linux-kernel - Re: [PATCH v2 1/3] lockdown: Switch implementation to using bitmap

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Message-ID: <68928324c4a32_cff991002@dwillia2-xfh.jf.intel.com.notmuch>
Date: Tue, 5 Aug 2025 15:18:12 -0700
From: <dan.j.williams@...el.com>
To: Nikolay Borisov <nik.borisov@...e.com>,
	<linux-security-module@...r.kernel.org>
CC: <linux-kernel@...r.kernel.org>, <paul@...l-moore.com>, <serge@...lyn.com>,
	<jmorris@...ei.org>, <dan.j.williams@...el.com>, Nikolay Borisov
	<nik.borisov@...e.com>
Subject: Re: [PATCH v2 1/3] lockdown: Switch implementation to using bitmap

Nikolay Borisov wrote:
> Tracking the lockdown at the depth granularity rather than at the
> individual is somewhat inflexible as it provides an "all or nothing"
> approach. Instead there are use cases where it  will be useful to be
> able to lockdown individual features - TDX for example wants to disable
> access to just /dev/mem.
> 
> To accommodate this use case switch the internal implementation to using
> a bitmap so that individual lockdown features can be turned on. At the
> same time retain the existing semantic where
> INTEGRITY_MAX/CONFIDENTIALITY_MAX are treated as wildcards meaning "lock
> everything below me".
> 
> Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
> ---
>  security/lockdown/lockdown.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index cf83afa1d879..5014d18c423f 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -10,12 +10,13 @@
>   * 2 of the Licence, or (at your option) any later version.
>   */
>  
> +#include <linux/bitmap.h>
>  #include <linux/security.h>
>  #include <linux/export.h>
>  #include <linux/lsm_hooks.h>
>  #include <uapi/linux/lsm.h>
>  
> -static enum lockdown_reason kernel_locked_down;
> +static DECLARE_BITMAP(kernel_locked_down, LOCKDOWN_CONFIDENTIALITY_MAX);
>  
>  static const enum lockdown_reason lockdown_levels[] = {LOCKDOWN_NONE,
>  						 LOCKDOWN_INTEGRITY_MAX,
> @@ -26,10 +27,15 @@ static const enum lockdown_reason lockdown_levels[] = {LOCKDOWN_NONE,
>   */
>  static int lock_kernel_down(const char *where, enum lockdown_reason level)
>  {
> -	if (kernel_locked_down >= level)
> -		return -EPERM;

So now attempts to reduce security return "success" where previously
they get permission denied?

I think that is an unforunate side effect of trying to have this one
function handle both levels and individual features.

> -	kernel_locked_down = level;
> +	if (level > LOCKDOWN_CONFIDENTIALITY_MAX)
> +		return -EINVAL;
> +
> +	if (level == LOCKDOWN_INTEGRITY_MAX || level == LOCKDOWN_CONFIDENTIALITY_MAX)
> +		bitmap_set(kernel_locked_down, 1, level);
> +	else
> +		bitmap_set(kernel_locked_down, level, 1);
> +

The individual case probably deserves its own interface given all
current kernels expect levels and the future callers probably want to
skip the pr_notice() below given only piecemeal features are being
disabled.

You might even special case just LOCKDOWN_DEV_MEM for now as the only
once that can be indepdently set by an internal caller.