linux-kernel - Re: [PATCH v2 4/5] mm, page_poison: remove CONFIG_PAGE_POISONING_NO

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Date:   Wed, 11 Nov 2020 16:43:43 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mateusz Nosek <mateusznosek0@...il.com>,
        Laura Abbott <labbott@...nel.org>
Subject: Re: [PATCH v2 4/5] mm, page_poison: remove
 CONFIG_PAGE_POISONING_NO_SANITY

On 03.11.20 16:22, Vlastimil Babka wrote:
> CONFIG_PAGE_POISONING_NO_SANITY skips the check on page alloc whether the
> poison pattern was corrupted, suggesting a use-after-free. The motivation to
> introduce it in commit 8823b1dbc05f ("mm/page_poison.c: enable PAGE_POISONING
> as a separate option") was to simply sanitize freed pages, optimally together
> with CONFIG_PAGE_POISONING_ZERO.
> 
> These days we have an init_on_free=1 boot option, which makes this use case of
> page poisoning redundant. For sanitizing, writing zeroes is sufficient, there
> is pretty much no benefit from writing the 0xAA poison pattern to freed pages,
> without checking it back on alloc. Thus, remove this option and suggest
> init_on_free instead in the main config's help.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>   drivers/virtio/virtio_balloon.c |  4 +---
>   mm/Kconfig.debug                | 15 ++++-----------
>   mm/page_poison.c                |  3 ---
>   3 files changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index e53faed6ba93..8985fc2cea86 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -1114,9 +1114,7 @@ static int virtballoon_validate(struct virtio_device *vdev)
>   	 * page reporting as it could potentially change the contents
>   	 * of our free pages.
>   	 */
> -	if (!want_init_on_free() &&
> -	    (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY) ||
> -	     !page_poisoning_enabled_static()))
> +	if (!want_init_on_free() && !page_poisoning_enabled_static())
>   		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
>   	else if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON))
>   		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index c57786ad5be9..14e29fe5bfa6 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -74,18 +74,11 @@ config PAGE_POISONING
>   	  Note that "poison" here is not the same thing as the "HWPoison"
>   	  for CONFIG_MEMORY_FAILURE. This is software poisoning only.
>   
> -	  If unsure, say N
> +	  If you are only interested in sanitization of freed pages without
> +	  checking the poison pattern on alloc, you can boot the kernel with
> +	  "init_on_free=1" instead of enabling this.
>   
> -config PAGE_POISONING_NO_SANITY
> -	depends on PAGE_POISONING
> -	bool "Only poison, don't sanity check"
> -	help
> -	   Skip the sanity checking on alloc, only fill the pages with
> -	   poison on free. This reduces some of the overhead of the
> -	   poisoning feature.
> -
> -	   If you are only interested in sanitization, say Y. Otherwise
> -	   say N.
> +	  If unsure, say N
>   
>   config PAGE_POISONING_ZERO
>   	bool "Use zero for poisoning instead of debugging value"
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index dd7aeada036f..084fc3ff4c15 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -51,9 +51,6 @@ static void check_poison_mem(unsigned char *mem, size_t bytes)
>   	unsigned char *start;
>   	unsigned char *end;
>   
> -	if (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY))
> -		return;
> -
>   	start = memchr_inv(mem, PAGE_POISON, bytes);
>   	if (!start)
>   		return;
> 

This clearly simplifies things.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb