[PATCH 2/6] landlock: Restrict capabilities guarding other LSMs' policies

["Mickaël Salaün" <mic@digikod.net>]

A process in a domain that denies CAP_MAC_ADMIN or CAP_MAC_OVERRIDE
could still use them for AppArmor, Smack or SELinux policy, and
CAP_SYS_PTRACE for Smack's ptrace rule, because these LSMs check them
with cap_capable() instead of capable().

Implement the capable_lsm hook with the same check as the capable hook,
so that such capability uses are restricted like any other.  The
following commits switch these LSMs to security_capable_lsm().

Cc: Christian Brauner <brauner@kernel.org>
Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge Hallyn <serge@hallyn.com>
Fixes: 97c8386f1d74 ("landlock: Enforce capability restrictions")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
---
 security/landlock/cap.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/security/landlock/cap.c b/security/landlock/cap.c
index c9a900888f3b..162edf689f29 100644
--- a/security/landlock/cap.c
+++ b/security/landlock/cap.c
@@ -91,6 +91,28 @@ static int hook_capable(const struct cred *cred, struct user_namespace *ns,
 	return check_cap(cred, cap, opts);
 }
 
+/**
+ * hook_capable_lsm - Deny capabilities guarding another LSM's policy
+ *
+ * @cred: Credentials being checked.
+ * @ns: Target user namespace (intentionally ignored, see hook_capable()).
+ * @cap: Capability number (CAP_*).
+ * @opts: Capability check options, see check_cap().
+ * @lsm_id: LSM_ID_* of the LSM whose policy @cap guards (unused: Landlock does
+ *          not call security_capable_lsm()).
+ *
+ * Other LSMs check the capabilities that guard their own policy (e.g.
+ * CAP_MAC_ADMIN) with security_capable_lsm() instead of capable().  Restrict
+ * them like any other capability use.
+ *
+ * Return: 0 if allowed, -EPERM if capability use is denied.
+ */
+static int hook_capable_lsm(const struct cred *cred, struct user_namespace *ns,
+			    int cap, unsigned int opts, u64 lsm_id)
+{
+	return check_cap(cred, cap, opts);
+}
+
 /**
  * hook_capset - Restrict inheritable capability changes
  *
@@ -163,6 +185,7 @@ static int hook_vm_enough_memory(struct mm_struct *mm, long pages)
 
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(capable, hook_capable),
+	LSM_HOOK_INIT(capable_lsm, hook_capable_lsm),
 	LSM_HOOK_INIT(capset, hook_capset),
 	LSM_HOOK_INIT(mmap_addr, hook_mmap_addr),
 	LSM_HOOK_INIT(vm_enough_memory, hook_vm_enough_memory),
-- 
2.56.0