LSM: SafeSetID: Add GID security policy handling

The SafeSetID LSM has functionality for restricting setuid() calls based
on its configured security policies. This patch adds the analogous
functionality for setgid() calls. This is mostly a copy-and-paste change
with some code deduplication, plus slight modifications/name changes to
the policy-rule-related structs (now contain GID rules in addition to
the UID ones) and some type generalization since SafeSetID now needs to
deal with kgid_t and kuid_t types.

Signed-off-by: Thomas Cedeno <thomascedeno@google.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>

1 files changed, 143 insertions, 47 deletions

diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
index 7760019ad35d..c08e67108a82 100644
--- a/security/safesetid/lsm.c
+++ b/security/safesetid/lsm.c
@@ -24,20 +24,36 @@
 /* Flag indicating whether initialization completed */
 int safesetid_initialized;
 
-struct setuid_ruleset __rcu *safesetid_setuid_rules;
+struct setid_ruleset __rcu *safesetid_setuid_rules;
+struct setid_ruleset __rcu *safesetid_setgid_rules;
+
 
 /* Compute a decision for a transition from @src to @dst under @policy. */
-enum sid_policy_type _setuid_policy_lookup(struct setuid_ruleset *policy,
-		kuid_t src, kuid_t dst)
+enum sid_policy_type _setid_policy_lookup(struct setid_ruleset *policy,
+		kid_t src, kid_t dst)
 {
-	struct setuid_rule *rule;
+	struct setid_rule *rule;
 	enum sid_policy_type result = SIDPOL_DEFAULT;
 
-	hash_for_each_possible(policy->rules, rule, next, __kuid_val(src)) {
-		if (!uid_eq(rule->src_uid, src))
-			continue;
-		if (uid_eq(rule->dst_uid, dst))
-			return SIDPOL_ALLOWED;
+	if (policy->type == UID) {
+		hash_for_each_possible(policy->rules, rule, next, __kuid_val(src.uid)) {
+			if (!uid_eq(rule->src_id.uid, src.uid))
+				continue;
+			if (uid_eq(rule->dst_id.uid, dst.uid))
+				return SIDPOL_ALLOWED;
+			result = SIDPOL_CONSTRAINED;
+		}
+	} else if (policy->type == GID) {
+		hash_for_each_possible(policy->rules, rule, next, __kgid_val(src.gid)) {
+			if (!gid_eq(rule->src_id.gid, src.gid))
+				continue;
+			if (gid_eq(rule->dst_id.gid, dst.gid)){
+				return SIDPOL_ALLOWED;
+			}
+			result = SIDPOL_CONSTRAINED;
+		}
+	} else {
+		/* Should not reach here, report the ID as contrainsted */
 		result = SIDPOL_CONSTRAINED;
 	}
 	return result;
@@ -47,15 +63,26 @@ enum sid_policy_type _setuid_policy_lookup(struct setuid_ruleset *policy,
  * Compute a decision for a transition from @src to @dst under the active
  * policy.
  */
-static enum sid_policy_type setuid_policy_lookup(kuid_t src, kuid_t dst)
+static enum sid_policy_type setid_policy_lookup(kid_t src, kid_t dst, enum setid_type new_type)
 {
 	enum sid_policy_type result = SIDPOL_DEFAULT;
-	struct setuid_ruleset *pol;
+	struct setid_ruleset *pol;
 
 	rcu_read_lock();
-	pol = rcu_dereference(safesetid_setuid_rules);
-	if (pol)
-		result = _setuid_policy_lookup(pol, src, dst);
+	if (new_type == UID)
+		pol = rcu_dereference(safesetid_setuid_rules);
+	else if (new_type == GID)
+		pol = rcu_dereference(safesetid_setgid_rules);
+	else { /* Should not reach here */
+		result = SIDPOL_CONSTRAINED;
+		rcu_read_unlock();
+		return result;
+	}
+
+	if (pol) {
+		pol->type = new_type;
+		result = _setid_policy_lookup(pol, src, dst);
+	}
 	rcu_read_unlock();
 	return result;
 }
@@ -65,57 +92,101 @@ static int safesetid_security_capable(const struct cred *cred,
 				      int cap,
 				      unsigned int opts)
 {
-	/* We're only interested in CAP_SETUID. */
-	if (cap != CAP_SETUID)
+	/* We're only interested in CAP_SETUID and CAP_SETGID. */
+	if (cap != CAP_SETUID && cap != CAP_SETGID)
 		return 0;
 
 	/*
-	 * If CAP_SETUID is currently used for a set*uid() syscall, we want to
+	 * If CAP_SET{U/G}ID is currently used for a setid() syscall, we want to
 	 * let it go through here; the real security check happens later, in the
-	 * task_fix_setuid hook.
+	 * task_fix_set{u/g}id hook.
+         *
+         * NOTE:
+         * Until we add support for restricting setgroups() calls, GID security
+         * policies offer no meaningful security since we always return 0 here
+         * when called from within the setgroups() syscall and there is no
+         * additional hook later on to enforce security policies for setgroups().
 	 */
 	if ((opts & CAP_OPT_INSETID) != 0)
 		return 0;
 
