[PATCH v2] LSM: add SafeSetID module that gates setid calls

[mortonm@chromium.org]

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From: Micah Morton <mortonm@chromium.org>

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>
---
Added a line in setuid_policy_warning to kill processes which violate
setid whitelist policies. This prevents potential security
vulnerabilities that could arise from a missing whitelist entry
preventing a privileged process from dropping to a lesser-privileged
one.

 Documentation/admin-guide/LSM/SafeSetID.rst |  94 ++++++
 Documentation/admin-guide/LSM/index.rst     |   1 +
 arch/Kconfig                                |   5 +
 arch/arm/Kconfig                            |   1 +
 arch/arm64/Kconfig                          |   1 +
 arch/x86/Kconfig                            |   1 +
 security/Kconfig                            |   1 +
 security/Makefile                           |   2 +
 security/safesetid/Kconfig                  |  13 +
 security/safesetid/Makefile                 |   7 +
 security/safesetid/lsm.c                    | 342 ++++++++++++++++++++
 security/safesetid/lsm.h                    |  30 ++
 security/safesetid/securityfs.c             | 189 +++++++++++
 13 files changed, 687 insertions(+)
 create mode 100644 Documentation/admin-guide/LSM/SafeSetID.rst
 create mode 100644 security/safesetid/Kconfig
 create mode 100644 security/safesetid/Makefile
 create mode 100644 security/safesetid/lsm.c
 create mode 100644 security/safesetid/lsm.h
 create mode 100644 security/safesetid/securityfs.c

diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst
new file mode 100644
index 000000000000..e7d072124424
--- /dev/null
+++ b/Documentation/admin-guide/LSM/SafeSetID.rst
@@ -0,0 +1,94 @@
+=========
+SafeSetID
+=========
+SafeSetID is an LSM module that 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.
+
+
+Background
+==========
+In absence of file capabilities, processes spawned on a Linux system that need
+to switch to a different user must be spawned with CAP_SETUID privileges.
+CAP_SETUID is granted to programs running as root or those running as a non-root
+user that have been explicitly given the CAP_SETUID runtime capability. It is
+often preferable to use Linux runtime capabilities rather than file
+capabilities, since using file capabilities to run a program with elevated
+privileges opens up possible security holes since any user with access to the
+file can exec() that program to gain the elevated privileges.
+
+While it is possible to implement a tree of processes by giving full
+CAP_SET{U/G}ID capabilities, this is often at odds with the goals of running a
+tree of processes under non-root user(s) in the first place. Specifically,
+since CAP_SETUID allows changing to any user on the system, including the root
+user, it is an overpowered capability for what is needed in this scenario,
+especially since programs often only call setuid() to drop privileges to a
+lesser-privileged user -- not elevate privileges. Unfortunately, there is no
+generally feasible way in Linux to restrict the potential UIDs that a user can
+switch to through setuid() beyond allowing a switch to any user on the system.
+This SafeSetID LSM seeks to provide a solution for restricting setid
+capabilities in such a way.
+
+
+Other Approaches Considered
+===========================
+
+Solve this problem in userspace
+-------------------------------
+For candidate applications that would like to have restricted setid capabilities
+as implemented in this LSM, an alternative option would be to simply take away
+setid capabilities from the application completely and refactor the process
+spawning semantics in the application (e.g. by using a privileged helper program
+to do process spawning and UID/GID transitions). Unfortunately, there are a
+number of semantics around process spawning that would be affected by this, such
+as fork() calls where the program doesn’t immediately call exec() after the
+fork(), parent processes specifying custom environment variables or command line
+args for spawned child processes, or inheritance of file handles across a
+fork()/exec(). Because of this, as solution that uses a privileged helper in
+userspace would likely be less appealing to incorporate into existing projects
+that rely on certain process-spawning semantics in Linux.
+
+Use user namespaces
+-------------------
+Another possible approach would be to run a given process tree in its own user
+namespace and give programs in the tree setid capabilities. In this way,
+programs in the tree could change to any desired UID/GID in the context of their
+own user namespace, and only approved UIDs/GIDs could be mapped back to the
+initial system user namespace, affectively preventing privilege escalation.
+Unfortunately, it is not generally feasible to use user namespaces in isolation,
+without pairing them with other namespace types, which is not always an option.
+Linux checks for capabilities based off of the user namespace that “owns” some
+entity. For example, Linux has the notion that network namespaces are owned by
+the user namespace in which they were created. A consequence of this is that
+capability checks for access to a given network namespace are done by checking
+whether a task has the given capability in the context of the user namespace
+that owns the network namespace -- not necessarily the user namespace under
+which the given task runs. Therefore spawning a process in a new user namespace
+effectively prevents it from accessing the network namespace owned by the
+initial namespace. This is a deal-breaker for any application that expects to
+retain the CAP_NET_ADMIN capability for the purpose of adjusting network
+configurations. Using user namespaces in isolation causes problems regarding
+other system interactions, including use of pid namespaces and device creation.
+
+Use an existing LSM
+-------------------
+None of the other in-tree LSMs have the capability to gate setid transitions, or
+even employ the security_task_fix_setuid hook at all. SELinux says of that hook:
+"Since setuid only affects the current process, and since the SELinux controls
+are not based on the Linux identity attributes, SELinux does not need to control
+this operation."
+
+
+Directions for use
+==================
+This LSM hooks the setid syscalls to make sure transitions are allowed if an
+applicable restriction policy is in place. Policies are configured through
+securityfs by writing to the safesetid/add_whitelist_policy and
+safesetid/flush_whitelist_policies files at the location where securityfs is
+mounted. The format for adding a policy is '<UID>:<UID>', using literal
+numbers, such as '123:456'. To flush the policies, any write to the file is
+sufficient. Again, configuring a policy for a UID will prevent that UID from
+obtaining auxiliary setid privileges, such as allowing a user to set up user
+namespace UID mappings.
diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst
index c980dfe9abf1..a0c387649e12 100644
--- a/Documentation/admin-guide/LSM/index.rst
+++ b/Documentation/admin-guide/LSM/index.rst
@@ -39,3 +39,4 @@ the one "major" module (e.g. SELinux) if there is one configured.
    Smack
    tomoyo
    Yama
+   SafeSetID
diff --git a/arch/Kconfig b/arch/Kconfig
index 1aa59063f1fd..c87070807ba2 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -381,6 +381,11 @@ config ARCH_WANT_OLD_COMPAT_IPC
 	select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
 	bool
 
