[kernel-hardening] Re: [PATCHv4 2/2] mm/page_poisoning.c: Allow for zero poisoning

[Laura Abbott <labbott@redhat.com>]

On 03/04/2016 04:07 PM, Andrew Morton wrote:
> On Fri,  4 Mar 2016 15:50:48 -0800 Laura Abbott <labbott@fedoraproject.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".
>
>

I didn't see that behavior when I tested, even with nonsense parameters.
It looks like it might fall back to some other behavior before giving
-ENOENT?

It's also worth noting the page_poison= option is also parsed in
mm/page_poison.c to do other on/off of the poisoning feature. The
option code supported it and it seemed to match better with what the
existing hibernate code was doing with turning off options.

Thanks,
Laura