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Date:   Thu, 20 Jun 2019 18:01:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, glider@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next v2] mm/page_alloc: fix a false memory corruption
Message-ID: <201906201801.9CFC9225@keescook>
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On Thu, Jun 20, 2019 at 04:46:06PM -0400, Qian Cai wrote:
> The linux-next commit "mm: security: introduce init_on_alloc=1 and
> init_on_free=1 boot options" [1] introduced a false positive when
> init_on_free=1 and page_poison=on, due to the page_poison expects the
> pattern 0xaa when allocating pages which were overwritten by
> init_on_free=1 with 0.
> 
> Fix it by switching the order between kernel_init_free_pages() and
> kernel_poison_pages() in free_pages_prepare().

Cool; this seems like the right approach. Alexander, what do you think?

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

-Kees

> 
> [1] https://patchwork.kernel.org/patch/10999465/
> 
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
> 
> v2: After further debugging, the issue after switching order is likely a
>     separate issue as clear_page() should not cause issues with future
>     accesses.
> 
>  mm/page_alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 54dacf35d200..32bbd30c5f85 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1172,9 +1172,10 @@ static __always_inline bool free_pages_prepare(struct page *page,
>  					   PAGE_SIZE << order);
>  	}
>  	arch_free_page(page, order);
> -	kernel_poison_pages(page, 1 << order, 0);
>  	if (want_init_on_free())
>  		kernel_init_free_pages(page, 1 << order);
> +
> +	kernel_poison_pages(page, 1 << order, 0);
>  	if (debug_pagealloc_enabled())
>  		kernel_map_pages(page, 1 << order, 0);
>  
> -- 
> 1.8.3.1
> 

-- 
Kees Cook