+config HAVE_SAFESETID
+	bool
+	help
+	  This option enables the SafeSetID LSM.
+
 config HAVE_ARCH_SECCOMP_FILTER
 	bool
 	help
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 843edfd000be..35b1a772c971 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -92,6 +92,7 @@ config ARM
 	select HAVE_RCU_TABLE_FREE if (SMP && ARM_LPAE)
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
+	select HAVE_SAFESETID
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UID16
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 42c090cf0292..2c6f5ec3a55e 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -127,6 +127,7 @@ config ARM64
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RCU_TABLE_FREE
+	select HAVE_SAFESETID
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_KPROBES
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 887d3a7bb646..a6527d6c0426 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -27,6 +27,7 @@ config X86_64
 	select ARCH_SUPPORTS_INT128
 	select ARCH_USE_CMPXCHG_LOCKREF
 	select HAVE_ARCH_SOFT_DIRTY
+	select HAVE_SAFESETID
 	select MODULES_USE_ELF_RELA
 	select NEED_DMA_MAP_STATE
 	select SWIOTLB
diff --git a/security/Kconfig b/security/Kconfig
index c4302067a3ad..7d9008ad5903 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -237,6 +237,7 @@ source security/tomoyo/Kconfig
 source security/apparmor/Kconfig
 source security/loadpin/Kconfig
 source security/yama/Kconfig
+source security/safesetid/Kconfig
 
 source security/integrity/Kconfig
 
diff --git a/security/Makefile b/security/Makefile
index 4d2d3782ddef..88209d827832 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -10,6 +10,7 @@ subdir-$(CONFIG_SECURITY_TOMOYO)        += tomoyo
 subdir-$(CONFIG_SECURITY_APPARMOR)	+= apparmor
 subdir-$(CONFIG_SECURITY_YAMA)		+= yama
 subdir-$(CONFIG_SECURITY_LOADPIN)	+= loadpin
+subdir-$(CONFIG_SECURITY_SAFESETID)	+= safesetid
 
