KASAN help, please (Re: [PATCH 00/16] Entry stuff, in decent shape now)

[Andy Lutomirski]

On Mon, Nov 20, 2017 at 9:07 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> This sets up stack switching, including for SYSCALL.  I think it's
> in decent shape.
>
> Known issues:
>  - KASAN is likely to be busted.  This could be fixed either by teaching
>    KASAN that cpu_entry_area contains valid stacks (I have no clue how
>    to go about doing this) or by rigging up the IST entry code to switch
>    RSP to point to the direct-mapped copy of the stacks before calling
>    into non-KASAN-excluded C code.
>

I tried to fix the KASAN issue, and I'm doing something wrong.  I'm
building this tree:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_stack&id=8319677bd04a1ab291ca71fe1da7aa023306e4a9

for 64 bits with KASAN on.  The relevant commit is:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_stack&id=a4bdb48c3469708b6b51e5ab90d27bf0c859000c

If I run tools/testing/selftests/single_step_syscall_32, then the
kernel goes into lala land and infinite loops.  The root cause seems
to we're hitting do_debug with RSP pointing into the fixmap,
specifically in the cpu_entry_area's exception stack, with a value of
roughly 0xffffffffff1bd108.  The KASAN instrumentation in do_debug is
then getting a page fault.  I think my KASAN setup code should be
populating the KASAN data there and, indeed, gdb seems to be able to
access the faulting address.  So I'm confused.

Help?