[PATCH 3/6] landlock: Mediate CAP_SETPCAP for inheritable capabilities

["Mickaël Salaün" <mic@digikod.net>]

A process in a domain that denies CAP_SETPCAP could still add
capabilities outside of its permitted set to its inheritable set with
capset(2), if it held CAP_SETPCAP.  cap_capset() checks CAP_SETPCAP with
cap_capable() in cap_inh_is_capped(), so the check never reaches
hook_capable().  Through file inheritable capabilities, such a set can
grant new permitted capabilities on execve() when no_new_privs is not
set, which is possible for a domain enforced by a task holding
CAP_SYS_ADMIN.

Add a Landlock capset hook that checks CAP_SETPCAP against the domain
under the same condition as cap_capset().  As with the mmap_addr hook,
LSMs implement their own policy in this hook: SELinux checks capset with
its own permission.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge Hallyn <serge@hallyn.com>
Reported-by: Ada Smith <adasmithgt@gmail.com>
Fixes: 97c8386f1d74 ("landlock: Enforce capability restrictions")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
---
 security/landlock/cap.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/security/landlock/cap.c b/security/landlock/cap.c
index 10e1ca772b9c..c89e8d76db8c 100644
--- a/security/landlock/cap.c
+++ b/security/landlock/cap.c
@@ -91,6 +91,35 @@ static int hook_capable(const struct cred *cred, struct user_namespace *ns,
 	return check_cap(cred, cap, opts);
 }
 
+/**
+ * hook_capset - Restrict inheritable capability changes
+ *
+ * @new: New credentials (unused).
+ * @old: Current credentials.
+ * @effective: New effective set (unused).
+ * @inheritable: New inheritable set.
+ * @permitted: New permitted set (unused).
+ *
+ * cap_capset() requires CAP_SETPCAP to add inheritable capabilities that are
+ * not in the old permitted set, but calls cap_capable() directly, which skips
+ * hook_capable().  Check the same condition against the domain.  commoncap runs
+ * first, so this only sees requests that commoncap allowed.  capset(2) discards
+ * @new if this hook fails, so nothing is committed.
+ *
+ * Return: 0 if allowed, -EPERM if CAP_SETPCAP use is denied.
+ */
+static int hook_capset(struct cred *new, const struct cred *old,
+		       const kernel_cap_t *effective,
+		       const kernel_cap_t *inheritable,
+		       const kernel_cap_t *permitted)
+{
+	if (cap_issubset(*inheritable,
+			 cap_combine(old->cap_inheritable, old->cap_permitted)))
+		return 0;
+
+	return check_cap(old, CAP_SETPCAP, CAP_OPT_NONE);
+}
+
 /**
  * hook_mmap_addr - Restrict mappings below dac_mmap_min_addr
  *
@@ -113,6 +142,7 @@ static int hook_mmap_addr(unsigned long addr)
 
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(capable, hook_capable),
+	LSM_HOOK_INIT(capset, hook_capset),
 	LSM_HOOK_INIT(mmap_addr, hook_mmap_addr),
 };
 
-- 
2.56.0