key_ref = lookup_user_key(keyid, KEY_LOOKUP_PARTIAL, 0);
> + key_ref = lookup_user_key(keyid, KEY_LOOKUP_PARTIAL,
> + KEY_AUTHTOKEN_OVERRIDE);
Ditto
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 0b4e32161b77..3ff6b6dfc5ca 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6541,20 +6541,31 @@ static void selinux_key_free(struct key *k)
>
> static int selinux_key_permission(key_ref_t key_ref,
> const struct cred *cred,
> - unsigned perm)
> + enum key_need_perm need_perm)
> {
> struct key *key;
> struct key_security_struct *ksec;
> - u32 sid;
> + u32 perm, sid;
>
> - /* if no specific permissions are requested, we skip the
> - permission check. No serious, additional covert channels
> - appear to be created. */
> - if (perm == 0)
> + switch (need_perm) {
> + case KEY_NEED_UNLINK:
> + case KEY_SYSADMIN_OVERRIDE:
> + case KEY_AUTHTOKEN_OVERRIDE:
> + case KEY_DEFER_PERM_CHECK:
> return 0;
We really shouldn't be skipping any/all checking on CAP_SYS_ADMIN or
an AUTHTOKEN; those should still be subject to MAC policy.
