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

[Dmitry Vyukov]

On Thu, Feb 28, 2019 at 4:46 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.