Re: [PATCH 5/5] x86/traps: avoid KMSAN bugs originating from handle_bug()

Peter Zijlstra
2022-11-03T11:19:03Z

On Wed, Nov 02, 2022 at 02:37:19PM +0100, Alexander Potapenko wrote:
> On Wed, Nov 2, 2022 at 1:51 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Nov 02, 2022 at 12:06:11PM +0100, Alexander Potapenko wrote:
> > > There is a case in exc_invalid_op handler that is executed outside the
> > > irqentry_enter()/irqentry_exit() region when an UD2 instruction is used
> > > to encode a call to __warn().
> > >
> > > In that case the `struct pt_regs` passed to the interrupt handler is
> > > never unpoisoned by KMSAN (this is normally done in irqentry_enter()),
> > > which leads to false positives inside handle_bug().
> > >
> > > Use kmsan_unpoison_entry_regs() to explicitly unpoison those registers
> > > before using them.
> >
> > As does poke_int3_handler(); does that need fixing up too? OTOH look
> > *very very* carefully at the contraints there.
>
> Fortunately poke_int3_handler() is a noinstr function, so KMSAN
> doesn't add any checks to it.
> It also does not pass regs to other instrumented functions, at least
> for now, so we're good.

Ah indeed; because it is fully noinstr, nothing will trigger the lack of
annotation.