[PATCH 01/12] Add BSD-style securelevel support

Matthew Garrett
2013-09-09T15:52:25Z

Provide a coarse-grained runtime configuration option for restricting
userspace's ability to modify the running kernel.

Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
---
Documentation/security/securelevel.txt | 23 +++++++
include/linux/security.h | 8 +++
security/Kconfig | 8 +++
security/Makefile | 1 +
security/securelevel.c | 116 +++++++++++++++++++++++++++++++++
5 files changed, 156 insertions(+)
create mode 100644 Documentation/security/securelevel.txt
create mode 100644 security/securelevel.c

diff --git a/Documentation/security/securelevel.txt b/Documentation/security/securelevel.txt
new file mode 100644
index 0000000..a1355a0
--- /dev/null
+++ b/Documentation/security/securelevel.txt
@@ -0,0 +1,23 @@
+Linux securelevel interface
+---------------------------
+
+The Linux securelevel interface (inspired by the BSD securelevel interface)
+is a runtime mechanism for configuring coarse-grained kernel-level security
+restrictions. It provides a runtime configuration variable at
+/sys/kernel/security/securelevel which can be written to by root. The
+following values are supported:
+
+-1: Permanently insecure mode. This level is equivalent to level 0, but once
+ set cannot be changed.
+
+0: Insecure mode (default). This level imposes no additional kernel
+ restrictions.
+
+1: Secure mode. If set, userspace will be unable to perform direct access
+ to PCI devices, port IO access, access system memory directly via
+ /dev/mem and /dev/kmem, perform kexec_load(), use the userspace
+ software suspend mechanism, insert new ACPI code at runtime via the
+ custom_method interface or modify CPU MSRs (on x86). Certain drivers
+ may also limit additional interfaces.
+
+Once the securelevel value is increased, it may not be decreased.
diff --git a/include/linux/security.h b/include/linux/security.h
index 9d37e2b..e8b69e0 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -3081,6 +3081,14 @@ static inline void security_audit_rule_free(void *lsmrule)
#endif /* CONFIG_SECURITY */
#endif /* CONFIG_AUDIT */

+#ifdef CONFIG_SECURITY_SECURELEVEL
+extern int get_securelevel(void);
+extern int set_securelevel(int new_securelevel);
+#else
+static inline int get_securelevel(void) { return 0; }
+static inline int set_securelevel(int new_securelevel) { return 0; }
+#endif /* CONFIG_SECURELEVEL */
+
#ifdef CONFIG_SECURITYFS

extern struct dentry *securityfs_create_file(const char *name, umode_t mode,
diff --git a/security/Kconfig b/security/Kconfig
index e9c6ac7..3605d24 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -70,6 +70,14 @@ config SECURITY_PATH
implement pathname based access controls.
If you are unsure how to answer this question, answer N.

+config SECURITY_SECURELEVEL
+ bool "Securelevel kernel restriction interface"
+ depends on SECURITY
+ help
+ This enables support for adding a set of additional kernel security
+ restrictions at runtime. See Documentation/security/securelevel.txt
+ for further information.
+
config INTEL_TXT
bool "Enable Intel(R) Trusted Execution Technology (Intel(R) TXT)"
depends on HAVE_INTEL_TXT
diff --git a/security/Makefile b/security/Makefile
index c26c81e..d6c98ab 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -16,6 +16,7 @@ obj-$(CONFIG_MMU) += min_addr.o
# Object file lists
obj-$(CONFIG_SECURITY) += security.o capability.o
obj-$(CONFIG_SECURITYFS) += inode.o
+obj-$(CONFIG_SECURITY_SECURELEVEL) += securelevel.o
# Must precede capability.o in order to stack properly.
obj-$(CONFIG_SECURITY_SELINUX) += selinux/built-in.o
obj-$(CONFIG_SECURITY_SMACK) += smack/built-in.o
diff --git a/security/securelevel.c b/security/securelevel.c
new file mode 100644
index 0000000..7128430
--- /dev/null
+++ b/security/securelevel.c
@@ -0,0 +1,116 @@
+/*
+ * securelevel.c - support for generic kernel lockdown
+ *
+ * Copyright Nebula, Inc <matthew.garrett@nebula.com>
+ *
+ * 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/fs.h>
+#include <linux/init.h>
+#include <linux/security.h>
+#include <linux/uaccess.h>
+
+static int securelevel;
+
+static DEFINE_SPINLOCK(securelevel_lock);
+
+#define MAX_SECURELEVEL 1
+
+int get_securelevel(void)
+{
+ return securelevel;
+}
+EXPORT_SYMBOL(get_securelevel);
+
+int set_securelevel(int new_securelevel)
+{
+ int ret = 0;
+
+ spin_lock(&securelevel_lock);
+
+ if ((securelevel == -1) || (new_securelevel < securelevel) ||
+ (new_securelevel > MAX_SECURELEVEL)) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ securelevel = new_securelevel;
+out:
+ spin_unlock(&securelevel_lock);
+ return ret;
+}
+EXPORT_SYMBOL(set_securelevel);
+
+static ssize_t securelevel_read(struct file *filp, char __user *buf,
+ size_t count, loff_t *ppos)
+{
+ char tmpbuf[12];
+ ssize_t length;
+
+ length = scnprintf(tmpbuf, sizeof(tmpbuf), "%d", securelevel);
+ return simple_read_from_buffer(buf, count, ppos, tmpbuf, length);
+}
+
+static ssize_t securelevel_write(struct file *file, const char __user *buf,
+ size_t count, loff_t *ppos)
+{
+ char *page = NULL;
+ ssize_t length;
+ int new_securelevel;
+
+ length = -ENOMEM;
+ if (count >= PAGE_SIZE)
+ goto out;
+
+ length = -EINVAL;
+ if (*ppos != 0)
+ goto out;
+
+ length = -ENOMEM;
+ page = (char *)get_zeroed_page(GFP_KERNEL);
+ if (!page)
+ goto out;
+
+ length = -EFAULT;
+ if (copy_from_user(page, buf, count))
+ goto out;
+
+ length = -EINVAL;
+ if (sscanf(page, "%d", &new_securelevel) != 1)
+ goto out;
+
+ length = set_securelevel(new_securelevel);
+ if (length)
+ goto out;
+
+ length = count;
+out:
+ free_page((unsigned long) page);
+ return length;
+}
+
+static const struct file_operations securelevel_fops = {
+ .read = securelevel_read,
+ .write = securelevel_write,
+ .llseek = generic_file_llseek,
+};
+
+static __init int setup_securelevel(void)
+{
+ struct dentry *securelevel_file;
+
+ securelevel_file = securityfs_create_file("securelevel",
+ S_IWUSR | S_IRUGO,
+ NULL, NULL,
+ &securelevel_fops);
+
+ if (IS_ERR(securelevel_file))
+ return PTR_ERR(securelevel_file);
+
+ return 0;
+}
+late_initcall(setup_securelevel);
--
1.8.3.1