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Date: Tue, 19 Jul 2016 22:44:58 +0200
From: Christian Borntraeger <borntraeger@...ibm.com>
To: Kees Cook <keescook@...omium.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Balbir Singh
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Subject: Re: [PATCH v3 02/11] mm: Hardened usercopy
On 07/19/2016 10:34 PM, Kees Cook wrote:
[...]
>>
>> So what about for the CONFIG text:
>>
>> An architecture should select this if the kernel mapping has a secondary
>> linear mapping of the kernel text - in other words more than one virtual
>> kernel address that points to the kernel image. This is used to verify
>> that kernel text exposures are not visible under CONFIG_HARDENED_USERCOPY.
>
> Sounds good, I've adjusted it for now.
>
>>> I wonder if I can avoid the CONFIG entirely if I just did a
>>> __va(__pa(_stext)) != _stext test... would that break anyone?
>>
>> Can this be resolved on all platforms at compile time?
>
> Well, I think it still needs a runtime check (compile-time may not be
> able to tell about kaslr, or who knows what else). I would really like
> to avoid the CONFIG if possible, though. Would this do the right thing
> on s390? This appears to work where I'm able to test it (32/64 x86,
> 32/64 arm):
>
> unsigned long textlow = (unsigned long)_stext;
> unsigned long texthigh = (unsigned long)_etext;
> unsigned long textlow_linear = (unsigned long)__va(__pa(textlow);
> unsigned long texthigh_linear = (unsigned long)__va(__pa(texthigh);
>
as we have
#define PAGE_OFFSET 0x0UL
#define __pa(x) (unsigned long)(x)
#define __va(x) (void *)(unsigned long)(x)
both should be identical on s390 as of today, so it should work fine and only
do the check once
> if (overlaps(ptr, n, textlow, texthigh))
> return "<kernel text>";
>
> /* Check against possible secondary linear mapping as well. */
> if (textlow != textlow_linear &&
> overlaps(ptr, n, textlow_linear, texthigh_linear))
> return "<linear kernel text>";
>
> return NULL;
>
>
> -Kees
>
PS: Not sure how useful and flexible this offers is but you can get some temporary
free access to an s390 on https://developer.ibm.com/linuxone/
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