linux-kernel - Re: [PATCH 5/5] x86/traps: avoid KMSAN bugs originating from handle

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Date:   Thu, 3 Nov 2022 12:18:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [PATCH 5/5] x86/traps: avoid KMSAN bugs originating from
 handle_bug()

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@...radead.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.