-	/*
-	 * If no policy applies to this task, allow the use of CAP_SETUID for
-	 * other purposes.
-	 */
-	if (setuid_policy_lookup(cred->uid, INVALID_UID) == SIDPOL_DEFAULT)
+	switch (cap) {
+	case CAP_SETUID:
+		/*
+		* If no policy applies to this task, allow the use of CAP_SETUID for
+		* other purposes.
+		*/
+		if (setid_policy_lookup((kid_t)cred->uid, INVALID_ID, UID) == SIDPOL_DEFAULT)
+			return 0;
+		/*
+		 * Reject use of CAP_SETUID for functionality other than calling
+		 * set*uid() (e.g. setting up userns uid mappings).
+		 */
+		pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
+			__kuid_val(cred->uid));
+		return -EPERM;
+		break;
+	case CAP_SETGID:
+		/*
+		* If no policy applies to this task, allow the use of CAP_SETGID for
+		* other purposes.
+		*/
+		if (setid_policy_lookup((kid_t)cred->gid, INVALID_ID, GID) == SIDPOL_DEFAULT)
+			return 0;
+		/*
+		 * Reject use of CAP_SETUID for functionality other than calling
+		 * set*gid() (e.g. setting up userns gid mappings).
+		 */
+		pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n",
+			__kuid_val(cred->uid));
+		return -EPERM;
+		break;
+	default:
+		/* Error, the only capabilities were checking for is CAP_SETUID/GID */
 		return 0;
-
-	/*
-	 * Reject use of CAP_SETUID for functionality other than calling
-	 * set*uid() (e.g. setting up userns uid mappings).
-	 */
-	pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
-		__kuid_val(cred->uid));
-	return -EPERM;
+		break;
+	}
+	return 0;
 }
 
 /*
  * Check whether a caller with old credentials @old is allowed to switch to
- * credentials that contain @new_uid.
+ * credentials that contain @new_id.
  */
-static bool uid_permitted_for_cred(const struct cred *old, kuid_t new_uid)
+static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type)
 {
 	bool permitted;
 
-	/* If our old creds already had this UID in it, it's fine. */
-	if (uid_eq(new_uid, old->uid) || uid_eq(new_uid, old->euid) ||
-	    uid_eq(new_uid, old->suid))
-		return true;
+	/* If our old creds already had this ID in it, it's fine. */
+	if (new_type == UID) {
+		if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) ||
+			uid_eq(new_id.uid, old->suid))
+			return true;
+	} else if (new_type == GID){
+		if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) ||
+			gid_eq(new_id.gid, old->sgid))
+			return true;
+	} else /* Error, new_type is an invalid type */
+		return false;
 
 	/*
 	 * Transitions to new UIDs require a check against the policy of the old
 	 * RUID.
 	 */
 	permitted =
-	    setuid_policy_lookup(old->uid, new_uid) != SIDPOL_CONSTRAINED;
+	    setid_policy_lookup((kid_t)old->uid, new_id, new_type) != SIDPOL_CONSTRAINED;
+
 	if (!permitted) {
-		pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
-			__kuid_val(old->uid), __kuid_val(old->euid),
-			__kuid_val(old->suid), __kuid_val(new_uid));
+		if (new_type == UID) {
+			pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
+				__kuid_val(old->uid), __kuid_val(old->euid),
+				__kuid_val(old->suid), __kuid_val(new_id.uid));
+		} else if (new_type == GID) {
+			pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n",
+				__kgid_val(old->gid), __kgid_val(old->egid),
+				__kgid_val(old->sgid), __kgid_val(new_id.gid));
+		} else /* Error, new_type is an invalid type */
+			return false;
 	}
 	return permitted;
 }
@@ -131,18 +202,42 @@ static int safesetid_task_fix_setuid(struct cred *new,
 {
 
 	/* Do nothing if there are no setuid restrictions for our old RUID. */
-	if (setuid_policy_lookup(old->uid, INVALID_UID) == SIDPOL_DEFAULT)
+	if (setid_policy_lookup((kid_t)old->uid, INVALID_ID, UID) == SIDPOL_DEFAULT)
+		return 0;
+
+	if (id_permitted_for_cred(old, (kid_t)new->uid, UID) &&
+	    id_permitted_for_cred(old, (kid_t)new->euid, UID) &&
+	    id_permitted_for_cred(old, (kid_t)new->suid, UID) &&
+	    id_permitted_for_cred(old, (kid_t)new->fsuid, UID))
+		return 0;
+
+	/*
+	 * Kill this process to avoid potential security vulnerabilities
+	 * that could arise from a missing allowlist entry preventing a
+	 * privileged process from dropping to a lesser-privileged one.
+	 */
+	force_sig(SIGKILL);
+	return -EACCES;
+}
+
+static int safesetid_task_fix_setgid(struct cred *new,
+				     const struct cred *old,
+				     int flags)
+{
+
+	/* Do nothing if there are no setgid restrictions for our old RGID. */
+	if (setid_policy_lookup((kid_t)old->gid, INVALID_ID, GID) == SIDPOL_DEFAULT)
 		return 0;
 
-	if (uid_permitted_for_cred(old, new->uid) &&
-	    uid_permitted_for_cred(old, new->euid) &&
-	    uid_permitted_for_cred(old, new->suid) &&
-	    uid_permitted_for_cred(old, new->fsuid))
+	if (id_permitted_for_cred(old, (kid_t)new->gid, GID) &&
+	    id_permitted_for_cred(old, (kid_t)new->egid, GID) &&
+	    id_permitted_for_cred(old, (kid_t)new->sgid, GID) &&
+	    id_permitted_for_cred(old, (kid_t)new->fsgid, GID))
 		return 0;
 
 	/*
 	 * Kill this process to avoid potential security vulnerabilities
-	 * that could arise from a missing whitelist entry preventing a
+	 * that could arise from a missing allowlist entry preventing a
 	 * privileged process from dropping to a lesser-privileged one.
 	 */
 	force_sig(SIGKILL);
@@ -151,6 +246,7 @@ static int safesetid_task_fix_setuid(struct cred *new,
 
 static struct security_hook_list safesetid_security_hooks[] = {
 	LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
+	LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
 	LSM_HOOK_INIT(capable, safesetid_security_capable)
 };