LSM: add SafeSetID module that gates setid calls

SafeSetID gates the setid family of syscalls to restrict UID/GID
transitions from a given UID/GID to only those approved by a
system-wide whitelist. These restrictions also prohibit the given
UIDs/GIDs from obtaining auxiliary privileges associated with
CAP_SET{U/G}ID, such as allowing a user to set up user namespace UID
mappings. For now, only gating the set*uid family of syscalls is
supported, with support for set*gid coming in a future patch set.

Signed-off-by: Micah Morton <mortonm@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>

1 files changed, 277 insertions, 0 deletions

diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
new file mode 100644
index 000000000000..3a2c75ac810c
--- /dev/null
+++ b/security/safesetid/lsm.c
@@ -0,0 +1,277 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * SafeSetID Linux Security Module
+ *
+ * Author: Micah Morton <mortonm@chromium.org>
+ *
+ * Copyright (C) 2018 The Chromium OS Authors.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ *
+ */
+
+#define pr_fmt(fmt) "SafeSetID: " fmt
+
+#include <asm/syscall.h>
+#include <linux/hashtable.h>
+#include <linux/lsm_hooks.h>
+#include <linux/module.h>
+#include <linux/ptrace.h>
+#include <linux/sched/task_stack.h>
+#include <linux/security.h>
+
+/* Flag indicating whether initialization completed */
+int safesetid_initialized;
+
+#define NUM_BITS 8 /* 128 buckets in hash table */
+
+static DEFINE_HASHTABLE(safesetid_whitelist_hashtable, NUM_BITS);
+
+/*
+ * Hash table entry to store safesetid policy signifying that 'parent' user
+ * can setid to 'child' user.
+ */
+struct entry {
+	struct hlist_node next;
+	struct hlist_node dlist; /* for deletion cleanup */
+	uint64_t parent_kuid;
+	uint64_t child_kuid;
+};
+
+static DEFINE_SPINLOCK(safesetid_whitelist_hashtable_spinlock);
+
+static bool check_setuid_policy_hashtable_key(kuid_t parent)
+{
+	struct entry *entry;
+
+	rcu_read_lock();
+	hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
+				   entry, next, __kuid_val(parent)) {
+		if (entry->parent_kuid == __kuid_val(parent)) {
+			rcu_read_unlock();
+			return true;
+		}
+	}
+	rcu_read_unlock();
+
+	return false;
+}
+
+static bool check_setuid_policy_hashtable_key_value(kuid_t parent,
+						    kuid_t child)
+{
+	struct entry *entry;
+
+	rcu_read_lock();
+	hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
+				   entry, next, __kuid_val(parent)) {
+		if (entry->parent_kuid == __kuid_val(parent) &&
+		    entry->child_kuid == __kuid_val(child)) {
+			rcu_read_unlock();
+			return true;
+		}
+	}
+	rcu_read_unlock();
+
+	return false;
+}
+
+static int safesetid_security_capable(const struct cred *cred,
+				      struct user_namespace *ns,
+				      int cap,
+				      unsigned int opts)
+{
+	if (cap == CAP_SETUID &&
+	    check_setuid_policy_hashtable_key(cred->uid)) {
+		if (!(opts & CAP_OPT_INSETID)) {
+			/*
+			 * Deny if we're not in a set*uid() syscall to avoid
+			 * giving powers gated by CAP_SETUID that are related
+			 * to functionality other than calling set*uid() (e.g.
+			 * allowing user to set up userns uid mappings).
+			 */
+			pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions",
+				__kuid_val(cred->uid));
+			return -1;
+		}
+	}
+	return 0;
+}
+
+static int check_uid_transition(kuid_t parent, kuid_t child)
+{
+	if (check_setuid_policy_hashtable_key_value(parent, child))
+		return 0;
+	pr_warn("UID transition (%d -> %d) blocked",
+		__kuid_val(parent),
+		__kuid_val(child));
+	/*
+	 * Kill this process to avoid potential security vulnerabilities
+	 * that could arise from a missing whitelist entry preventing a
+	 * privileged process from dropping to a lesser-privileged one.
+	 */
+	force_sig(SIGKILL, current);
+	return -EACCES;
+}
+
+/*
+ * Check whether there is either an exception for user under old cred struct to
+ * set*uid to user under new cred struct, or the UID transition is allowed (by
+ * Linux set*uid rules) even without CAP_SETUID.
+ */
+static int safesetid_task_fix_setuid(struct cred *new,
+				     const struct cred *old,
+				     int flags)
+{
+
+	/* Do nothing if there are no setuid restrictions for this UID. */
+	if (!check_setuid_policy_hashtable_key(old->uid))
+		return 0;
+
+	switch (flags) {
+	case LSM_SETID_RE:
+		/*
+		 * Users for which setuid restrictions exist can only set the
