[PATCH] LSM: add SafeSetID module that gates setid calls
Stephen Smalley
sds at tycho.nsa.gov
Fri Nov 2 18:07:01 UTC 2018
On 10/31/18 11:28 AM, mortonm at chromium.org wrote:
> From: Micah Morton <mortonm at 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 at 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.
If it were me, I'd modify the callers of ns_capable(..., CAP_SETUID) in
some manner to let you distinguish rather than trying to test the
current syscall within the capable hook. Modify the set*id system calls
to use a variant interface that passes flags or something; there is
already precedent for the _noaudit case but it isn't general. More
generally, extending ns_capable() and friends to take a variety of
additional inputs would be useful, e.g. to allow one to pass down the
inode for CAP_DAC_OVERRIDE/READ_SEARCH checks so that one could
authorize it for specific files rather than all or nothing. This is
already partly done via capable_wrt_inode_uidgid() but the inode isn't
propagated down to ns_capable() and thus cannot be passed down to the
security hook currently.
>
> 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 at 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 at 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 at 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);
>
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