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* [PATCH] LSM: add SafeSetID module that gates setid calls
@ 2018-10-31 15:28 mortonm
  2018-10-31 21:02 ` Serge E. Hallyn
  2018-11-02 18:07 ` [PATCH] " Stephen Smalley
  0 siblings, 2 replies; 88+ messages in thread
From: mortonm @ 2018-10-31 15:28 UTC (permalink / raw)
  To: jmorris, serge, keescook, linux-security-module; +Cc: Micah Morton

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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>
---

NOTE: See the TODO above setuid_syscall() in lsm.c for an aspect of this
code that likely needs improvement before being an acceptable approach.
I'm specifically interested to see if there are better ideas for how
this could be done.

 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                    | 334 ++++++++++++++++++++
 security/safesetid/lsm.h                    |  30 ++
 security/safesetid/securityfs.c             | 189 +++++++++++
 13 files changed, 679 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..e30ff06d8e07
--- /dev/null
+++ b/security/safesetid/lsm.c
@@ -0,0 +1,334 @@
+// 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));
+}
+
+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.
+	 */
+	hash_for_each_safe(safesetid_whitelist_hashtable, bkt_loop_cursor,
+			   hlist_node, entry, next) {
+		spin_lock(&safesetid_whitelist_hashtable_spinlock);
+		hash_del_rcu(&entry->next);
+		spin_unlock(&safesetid_whitelist_hashtable_spinlock);
+		hlist_add_head(&entry->dlist, &free_list);
+	}
+	synchronize_rcu();
+	hlist_for_each_entry_safe(entry, hlist_node, &free_list, 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);
-- 
2.19.1.568.g152ad8e336-goog


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