[PATCH 01/12] Add support for indicating that the booted kernel is externally trusted

[Matthew Garrett]

Provide a boolean runtime configuration option for restricting userspace's
ability to modify the running kernel. This can be used when some external
validation of the kernel's state has been performed.

Signed-off-by: Matthew Garrett <matthew.garrett@xxxxxxxxxx>
---
 Documentation/kernel-parameters.txt       |   6 ++
 Documentation/security/trusted_kernel.txt |  35 ++++++++++
 include/linux/security.h                  |  10 +++
 security/Kconfig                          |   9 +++
 security/Makefile                         |   1 +
 security/trusted_kernel.c                 | 111 ++++++++++++++++++++++++++++++
 6 files changed, 172 insertions(+)
 create mode 100644 Documentation/security/trusted_kernel.txt
 create mode 100644 security/trusted_kernel.c

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index bfcb1a6..2d2aa946 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3647,6 +3647,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			with respect to transparent hugepages.
 			See Documentation/vm/transhuge.txt for more details.
 
+	trusted_kernel	Indicate that the booted kernel has been verified to
+			be trustworthy and that userspace should be forbidden
+			from modifying it at runtime.
+			See Documentation/security/trusted_kernel.txt for more
+			details.
+
 	tsc=		Disable clocksource stability checks for TSC.
 			Format: <string>
 			[x86] reliable: mark tsc clocksource as reliable, this
diff --git a/Documentation/security/trusted_kernel.txt b/Documentation/security/trusted_kernel.txt
new file mode 100644
index 0000000..95300bb
--- /dev/null
+++ b/Documentation/security/trusted_kernel.txt
@@ -0,0 +1,35 @@
+Linux trusted kernel support
+----------------------------
+
+Various mechanisms exist to ensure that a booted kernel is trusted by the
+user or some external party (UEFI Secure Boot, Intel TXT, embedded platform
+bootloaders). If userspace is able to modify the running kernel then this
+trust can be subverted.
+
+The trusted kernel support modifies certain kernel interfaces such that
+userspace is restricted from performing acts that would allow it to inject
+untrusted code into the kernel. 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.
+
+The trusted kernel feature may be enabled in multiple ways:
+
+1) Platform-specific code may automatically enable it when it detects that
+the system has been booted appropriately
+
+2) The user or bootloader may pass the "trusted_kernel" kernel parameter
+
+3) Userspace may write "1" to the /sys/kernel/security/trusted_kernel
+node. This must be done sufficiently early in the boot process that
+untrusted userspace has no opportunity to modify the kernel.
+
+Once enabled, trusted kernel support may not be disabled without rebooting
+the system.
+
+Note that this is a mechanism for the kernel to determine whether or not
+it is externally trusted. Untrusted userspace can enable this option even
+if the kernel is not trusted, and therefore userspace should not use this
+value as an indication of whether or not the kernel is trustworthy.
diff --git a/include/linux/security.h b/include/linux/security.h
index a1b7dbd..87bbbc0 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -3174,6 +3174,16 @@ static inline void security_audit_rule_free(void *lsmrule)
 #endif /* CONFIG_SECURITY */
 #endif /* CONFIG_AUDIT */
 
+#ifdef CONFIG_SECURITY_TRUSTED_KERNEL
+extern bool get_trusted_kernel(void);
+extern int set_trusted_kernel(bool new_trusted_kernel);
+#else
+static inline bool get_trusted_kernel(void) { return 0; }
+static inline int set_trusted_kernel(bool new_trusted_kernel) {
+	return -ENOTSUPP;
+}
+#endif /* CONFIG_SECURITY_TRUSTED_KERNEL */
+
 #ifdef CONFIG_SECURITYFS
 
 extern struct dentry *securityfs_create_file(const char *name, umode_t mode,
diff --git a/security/Kconfig b/security/Kconfig
index beb86b5..c0462c9 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -70,6 +70,15 @@ config SECURITY_PATH
 	  implement pathname based access controls.
 	  If you are unsure how to answer this question, answer N.
 
+config SECURITY_TRUSTED_KERNEL
+        bool "Support for indicating that the kernel is trusted"
+	depends on SECURITY
+	help
+	  This enables support for adding a set of additional kernel security
+	  restrictions at runtime.
+	  See Documentation/security/trusted_kernel.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 05f1c93..0d5d689 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_TRUSTED_KERNEL)	+= trusted_kernel.o
 obj-$(CONFIG_SECURITY_SELINUX)		+= selinux/
 obj-$(CONFIG_SECURITY_SMACK)		+= smack/
 obj-$(CONFIG_AUDIT)			+= lsm_audit.o
diff --git a/security/trusted_kernel.c b/security/trusted_kernel.c
new file mode 100644
index 0000000..2808113
--- /dev/null
+++ b/security/trusted_kernel.c
@@ -0,0 +1,111 @@
+/*
+ *  trusted_kernel.c - support for generic kernel lockdown
+ *
+ *  Copyright Nebula, Inc <matthew.garrett@xxxxxxxxxx>
+ *
+ *  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 bool trusted_kernel;
+
+bool get_trusted_kernel(void)
+{
+	return trusted_kernel;
+}
+EXPORT_SYMBOL(get_trusted_kernel);
+
+int set_trusted_kernel(bool new_trusted_kernel)
+{
+	if (trusted_kernel == true && new_trusted_kernel == false)
+		return -EINVAL;
+
+	trusted_kernel = new_trusted_kernel;
+
+	return 0;
+}
+EXPORT_SYMBOL(set_trusted_kernel);
+
+static ssize_t trusted_kernel_read(struct file *filp, char __user *buf,
+				   size_t count, loff_t *ppos)
+{
+	char tmpbuf[2];
+	ssize_t length;
+
+	length = scnprintf(tmpbuf, sizeof(tmpbuf), "%d", trusted_kernel);
+	return simple_read_from_buffer(buf, count, ppos, tmpbuf, length);
+}
+
+static ssize_t trusted_kernel_write(struct file *file, const char __user *buf,
+				    size_t count, loff_t *ppos)
+{
+	char *page = NULL;
+	ssize_t length;
+	int new_trusted_kernel;
+
+	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_trusted_kernel) != 1)
+		goto out;
+
+	length = set_trusted_kernel(!!new_trusted_kernel);
+	if (length)
+		goto out;
+
+	length = count;
+out:
+	free_page((unsigned long) page);
+	return length;
+}
+
+static const struct file_operations trusted_kernel_fops = {
+	.read	= trusted_kernel_read,
+	.write	= trusted_kernel_write,
+	.llseek	= generic_file_llseek,
+};
+
+static __init int setup_trusted_kernel(void)
+{
+	struct dentry *trusted_kernel_file;
+
+	trusted_kernel_file = securityfs_create_file("trusted_kernel",
+						     S_IWUSR | S_IRUGO,
+						     NULL, NULL,
+						     &trusted_kernel_fops);
+
+	if (IS_ERR(trusted_kernel_file))
+		return PTR_ERR(trusted_kernel_file);
+
+	return 0;
+}
+late_initcall(setup_trusted_kernel);
+
+static int __init enable_trusted_kernel(char *__str)
+{
+	trusted_kernel = true;
+	return 1;
+}
+__setup("trusted_kernel", enable_trusted_kernel);
-- 
2.1.0

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