 # always enable default capabilities
 obj-y					+= commoncap.o
@@ -25,6 +26,7 @@ obj-$(CONFIG_SECURITY_TOMOYO)		+= tomoyo/
 obj-$(CONFIG_SECURITY_APPARMOR)		+= apparmor/
 obj-$(CONFIG_SECURITY_YAMA)		+= yama/
 obj-$(CONFIG_SECURITY_LOADPIN)		+= loadpin/
+obj-$(CONFIG_SECURITY_SAFESETID)	+= safesetid/
 obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
 
 # Object integrity file lists
diff --git a/security/safesetid/Kconfig b/security/safesetid/Kconfig
new file mode 100644
index 000000000000..4ff82c7ed273
--- /dev/null
+++ b/security/safesetid/Kconfig
@@ -0,0 +1,13 @@
+config SECURITY_SAFESETID
+        bool "Gate setid transitions to limit CAP_SET{U/G}ID capabilities"
+        depends on HAVE_SAFESETID
+        default n
+        help
+          SafeSetID is an LSM module that 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.
+
+          If you are unsure how to answer this question, answer N.
diff --git a/security/safesetid/Makefile b/security/safesetid/Makefile
new file mode 100644
index 000000000000..6b0660321164
--- /dev/null
+++ b/security/safesetid/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for the safesetid LSM.
+#
+
+obj-$(CONFIG_SECURITY_SAFESETID) := safesetid.o
+safesetid-y := lsm.o securityfs.o
diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
new file mode 100644
index 000000000000..32040f8db7ce
--- /dev/null
+++ b/security/safesetid/lsm.c
@@ -0,0 +1,342 @@
+// 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>
+
+#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;
+}
+
+/*
+ * TODO: Figuring out whether the current syscall number (saved on the kernel
+ * stack) is one of the set*uid syscalls is an operation that requires checking
+ * the number against arch-specific constants as seen below. The need for this
+ * LSM to know about arch-specific syscall stuff is not ideal. Is it better to
+ * implement an arch-specific function that gets called from this file and
+ * update arch/Kconfig to mention that the HAVE_SAFESETID symbol should only be
+ * selected for architectures that implement the function? Any other ideas?
+ */
+static bool setuid_syscall(int num)
+{
+#ifdef CONFIG_X86_64
+#ifdef CONFIG_COMPAT
+	if (!(num == __NR_setreuid ||
+	      num == __NR_setuid ||
+	      num == __NR_setresuid ||
+	      num == __NR_setfsuid ||
+	      num == __NR_ia32_setreuid32 ||
+	      num == __NR_ia32_setuid ||
+	      num == __NR_ia32_setresuid ||
+	      num == __NR_ia32_setresuid ||
+	      num == __NR_ia32_setuid32))
+		return false;
+#else
+	if (!(num == __NR_setreuid ||
+	      num == __NR_setuid ||
+	      num == __NR_setresuid ||
+	      num == __NR_setfsuid))
+		return false;
+#endif /* CONFIG_COMPAT */
+#elif defined CONFIG_ARM64
+#ifdef CONFIG_COMPAT
+	if (!(num == __NR_setuid ||
+	      num == __NR_setreuid ||
+	      num == __NR_setfsuid ||
+	      num == __NR_setresuid ||
+	      num == __NR_setreuid32 ||
+	      num == __NR_setresuid32 ||
+	      num == __NR_setuid32 ||
+	      num == __NR_setfsuid32 ||
+	      num == __NR_compat_setuid ||
+	      num == __NR_compat_setreuid ||
+	      num == __NR_compat_setfsuid ||
+	      num == __NR_compat_setresuid ||
+	      num == __NR_compat_setreuid32 ||
+	      num == __NR_compat_setresuid32 ||
+	      num == __NR_compat_setuid32 ||
+	      num == __NR_compat_setfsuid32))
+		return false;
+#else
+	if (!(num == __NR_setuid ||