+		 * real UID to the real UID or the effective UID, unless an
+		 * explicit whitelist policy allows the transition.
+		 */
+		if (!uid_eq(old->uid, new->uid) &&
+			!uid_eq(old->euid, new->uid)) {
+			return check_uid_transition(old->uid, new->uid);
+		}
+		/*
+		 * Users for which setuid restrictions exist can only set the
+		 * effective UID to the real UID, the effective UID, or the
+		 * saved set-UID, unless an explicit whitelist policy allows
+		 * the transition.
+		 */
+		if (!uid_eq(old->uid, new->euid) &&
+			!uid_eq(old->euid, new->euid) &&
+			!uid_eq(old->suid, new->euid)) {
+			return check_uid_transition(old->euid, new->euid);
+		}
+		break;
+	case LSM_SETID_ID:
+		/*
+		 * Users for which setuid restrictions exist cannot change the
+		 * real UID or saved set-UID unless an explicit whitelist
+		 * policy allows the transition.
+		 */
+		if (!uid_eq(old->uid, new->uid))
+			return check_uid_transition(old->uid, new->uid);
+		if (!uid_eq(old->suid, new->suid))
+			return check_uid_transition(old->suid, new->suid);
+		break;
+	case LSM_SETID_RES:
+		/*
+		 * Users for which setuid restrictions exist cannot change the
+		 * real UID, effective UID, or saved set-UID to anything but
+		 * one of: the current real UID, the current effective UID or
+		 * the current saved set-user-ID unless an explicit whitelist
+		 * policy allows the transition.
+		 */
+		if (!uid_eq(new->uid, old->uid) &&
+			!uid_eq(new->uid, old->euid) &&
+			!uid_eq(new->uid, old->suid)) {
+			return check_uid_transition(old->uid, new->uid);
+		}
+		if (!uid_eq(new->euid, old->uid) &&
+			!uid_eq(new->euid, old->euid) &&
+			!uid_eq(new->euid, old->suid)) {
+			return check_uid_transition(old->euid, new->euid);
+		}
+		if (!uid_eq(new->suid, old->uid) &&
+			!uid_eq(new->suid, old->euid) &&
+			!uid_eq(new->suid, old->suid)) {
+			return check_uid_transition(old->suid, new->suid);
+		}
+		break;
+	case LSM_SETID_FS:
+		/*
+		 * Users for which setuid restrictions exist cannot change the
+		 * filesystem UID to anything but one of: the current real UID,
+		 * the current effective UID or the current saved set-UID
+		 * unless an explicit whitelist policy allows the transition.
+		 */
+		if (!uid_eq(new->fsuid, old->uid)  &&
+			!uid_eq(new->fsuid, old->euid)  &&
+			!uid_eq(new->fsuid, old->suid) &&
+			!uid_eq(new->fsuid, old->fsuid)) {
+			return check_uid_transition(old->fsuid, new->fsuid);
+		}
+		break;
+	default:
+		pr_warn("Unknown setid state %d\n", flags);
+		force_sig(SIGKILL, current);
+		return -EINVAL;
+	}
+	return 0;
+}
+
+int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child)
+{
+	struct entry *new;
+
+	/* Return if entry already exists */
+	if (check_setuid_policy_hashtable_key_value(parent, child))
+		return 0;
+
+	new = kzalloc(sizeof(struct entry), GFP_KERNEL);
+	if (!new)
+		return -ENOMEM;
+	new->parent_kuid = __kuid_val(parent);
+	new->child_kuid = __kuid_val(child);
+	spin_lock(&safesetid_whitelist_hashtable_spinlock);
+	hash_add_rcu(safesetid_whitelist_hashtable,
+		     &new->next,
+		     __kuid_val(parent));
+	spin_unlock(&safesetid_whitelist_hashtable_spinlock);
+	return 0;
+}
+
+void flush_safesetid_whitelist_entries(void)
+{
+	struct entry *entry;
+	struct hlist_node *hlist_node;
+	unsigned int bkt_loop_cursor;
+	HLIST_HEAD(free_list);
+
+	/*
+	 * Could probably use hash_for_each_rcu here instead, but this should
+	 * be fine as well.
+	 */
+	spin_lock(&safesetid_whitelist_hashtable_spinlock);
+	hash_for_each_safe(safesetid_whitelist_hashtable, bkt_loop_cursor,
+			   hlist_node, entry, next) {
+		hash_del_rcu(&entry->next);
+		hlist_add_head(&entry->dlist, &free_list);
+	}
+	spin_unlock(&safesetid_whitelist_hashtable_spinlock);
+	synchronize_rcu();
+	hlist_for_each_entry_safe(entry, hlist_node, &free_list, dlist) {
+		hlist_del(&entry->dlist);
+		kfree(entry);
+	}
+}
+
+static struct security_hook_list safesetid_security_hooks[] = {
+	LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
+	LSM_HOOK_INIT(capable, safesetid_security_capable)
+};
+
+static int __init safesetid_security_init(void)
+{
+	security_add_hooks(safesetid_security_hooks,
+			   ARRAY_SIZE(safesetid_security_hooks), "safesetid");
+
+	/* Report that SafeSetID successfully initialized */
+	safesetid_initialized = 1;
+
+	return 0;
+}
+
+DEFINE_LSM(safesetid_security_init) = {
+	.init = safesetid_security_init,
+};