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

From: Dmitry Vyukov
Date: Tue Nov 21 2017 - 04:09:40 EST

On Mon, Nov 20, 2017 at 10:44 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 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:
> for 64 bits with KASAN on. The relevant commit is:
> 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.


I don't have any great insights.

You have stack instrumentation turned on, right? And the fault happens
on stack instrumentation?
Stack instrumentation is turned on with gcc7+ I think. And as the
result compiler adds redzones on stack and poisons/unpoisons shadow
for them in function prologue/epilogue.

The fact that KASAN instrumentation faults, but gdb can access it
sounds strange. KASAN instrumentation is no magic, it just does not a
normal memory load. Please check exact faulting address. KASAN can do
accesses with large offset from RSP.

Does the fault happen before/after kasan_early_init? Before that there
is a different bootstrap shadow mapped by kasan_map_early_shadow.

Does the fault happen on read access or write access? Stack
instrumentation does write into shadow, but some parts of shadow are
mapped with a single read-only page. Can gdb write to that address?

Is it possible that the stack has overflowed? I see that we increase
EXCEPTION_STACK_ORDER by order 1 under KASAN (from 4k page to 8k
pages), but it may be not enough. Normal stacks are increased from 16k
to 32k.

Last stupid question: why is it -1 here:
Say CPU_ENTRY_AREA_PAGES=1 (we need only 1 page) and NR_CPUS=1, then
the increment will be 0, which looks wrong for any case (must be at
least 1, right?).