Re: [RFC] selftests/x86: Add test_vsyscall

[Andy Lutomirski]

On Fri, Jan 5, 2018 at 10:30 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Fri, Jan 05, 2018 at 10:01:23AM -0800, Andy Lutomirski wrote:
>> Yes.  There are very clever tools like 'pin' that instrument a binary
>> by decoding all the instructions it executes and generating an
>> instrumented copy.  If that binary calls into the vDSO, the vDSO gets
>> decoded and instrumented (which works fine).  If the binary calls into
>> the vsyscall page, it still needs to work.  So the vsyscall page
>> contains machine code that actually works (even if it's NX) to support
>> these tools.  The authors and users of the tools yelled loudly in an
>> earlier version of the vsyscall emulation code that didn't support
>> this use case.
>
> It rings a bell...
>
>> The root cause here is that 4.4 is KAISER, not KPTI.  The
>> kaiser_set_shadow_pgd() function is a steaming pile of shit, and this
>> is a known bug in it.
>
> Tell me about it.
>
> We found out last night it breaks EFI too, see:
>
> https://lkml.kernel.org/r/alpine.LRH.2.00.1801051909160.27010@xxxxxxxxxxxxx
>
> To put it mildly, this new PTI et al crap will bring us a lot of fun in
> the coming year. I tell ya, a year from now we'll be dealing with the
> fallout from this.
>
>> I have zero desire to hack up some stupid special case in there. For
>> the modern KPTI kernels, I rewrote that function entirely to be much
>> simpler and much more correct.
>>
>> It should be straightforward to kludge something up, though, but I'm
>> not volunteering.
>
> Yeah, I think adding _PAGE_RW into the mix should fix it but I need to
> give it a try first.
>

Not _PAGE_RW.  Probably _PAGE_USER somewhere in the hierarchy.