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Message-ID: <80bdc6c9-004b-800f-ffd0-4b5ebf8cdeba@schaufler-ca.com>
Date: Wed, 24 May 2017 08:36:49 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: David Howells <dhowells@...hat.com>, ard.biesheuvel@...aro.org
Cc: matthew.garrett@...ula.com, linux-security-module@...r.kernel.org,
linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] Add the ability to lock down access to the running
kernel image
On 5/24/2017 7:45 AM, David Howells wrote:
> Provide a single call to allow kernel code to determine whether the system
> should be locked down, thereby disallowing various accesses that might
> allow the running kernel image to be changed including the loading of
> modules that aren't validly signed with a key we recognise, fiddling with
> MSR registers and disallowing hibernation,
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> Acked-by: James Morris <james.l.morris@...cle.com>
> ---
>
> include/linux/kernel.h | 9 +++++++++
> include/linux/security.h | 11 +++++++++++
> security/Kconfig | 15 +++++++++++++++
> security/Makefile | 3 +++
> security/lock_down.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 84 insertions(+)
> create mode 100644 security/lock_down.c
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 13bc08aba704..282a1684d6e8 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -276,6 +276,15 @@ extern int oops_may_print(void);
> void do_exit(long error_code) __noreturn;
> void complete_and_exit(struct completion *, long) __noreturn;
>
> +#ifdef CONFIG_LOCK_DOWN_KERNEL
> +extern bool kernel_is_locked_down(void);
> +#else
> +static inline bool kernel_is_locked_down(void)
Should this be a bool or an int? I can imagine that
someone is going to want various different degrees
of lock down for kernels. As an int you could return
a bitmap indicating which features were locked. This
would allow additional things to be locked down
without changing the interface.
> +{
> + return false;
> +}
> +#endif
> +
> /* Internal, do not use. */
> int __must_check _kstrtoul(const char *s, unsigned int base, unsigned long *res);
> int __must_check _kstrtol(const char *s, unsigned int base, long *res);
> diff --git a/include/linux/security.h b/include/linux/security.h
> index af675b576645..8db2d886aa90 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -1698,5 +1698,16 @@ static inline void free_secdata(void *secdata)
> { }
> #endif /* CONFIG_SECURITY */
>
> +#ifdef CONFIG_LOCK_DOWN_KERNEL
> +extern void __init lock_kernel_down(void);
> +#ifdef CONFIG_ALLOW_LOCKDOWN_LIFT
> +extern void lift_kernel_lockdown(void);
> +#endif
> +#else
> +static inline void lock_kernel_down(void)
> +{
> +}
> +#endif
> +
> #endif /* ! __LINUX_SECURITY_H */
>
> diff --git a/security/Kconfig b/security/Kconfig
> index 93027fdf47d1..4baac4aab277 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -189,6 +189,21 @@ config STATIC_USERMODEHELPER_PATH
> If you wish for all usermode helper programs to be disabled,
> specify an empty string here (i.e. "").
>
> +config LOCK_DOWN_KERNEL
> + bool "Allow the kernel to be 'locked down'"
> + help
> + Allow the kernel to be locked down under certain circumstances, for
> + instance if UEFI secure boot is enabled. Locking down the kernel
> + turns off various features that might otherwise allow access to the
> + kernel image (eg. setting MSR registers).
> +
> +config ALLOW_LOCKDOWN_LIFT
> + bool
> + help
> + Allow the lockdown on a kernel to be lifted, thereby restoring the
> + ability of userspace to access the kernel image (eg. by SysRq+x under
> + x86).
> +
> source security/selinux/Kconfig
> source security/smack/Kconfig
> source security/tomoyo/Kconfig
> diff --git a/security/Makefile b/security/Makefile
> index f2d71cdb8e19..8c4a43e3d4e0 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -29,3 +29,6 @@ obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
> # Object integrity file lists
> subdir-$(CONFIG_INTEGRITY) += integrity
> obj-$(CONFIG_INTEGRITY) += integrity/
> +
> +# Allow the kernel to be locked down
> +obj-$(CONFIG_LOCK_DOWN_KERNEL) += lock_down.o
> diff --git a/security/lock_down.c b/security/lock_down.c
> new file mode 100644
> index 000000000000..dd98422fbda7
> --- /dev/null
> +++ b/security/lock_down.c
> @@ -0,0 +1,46 @@
> +/* Lock down the kernel
> + *
> + * Copyright (C) 2016 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@...hat.com)
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public Licence
> + * as published by the Free Software Foundation; either version
> + * 2 of the Licence, or (at your option) any later version.
> + */
> +
> +#include <linux/security.h>
> +#include <linux/export.h>
> +
> +#ifdef CONFIG_ALLOW_LOCKDOWN_LIFT
> +static __read_mostly bool kernel_locked_down;
> +#else
> +static __ro_after_init bool kernel_locked_down;
> +#endif
> +
> +/*
> + * Put the kernel into lock-down mode.
> + */
> +void __init lock_kernel_down(void)
> +{
> + kernel_locked_down = true;
> +}
> +
> +/*
> + * Take the kernel out of lockdown mode.
> + */
> +#ifdef CONFIG_ALLOW_LOCKDOWN_LIFT
> +void lift_kernel_lockdown(void)
> +{
> + kernel_locked_down = false;
> +}
> +#endif
> +
> +/**
> + * kernel_is_locked_down - Find out if the kernel is locked down
> + */
> +bool kernel_is_locked_down(void)
> +{
> + return kernel_locked_down;
> +}
> +EXPORT_SYMBOL(kernel_is_locked_down);
>
> --
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