[PATCH v2] LSM: add SafeSetID module that gates setid calls
Kees Cook
keescook at chromium.org
Tue Jan 15 00:38:16 UTC 2019
On Fri, Jan 11, 2019 at 9:13 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>
> ---
> Changes since the last patch set: Rebase after commit
> a35ce66b801631823fc78c8a78d104f9c0976867 got applied to next-general.
> As a result of that commit, we can remove the changes in arch/ and the
> setuid_syscall function in lsm.c, since this code no longer needs to do
> arch-specific operations to see if security_capable is being called from
> a setid syscall. Instead, we add the ns_capable_insetid function and
> call it from the setid syscalls in kernel/sys.c (rather than calling the
> original ns_capable function), which allows us to signal to the
> security_capable hooks whether the hook is being called from within a
> setid syscall.
I would split this patch into two halfs: the "no op" change that
"marks" all the setid call sites in the first patch, then the LSM
itself in the second patch.
> +bool ns_capable_insetid(struct user_namespace *ns, int cap)
> +{
> + return ns_capable_common(ns, cap, CAP_OPT_INSETID);
> +}
> +EXPORT_SYMBOL(ns_capable_insetid);
Since we have the noaudit helper still, using this one seems fine to
me. I might bikeshed the name to "ns_capable_setid()". If others don't
want a new helper, then it should be fine to just change the callsites
to the direct ns_capable_common(ns, cap, CAP_OPT_INSETID).
> +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;
> +}
Much cleaner than the per-arch syscall tests. :)
> +static void setuid_policy_violation(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);
I think I asked earlier if this should be an unblockable signal raise
instead of a do_exit(). I don't remember if that got answered?
> +}
> +
> +static int check_uid_transition(kuid_t parent, kuid_t child)
> +{
> + if (check_setuid_policy_hashtable_key_value(parent, child))
> + return 0;
> + setuid_policy_violation(parent, child);
> + return -1;
> +}
Any reason not to just collapse setuid_policy_violation() into this function?
> +
> +/*
> + * 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;
> +}
> +
> +DEFINE_LSM(safesetid_security_init) = {
> + .init = 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))
Maybe CAP_MAC_ADMIN instead of (the overloaded) 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
It seems a bit silly with only two options here, but it'll change for
gids, yes? How about just building a switch() around file_entry->type
instead and avoid needing to refactor this later?
> + */
> + 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);
> --
> 2.20.1.97.g81188d93c3-goog
>
But overall, it looks good to me. :)
--
Kees Cook
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