linux-kernel - Re: [PATCH] x86/entry/64: wipe KASAN stack shadow in rewind_stack_do

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Date:   Tue, 28 Aug 2018 12:57:36 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86/entry/64: wipe KASAN stack shadow in rewind_stack_do_exit()



> On Aug 28, 2018, at 4:33 AM, Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
> 
> 
> 
> On 08/28/2018 01:38 PM, Jann Horn wrote:
> 
>>> 
>>> 
>>> Why this has to be done in the rewind_stack_do_exit()?
>>> Are there any problems with calling the kasan_unpoison_task_stack(current) from oops_end(), before the rewind_stack_do_exit()?
>> 
>> Ooh, good point! I didn't see that KASAN instrumentation is disabled
>> for dumpstack.c. 
> 
> It doesn't really matter. This would work with instrumented oops_end() as well.
> kasan_unpoison_task_stack() will unpoison everything including oops_end's stack.
> It would be also ok if kasan_unpoison_task_stack() instrumented, or calling any number of instrumented functions
> in between kasan_unpoison_task_stack() and rewind_stack_do_exit(). As long as we return from these functions before
> the rewind_stack_do_exit(), the stack will be unpoisoned on return.

I think that, if all this is in C, we need rewind_stack_do_exit()’s caller to be uninstrumented. Which it is.