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Date: Thu, 27 Feb 2014 11:02:07 -0800
From: Kees Cook <keescook@...omium.org>
To: Matthew Garrett <matthew.garrett@...ula.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Greg KH <gregkh@...uxfoundation.org>,
"H. Peter Anvin" <hpa@...or.com>,
"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
James Morris <jmorris@...ei.org>,
linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 01/12] Add support for indicating that the booted kernel
is externally trusted
On Wed, Feb 26, 2014 at 12:11 PM, Matthew Garrett
<matthew.garrett@...ula.com> wrote:
> 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@...ula.com>
> ---
> 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; }
Should set_trusted_kernel return -ENOTSUPP when the config is off?
> +#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@...ula.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 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
>
Thanks for reposting this. I'm looking forward to adding
"trusted_kernel" to the Chrome OS kernel cmdline. :)
Please consider the series:
Acked-by: Kees Cook <keescook@...omium.org>
-Kees
--
Kees Cook
Chrome OS Security
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