Re: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception

From: Andrey Ryabinin <aryabinin@virtuozzo.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Julien Thierry <julien.thierry@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>,
	valentin.schneider@arm.com, Brian Gerst <brgerst@gmail.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception
Date: Thu, 28 Feb 2019 19:01:49 +0300	[thread overview]
Message-ID: <a8b9e933-d7e5-e34f-b395-0be48347f9cc@virtuozzo.com> (raw)
In-Reply-To: <CACT4Y+aEx5-8g-YDoK37gtntCxy8_8sSWdG7FeU+fc=B3C3axA@mail.gmail.com>

On 2/28/19 6:52 PM, Dmitry Vyukov wrote:
> On Thu, Feb 28, 2019 at 4:46 PM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> On Thu, Feb 28, 2019 at 04:22:04PM +0100, Dmitry Vyukov wrote:
>>> On Thu, Feb 28, 2019 at 4:05 PM Peter Zijlstra <peterz@infradead.org> wrote:
>>>>
>>>> Because __asan_{load,store}{N,1,2,4,8,16}_noabort() get called from
>>>> UACCESS context, and kasan_report() is most definitely _NOT_ safe to
>>>> be called from there, move it into an exception much like BUG/WARN.
>>>>
>>>> *compile tested only*
>>>
>>>
>>> Please test it by booting KASAN kernel and then loading module
>>> produced by CONFIG_TEST_KASAN=y. There are too many subtle aspects to
>>> rely on "compile tested only", reviewers can't catch all of them
>>> either.
>>
>> Sure, I'll do that. I just wanted to share the rest of the patches.
>>
>> A quick test shows it dies _REAAAAAAAALY_ early, as in:
>>
>> "Booting the kernel."
>>
>> is the first and very last thing it says... I wonder how I did that :-)
> 
> One thing is that during early boot kasan_report is called multiple
> times, but these are false positives related to the fact that we don't
> have a proper shadow yet (setup later). So during early boot we set
> kasan_disable=1 (or some global or per-task flag), and then
> kasan_report checks it and returns.
> Once we setup proper shadow, the flag is reset and from now on
> kasan_report actually reports bug.
> 

Yup, see report_enabled() function.