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From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Tue, 19 Jul 2016 22:14:26 +0200
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Subject: [kernel-hardening] Re: [PATCH v3 02/11] mm: Hardened usercopy
To: Kees Cook <keescook@chromium.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Balbir Singh <bsingharora@gmail.com>, Daniel Micay <danielmicay@gmail.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Rik van Riel <riel@redhat.com>, Casey Schaufler <casey@schaufler-ca.com>, PaX Team <pageexec@freemail.hu>, Brad Spengler <spender@grsecurity.net>, Russell King <linux@armlinux.org.uk>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will.deacon@arm.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Michael Ellerman <mpe@ellerman.id.au>, Tony Luck <tony.luck@intel.com>, Fenghua Yu <fenghua.yu@intel.com>, "David S. Miller" <davem@davemloft.net>, "x86@kernel.org" <x86@kernel.org>, Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>, Mathias Krause <minipli@googlemail.com>, Jan Kara <jack@suse.cz>, Vitaly Wool <vitalywool@gmail.com>, Andrea Arcangeli <aarcange@redhat.com>, Dmitry Vyukov <dvyukov@google.com>, Laura Abbott <labbott@fedoraproject.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-ia64@vger.kernel.org, "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>, sparclinux <sparclinux@vger.kernel.org>, linux-arch <linux-arch@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, "kernel-hardening@lists.openwall.com" <kernel-hardening@lists.openwall.com>
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On 07/19/2016 09:31 PM, Kees Cook wrote:
> On Tue, Jul 19, 2016 at 2:21 AM, Christian Borntraeger
> <borntraeger@de.ibm.com> wrote:
>> On 07/15/2016 11:44 PM, Kees Cook wrote:
>>> +config HAVE_ARCH_LINEAR_KERNEL_MAPPING
>>> +     bool
>>> +     help
>>> +       An architecture should select this if it has a secondary linear
>>> +       mapping of the kernel text. This is used to verify that kernel
>>> +       text exposures are not visible under CONFIG_HARDENED_USERCOPY.
>>
>> I have trouble parsing this. (What does secondary linear mapping mean?)
> 
> I likely need help clarifying this language...
> 
>> So let me give an example below
>>
>>> +
>> [...]
>>> +/* Is this address range in the kernel text area? */
>>> +static inline const char *check_kernel_text_object(const void *ptr,
>>> +                                                unsigned long n)
>>> +{
>>> +     unsigned long textlow = (unsigned long)_stext;
>>> +     unsigned long texthigh = (unsigned long)_etext;
>>> +
>>> +     if (overlaps(ptr, n, textlow, texthigh))
>>> +             return "<kernel text>";
>>> +
>>> +#ifdef HAVE_ARCH_LINEAR_KERNEL_MAPPING
>>> +     /* Check against linear mapping as well. */
>>> +     if (overlaps(ptr, n, (unsigned long)__va(__pa(textlow)),
>>> +                  (unsigned long)__va(__pa(texthigh))))
>>> +             return "<linear kernel text>";
>>> +#endif
>>> +
>>> +     return NULL;
>>> +}
>>
>> s390 has an address space for user (primary address space from 0..4TB/8PB) and a separate
>> address space (home space from 0..4TB/8PB) for the kernel. In this home space the kernel
>> mapping is virtual containing the physical memory as well as vmalloc memory (creating aliases
>> into the physical one). The kernel text is mapped from _stext to _etext in this mapping.
>> So I assume this would qualify for HAVE_ARCH_LINEAR_KERNEL_MAPPING ?
> 
> If I understand your example, yes. In the home space you have two
> addresses that reference the kernel image?

No, there is only one address that points to the kernel.
As we have no kernel ASLR yet, and the kernel mapping is 
a 1:1 mapping from 0 to memory end and the kernel is only
from _stext to _etext. The vmalloc area contains modules
and vmalloc but not a 2nd kernel mapping.

But thanks for your example, now I understood. If we have only
one address 
>>> +     if (overlaps(ptr, n, textlow, texthigh))
>>> +             return "<kernel text>";

This is just enough.

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.


> 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?