Linux Virtualization - Mar 2015 - [Xen-devel] kasan_map_early_shadow() on Xen

On 03/03/2015 12:40 PM, Luis R. Rodriguez wrote:
> Andrey,
> 
> I believe that on Xen we should disable kasan, would like confirmation
I guess Xen guests won't work with kasan because Xen guests doesn't
setup shadow
(kasan_map_early_shadow() is not called in xen guests).

Disabling kasan for Xen in Kconfig is undesirable because that will disable
kasan
for allmodconfig and allyesconfig builds, but I don't see other option for
now.

> from someone on xen-devel though. Here's the thing though -- if true
> -- I'd like to do it *properly*, where *properly* means addressing a
> bit of architecture. A simple Kconfig slap seems rather reactive. I'd
> like to address a way to properly ensure we don't run into this and
> other similar issues in the future. The CR4 shadow issue was another
> recent example issue, also introduced via v4.0 [0]. We can't keep
> doing this reactively.
> 
> Let's go down the rabbit hole for a bit. HAVE_ARCH_KASAN will be
> selected on x86 when:
> 
> if X86_64 && SPARSEMEM_VMEMMAP
> 
> Now Xen should not have SPARSEMEM_VMEMMAP but PVOPs' goal is to enable
> distributions to be able to have a single binary kernels and let the
> rest be figured out, so we can't just disable SPARSEMEM_VMEMMAP for
> Xen alone, we want to build Xen.. or part of Xen and perhaps keep
> SPARSEMEM_VMEMMAP, and only later figure things out.
> 
> How do we do this cleanly and avoid future reactive measures? If the
> answer is not upon us, I'd like to at least highlight the issue so
> that in case we do come up with something its no surprise PVOPs is
> falling short for that single binary pipe dream right now.
> 
> [0] https://lkml.org/lkml/2015/2/23/328
> 
>  Luis
>

On 03/03/2015 05:16 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 03, 2015 at 04:15:06PM +0300, Andrey Ryabinin wrote:
>> On 03/03/2015 12:40 PM, Luis R. Rodriguez wrote:
>>> Andrey,
>>>
>>> I believe that on Xen we should disable kasan, would like
confirmation
>>
>> I guess Xen guests won't work with kasan because Xen guests
doesn't setup shadow
>> (kasan_map_early_shadow() is not called in xen guests).
>>
>> Disabling kasan for Xen in Kconfig is undesirable because that will
disable kasan
>> for allmodconfig and allyesconfig builds, but I don't see other
option for now.
> 
> Was there an bug reported for this? It would be good to CC the maintainers
> of Xen on that sort of thing.
> 
There was no report for this. I just looked at Xen code because of this
Luis's mail
and I don't see how it could work with kasan.
Fixing this might be not trivial. We need to setup shadow memory before any
kasan instrumented
function will run.
This is like with -fstack-protector (you need to setup gdt before calling any
stack protected function).