Re: [PATCH 1/2] staging: comedi: Restrict COMEDI_DEVCONFIG when the kernel is locked down

[Ian Abbott <abbotti@mev.co.uk>]

On 30/08/2019 16:45, Ben Hutchings wrote:
> The COMEDI_DEVCONFIG ioctl can be used to configure I/O addresses and
> other hardware settings for non plug-and-play devices such as ISA
> cards.  This should be disabled to preserve the kernel's integrity
> when it is locked down.

I haven't boned up on the lockdown mechanism yet, but just FYI, this is 
only possible if the "comedi_num_legacy_minors" module parameter is 
non-zero (which it isn't by default).

> 
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> Cc: Matthew Garrett <mjg59@google.com>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Ian Abbott <abbotti@mev.co.uk>
> Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
> ---
>   drivers/staging/comedi/comedi_fops.c | 6 ++++++
>   include/linux/security.h             | 1 +
>   security/lockdown/lockdown.c         | 1 +
>   3 files changed, 8 insertions(+)
> 
> diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
> index f6d1287c7b83..fdf030e53035 100644
> --- a/drivers/staging/comedi/comedi_fops.c
> +++ b/drivers/staging/comedi/comedi_fops.c
> @@ -27,6 +27,7 @@
>   
>   #include <linux/io.h>
>   #include <linux/uaccess.h>
> +#include <linux/security.h>
>   
>   #include "comedi_internal.h"
>   
> @@ -813,11 +814,16 @@ static int do_devconfig_ioctl(struct comedi_device *dev,
>   			      struct comedi_devconfig __user *arg)
>   {
>   	struct comedi_devconfig it;
> +	int ret;
>   
>   	lockdep_assert_held(&dev->mutex);
>   	if (!capable(CAP_SYS_ADMIN))
>   		return -EPERM;
>   
> +	ret = security_locked_down(LOCKDOWN_COMEDI_DEVCONFIG);
> +	if (ret)
> +		return ret;
> +

You might consider moving that check to be done after the following 'if 
(!arg)' block, since that should be safe.  (It detaches an already 
configured device from the comedi core.)

>   	if (!arg) {
>   		if (is_device_busy(dev))
>   			return -EBUSY;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 429f9f03372b..b16365dccfc5 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -113,6 +113,7 @@ enum lockdown_reason {
>   	LOCKDOWN_ACPI_TABLES,
>   	LOCKDOWN_PCMCIA_CIS,
>   	LOCKDOWN_TIOCSSERIAL,
> +	LOCKDOWN_COMEDI_DEVCONFIG,
>   	LOCKDOWN_MODULE_PARAMETERS,
>   	LOCKDOWN_MMIOTRACE,
>   	LOCKDOWN_DEBUGFS,
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 0068cec77c05..971bb99b9051 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -28,6 +28,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>   	[LOCKDOWN_ACPI_TABLES] = "modifying ACPI tables",
>   	[LOCKDOWN_PCMCIA_CIS] = "direct PCMCIA CIS storage",
>   	[LOCKDOWN_TIOCSSERIAL] = "reconfiguration of serial port IO",
> +	[LOCKDOWN_COMEDI_DEVCONFIG] = "reconfiguration of Comedi legacy device",
>   	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
>   	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
>   	[LOCKDOWN_DEBUGFS] = "debugfs access",
> 


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