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

From: Dmitry Vyukov <dvyukov@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception
Date: Thu, 28 Feb 2019 16:52:30 +0100	[thread overview]
Message-ID: <CACT4Y+aEx5-8g-YDoK37gtntCxy8_8sSWdG7FeU+fc=B3C3axA@mail.gmail.com> (raw)
In-Reply-To: <20190228154551.GE32494@hirez.programming.kicks-ass.net>

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.

> > > +static __always_inline void
> > > +kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip)
> > > +{
> > > +       unsigned long rdi = addr, rsi = size, rdx = is_write, rcx = ip;
> > > +
> > > +       _BUG_FLAGS(ASM_UD2, BUGFLAG_KASAN,
> > > +                  "D" (rdi), "S" (rsi), "d" (rdx), "c" (rcx));
> >
> > Can BUG return?
>
> Yes. Also see the annotate_reachable().
>
> > This should be able to return.
> > We also have other tools coming (KMSAN/KTSAN) where distinction
> > between fast path that does nothing and slower-paths are very blurred
> > and there are dozens of them, I don't think this BUG thunk will be
> > sustainable. What does BUG do what a normal call can't do?
>
> It keeps the SMAP validation rules nice and tight. If we were to add
> (and allow) things like pushf;clac;call ponies;popf or similar things,
> it all becomes complicated real quick.
>
> How would KMSAN/KTSAN interact with SMAP ?
>
> > > +       annotate_reachable();
> > > +}
> > > @@ -228,7 +228,7 @@ void __asan_unregister_globals(struct ka
> > >  EXPORT_SYMBOL(__asan_unregister_globals);
> > >
> > >  #define DEFINE_ASAN_LOAD_STORE(size)                                   \
> > > -       void __asan_load##size(unsigned long addr)                      \
> > > +       notrace void __asan_load##size(unsigned long addr)              \
> >
> >
> > We already have:
> > CFLAGS_REMOVE_generic.o = -pg
> > Doesn't it imply notrace for all functions?
>
> Indeed so, I'll make these hunks go away.