Re: [RFC] selftests/x86: Add test_vsyscall

[Borislav Petkov]

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.

Thanks for the test!

-- 
Regards/Gruss,
    Boris.

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