+	      num == __NR_setreuid ||
+	      num == __NR_setfsuid ||
+	      num == __NR_setresuid))
+		return false;
+#endif /* CONFIG_COMPAT */
+#elif defined CONFIG_ARM
+	if (!(num == __NR_setreuid32 ||
+	      num == __NR_setuid32 ||
+	      num == __NR_setresuid32 ||
+	      num == __NR_setfsuid32))
+		return false;
+#else
+	BUILD_BUG();
+#endif
+	return true;
+}
+
+static int safesetid_security_capable(const struct cred *cred,
+				      struct user_namespace *ns,
+				      int cap,
+				      int audit)
+{
+	/* The current->mm check will fail if this is a kernel thread. */
+	if (cap == CAP_SETUID &&
+	    current->mm &&
+	    check_setuid_policy_hashtable_key(cred->uid)) {
+		/*
+		 * syscall_get_nr can theoretically return 0 or -1, but that
+		 * would signify that the syscall is being aborted due to a
+		 * signal, so we don't need to check for this case here.
+		 */
+		if (!(setuid_syscall(syscall_get_nr(current,
+						    current_pt_regs()))))
+			/*
+			 * 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).
+			 */
+			return -1;
+	}
+	return 0;
+}
+
+static void setuid_policy_warning(kuid_t parent, kuid_t child)
+{
+	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.
+         */
+        do_exit(SIGKILL);
+}
+
+static int check_uid_transition(kuid_t parent, kuid_t child)
+{
+	if (check_setuid_policy_hashtable_key_value(parent, child))
+		return 0;
+	setuid_policy_warning(parent, child);
+	return -1;
+}
+
+/*
+ * 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;
+	}
+	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");
+
+	return 0;
+}
+security_initcall(safesetid_security_init);
diff --git a/security/safesetid/lsm.h b/security/safesetid/lsm.h
new file mode 100644
index 000000000000..bf78af9bf314
--- /dev/null
+++ b/security/safesetid/lsm.h
@@ -0,0 +1,30 @@
+/* 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.
+ *
+ */
+#ifndef _SAFESETID_H
+#define _SAFESETID_H
+
+#include <linux/types.h>
+
+/* Function type. */
+enum safesetid_whitelist_file_write_type {
+	SAFESETID_WHITELIST_ADD, /* Add whitelist policy. */
+	SAFESETID_WHITELIST_FLUSH, /* Flush whitelist policies. */
+};
+
+/* Add entry to safesetid whitelist to allow 'parent' to setid to 'child'. */
+int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child);
+
+void flush_safesetid_whitelist_entries(void);
+
+#endif /* _SAFESETID_H */
diff --git a/security/safesetid/securityfs.c b/security/safesetid/securityfs.c
new file mode 100644
index 000000000000..ff5fcf2c1b37
--- /dev/null
+++ b/security/safesetid/securityfs.c
@@ -0,0 +1,189 @@
+// 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.
+ *
+ */
+#include <linux/security.h>
+#include <linux/cred.h>
+
+#include "lsm.h"
+
+static struct dentry *safesetid_policy_dir;
+
+struct safesetid_file_entry {
+	const char *name;
+	enum safesetid_whitelist_file_write_type type;
+	struct dentry *dentry;
+};
+
+static struct safesetid_file_entry safesetid_files[] = {
+	{.name = "add_whitelist_policy",
+	 .type = SAFESETID_WHITELIST_ADD},
+	{.name = "flush_whitelist_policies",
+	 .type = SAFESETID_WHITELIST_FLUSH},
+};
+
+/*
+ * In the case the input buffer contains one or more invalid UIDs, the kuid_t
+ * variables pointed to by 'parent' and 'child' will get updated but this
+ * function will return an error.
+ */
+static int parse_safesetid_whitelist_policy(const char __user *buf,
