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

[Peter Zijlstra]

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 :-)

> > +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.