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Date:   Thu, 3 Nov 2022 12:18:41 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexander Potapenko <glider@google.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org
Subject: Re: [PATCH 5/5] x86/traps: avoid KMSAN bugs originating from
 handle_bug()
Message-ID: <Y2OjkZTOoDa+mQrS@hirez.programming.kicks-ass.net>
References: <20221102110611.1085175-1-glider@google.com>
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 <Y2JnsG5QZoO4iazD@hirez.programming.kicks-ass.net>
 <CAG_fn=X_92f1w9=Xuj6eYcF9Za3rbpeGe+P79fJ8mkgEQO1XYA@mail.gmail.com>
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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.