linux-kernel - Re: [PATCH v2 1/2] mm/mm_init: Introduce a boot parameter for check

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Message-ID: <aSWDkuJEdk9cdVCK@kernel.org>
Date: Tue, 25 Nov 2025 12:23:14 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jonathan Corbet <corbet@....net>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel-team@...a.com
Subject: Re: [PATCH v2 1/2] mm/mm_init: Introduce a boot parameter for
 check_pages

On Mon, Nov 24, 2025 at 02:54:06PM -0800, Joshua Hahn wrote:
> Use-after-free and double-free bugs can be very difficult to track down.
> The kernel is good at tracking these and preventing bad pages from being
> used/created through simple checks gated behind "check_pages_enabled".
> 
> Currently, the only ways to enable this flag is by building with
> CONFIG_DEBUG_VM, or as a side effect of other checks such as
> init_on_{alloc, free}, page_poisoning, or debug_pagealloc among others.
> These solutions are powerful, but may often be too coarse in balancing
> the performance vs. safety that a user may want, particularly in
> latency-sensitive production environments.
> 
> Introduce a new boot parameter "check_pages", which enables page checking
> with no other side effects. It takes kstrbool-able inputs as an argument
> (i.e. 0/1, true/false, on/off, ...). This patch is backwards-compatible;
> setting CONFIG_DEBUG_VM still enables page checking.
> 
> Signed-off-by: Joshua Hahn <joshua.hahnjy@...il.com>
> ---
> v1 --> v2:
> - Changed check_pages from a build config into a boot config, as suggested
>   by Vlastimil.
> - Introduced the second patch, which decouples page checking from 
>   init_on_page_alloc and init_on_page_free.
> ---
> 
>  Documentation/admin-guide/kernel-parameters.txt |  8 ++++++++
>  mm/mm_init.c                                    | 11 ++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 6c42061ca20e..0ba9561440a7 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -669,6 +669,14 @@
>  			nokmem -- Disable kernel memory accounting.
>  			nobpf -- Disable BPF memory accounting.
>  
> +	check_pages=	[MM,EARLY] Enable sanity checking of pages after
> +			allocations / before freeing. This adds checks to catch
> +			double-frees, use-after-frees, and other sources of
> +			page corruption by inspecting page internals (flags,
> +			mapcount/refcount, memcg_data, etc.).
> +			Format: { "0" | "1" }
> +			Default: 0 (1 if CONFIG_DEBUG_VM is set)
> +
>  	checkreqprot=	[SELINUX] Set initial checkreqprot flag value.
>  			Format: { "0" | "1" }
>  			See security/selinux/Kconfig help text.
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index c6812b4dbb2e..01d46efc42b4 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2525,6 +2525,14 @@ early_param("init_on_free", early_init_on_free);
>  
>  DEFINE_STATIC_KEY_MAYBE(CONFIG_DEBUG_VM, check_pages_enabled);
>  
> +static bool _check_pages_enabled_early __initdata;

No need in the leading underscore.

> +
> +static int __init early_check_pages(char *buf)
> +{
> +	return kstrtobool(buf, &_check_pages_enabled_early);
> +}
> +early_param("check_pages", early_check_pages);
> +
>  /*
>   * Enable static keys related to various memory debugging and hardening options.
>   * Some override others, and depend on early params that are evaluated in the
> @@ -2591,7 +2599,8 @@ static void __init mem_debugging_and_hardening_init(void)
>  	 * of struct pages being allocated or freed. With CONFIG_DEBUG_VM it's
>  	 * enabled already.
>  	 */
> -	if (!IS_ENABLED(CONFIG_DEBUG_VM) && want_check_pages)
> +	if (!IS_ENABLED(CONFIG_DEBUG_VM) && (_check_pages_enabled_early ||
> +					     want_check_pages))

You can initialize want_check_pages to check_pages_enabled_early, would be
clearer IMO.

>  		static_branch_enable(&check_pages_enabled);
>  }
>  
> -- 
> 2.47.3

-- 
Sincerely yours,
Mike.