Re: [PATCH V34 20/29] x86/mmiotrace: Lock down the testmmiotrace module

[Kees Cook <keescook@chromium.org>]

On Fri, Jun 21, 2019 at 05:03:49PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@redhat.com>
> 
> The testmmiotrace module shouldn't be permitted when the kernel is locked
> down as it can be used to arbitrarily read and write MMIO space. This is
> a runtime check rather than buildtime in order to allow configurations
> where the same kernel may be run in both locked down or permissive modes
> depending on local policy.
> 
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: David Howells <dhowells@redhat.com

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Signed-off-by: Matthew Garrett <mjg59@google.com>
> cc: Thomas Gleixner <tglx@linutronix.de>
> cc: Steven Rostedt <rostedt@goodmis.org>
> cc: Ingo Molnar <mingo@kernel.org>
> cc: "H. Peter Anvin" <hpa@zytor.com>
> cc: x86@kernel.org
> ---
>  arch/x86/mm/testmmiotrace.c  | 5 +++++
>  include/linux/security.h     | 1 +
>  security/lockdown/lockdown.c | 1 +
>  3 files changed, 7 insertions(+)
> 
> diff --git a/arch/x86/mm/testmmiotrace.c b/arch/x86/mm/testmmiotrace.c
> index f6ae6830b341..6b9486baa2e9 100644
> --- a/arch/x86/mm/testmmiotrace.c
> +++ b/arch/x86/mm/testmmiotrace.c
> @@ -7,6 +7,7 @@
>  #include <linux/module.h>
>  #include <linux/io.h>
>  #include <linux/mmiotrace.h>
> +#include <linux/security.h>
>  
>  static unsigned long mmio_address;
>  module_param_hw(mmio_address, ulong, iomem, 0);
> @@ -114,6 +115,10 @@ static void do_test_bulk_ioremapping(void)
>  static int __init init(void)
>  {
>  	unsigned long size = (read_far) ? (8 << 20) : (16 << 10);
> +	int ret = security_locked_down(LOCKDOWN_MMIOTRACE);
> +
> +	if (ret)
> +		return ret;
>  
>  	if (mmio_address == 0) {
>  		pr_err("you have to use the module argument mmio_address.\n");
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 88064d7f6827..c649cb91e762 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -93,6 +93,7 @@ enum lockdown_reason {
>  	LOCKDOWN_PCMCIA_CIS,
>  	LOCKDOWN_TIOCSSERIAL,
>  	LOCKDOWN_MODULE_PARAMETERS,
> +	LOCKDOWN_MMIOTRACE,
>  	LOCKDOWN_INTEGRITY_MAX,
>  	LOCKDOWN_CONFIDENTIALITY_MAX,
>  };
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index d03c4c296af7..cd86ed9f4d4b 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -29,6 +29,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>  	[LOCKDOWN_PCMCIA_CIS] = "direct PCMCIA CIS storage",
>  	[LOCKDOWN_TIOCSSERIAL] = "reconfiguration of serial port IO",
>  	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
> +	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
>  	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
>  	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
>  };
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
> 

-- 
Kees Cook