+					    size_t len,
+					    kuid_t *parent,
+					    kuid_t *child)
+{
+	char *kern_buf;
+	char *parent_buf;
+	char *child_buf;
+	const char separator[] = ":";
+	int ret;
+	size_t first_substring_length;
+	long parsed_parent;
+	long parsed_child;
+
+	/* Duplicate string from user memory and NULL-terminate */
+	kern_buf = memdup_user_nul(buf, len);
+	if (IS_ERR(kern_buf))
+		return PTR_ERR(kern_buf);
+
+	/*
+	 * Format of |buf| string should be <UID>:<UID>.
+	 * Find location of ":" in kern_buf (copied from |buf|).
+	 */
+	first_substring_length = strcspn(kern_buf, separator);
+	if (first_substring_length == 0 || first_substring_length == len) {
+		ret = -EINVAL;
+		goto free_kern;
+	}
+
+	parent_buf = kmemdup_nul(kern_buf, first_substring_length, GFP_KERNEL);
+	if (!parent_buf) {
+		ret = -ENOMEM;
+		goto free_kern;
+	}
+
+	ret = kstrtol(parent_buf, 0, &parsed_parent);
+	if (ret)
+		goto free_both;
+
+	child_buf = kern_buf + first_substring_length + 1;
+	ret = kstrtol(child_buf, 0, &parsed_child);
+	if (ret)
+		goto free_both;
+
+	*parent = make_kuid(current_user_ns(), parsed_parent);
+	if (!uid_valid(*parent)) {
+		ret = -EINVAL;
+		goto free_both;
+	}
+
+	*child = make_kuid(current_user_ns(), parsed_child);
+	if (!uid_valid(*child)) {
+		ret = -EINVAL;
+		goto free_both;
+	}
+
+free_both:
+	kfree(parent_buf);
+free_kern:
+	kfree(kern_buf);
+	return ret;
+}
+
+static ssize_t safesetid_file_write(struct file *file,
+				    const char __user *buf,
+				    size_t len,
+				    loff_t *ppos)
+{
+	struct safesetid_file_entry *file_entry =
+		file->f_inode->i_private;
+	kuid_t parent;
+	kuid_t child;
+	int ret;
+
+	if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (*ppos != 0)
+		return -EINVAL;
+
+	if (file_entry->type == SAFESETID_WHITELIST_FLUSH) {
+		flush_safesetid_whitelist_entries();
+		return len;
+	}
+
+	/*
+	 * If we get to here, must be the case that file_entry->type equals
+	 * SAFESETID_WHITELIST_ADD
+	 */
+	ret = parse_safesetid_whitelist_policy(buf, len, &parent,
+							 &child);
+	if (ret)
+		return ret;
+
+	ret = add_safesetid_whitelist_entry(parent, child);
+	if (ret)
+		return ret;
+
+	/* Return len on success so caller won't keep trying to write */
+	return len;
+}
+
+static const struct file_operations safesetid_file_fops = {
+	.write = safesetid_file_write,
+};
+
+static void safesetid_shutdown_securityfs(void)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(safesetid_files); ++i) {
+		struct safesetid_file_entry *entry =
+			&safesetid_files[i];
+		securityfs_remove(entry->dentry);
+		entry->dentry = NULL;
+	}
+
+	securityfs_remove(safesetid_policy_dir);
+	safesetid_policy_dir = NULL;
+}
+
+static int __init safesetid_init_securityfs(void)
+{
+	int i;
+	int ret;
+
+	safesetid_policy_dir = securityfs_create_dir("safesetid", NULL);
+	if (!safesetid_policy_dir) {
+		ret = PTR_ERR(safesetid_policy_dir);
+		goto error;
+	}
+
+	for (i = 0; i < ARRAY_SIZE(safesetid_files); ++i) {
+		struct safesetid_file_entry *entry =
+			&safesetid_files[i];
+		entry->dentry = securityfs_create_file(
+			entry->name, 0200, safesetid_policy_dir,
+			entry, &safesetid_file_fops);
+		if (IS_ERR(entry->dentry)) {
+			ret = PTR_ERR(entry->dentry);
+			goto error;
+		}
+	}
+
+	return 0;
+
+error:
+	safesetid_shutdown_securityfs();
+	return ret;
+}
+fs_initcall(safesetid_init_securityfs);
-- 
2.19.1.930.g4563a0d9d0-goog