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 | 8 +++
security/Kconfig | 9 +++
security/Makefile | 1 +
security/trusted_kernel.c | 111 ++++++++++++++++++++++++++++++
6 files changed, 170 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 7116fda..d82ba9e 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3271,6 +3271,12 @@ 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..538d21d
--- /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 5623a7f..3415968 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -3091,6 +3091,14 @@ 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 0; }
+#endif /* CONFIG_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 a5918e0..72af305 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-$(CONFIG_SECURITY_SELINUX) += selinux/built-in.o
obj-$(CONFIG_SECURITY_SMACK) += smack/built-in.o
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);
--
1.8.5.3
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