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Subject: Re: [PATCH] x86/entry/64: wipe KASAN stack shadow in rewind_stack_do_exit()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <daba9c5b-b5c6-beaa-ca96-81cf6af760b9@virtuozzo.com>
Date:   Tue, 28 Aug 2018 12:57:36 -0700
Cc:     Jann Horn <jannh@google.com>, Andy Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>, kasan-dev@googlegroups.com,
        Alexander Potapenko <glider@google.com>,
        Kees Cook <keescook@chromium.org>
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Message-Id: <CA9DE24E-D1DC-4000-A9E5-0905C6433F04@amacapital.net>
References: <20180824235826.62741-1-jannh@google.com> <0897d173-6a30-09df-f16a-76322384fe0d@virtuozzo.com> <CAG48ez1zYqx8-1We3JkDBmYREksUMoUe7VBeAOv2JH98gVWcKw@mail.gmail.com> <daba9c5b-b5c6-beaa-ca96-81cf6af760b9@virtuozzo.com>
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Aug 28, 2018, at 4:33 AM, Andrey Ryabinin <aryabinin@virtuozzo.com> wro=
te:
>=20
>=20
>=20
> On 08/28/2018 01:38 PM, Jann Horn wrote:
>=20
>>>=20
>>>=20
>>> Why this has to be done in the rewind_stack_do_exit()?
>>> Are there any problems with calling the kasan_unpoison_task_stack(curren=
t) from oops_end(), before the rewind_stack_do_exit()?
>>=20
>> Ooh, good point! I didn't see that KASAN instrumentation is disabled
>> for dumpstack.c.=20
>=20
> It doesn't really matter. This would work with instrumented oops_end() as w=
ell.
> kasan_unpoison_task_stack() will unpoison everything including oops_end's s=
tack.
> It would be also ok if kasan_unpoison_task_stack() instrumented, or callin=
g 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()=E2=80=99s c=
aller to be uninstrumented. Which it is.