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Date: Fri, 4 Mar 2016 16:07:51 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Laura Abbott <labbott@...oraproject.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Vlastimil Babka
<vbabka@...e.cz>, Michal Hocko <mhocko@...e.com>, Kees Cook
<keescook@...omium.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCHv4 2/2] mm/page_poisoning.c: Allow for zero poisoning
On Fri, 4 Mar 2016 15:50:48 -0800 Laura Abbott <labbott@...oraproject.org> wrote:
>
> By default, page poisoning uses a poison value (0xaa) on free. If this
> is changed to 0, the page is not only sanitized but zeroing on alloc
> with __GFP_ZERO can be skipped as well. The tradeoff is that detecting
> corruption from the poisoning is harder to detect. This feature also
> cannot be used with hibernation since pages are not guaranteed to be
> zeroed after hibernation.
>
> Credit to Grsecurity/PaX team for inspiring this work
>
> --- a/kernel/power/hibernate.c
> +++ b/kernel/power/hibernate.c
> @@ -1158,6 +1158,22 @@ static int __init kaslr_nohibernate_setup(char *str)
> return nohibernate_setup(str);
> }
>
> +static int __init page_poison_nohibernate_setup(char *str)
> +{
> +#ifdef CONFIG_PAGE_POISONING_ZERO
> + /*
> + * The zeroing option for page poison skips the checks on alloc.
> + * since hibernation doesn't save free pages there's no way to
> + * guarantee the pages will still be zeroed.
> + */
> + if (!strcmp(str, "on")) {
> + pr_info("Disabling hibernation due to page poisoning\n");
> + return nohibernate_setup(str);
> + }
> +#endif
> + return 1;
> +}
It seems a bit unfriendly to silently accept the boot option but not
actually do anything with it. Perhaps a `#else pr_info("sorry")' is
needed.
But I bet we made the same mistake in 1000 other places.
What happens if page_poison_nohibernate_setup() simply doesn't exist
when CONFIG_PAGE_POISONING_ZERO=n? It looks like
kernel/params.c:parse_args() says "Unknown parameter".
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