Re: [PATCH v3 5/6] kasan: Unset panic_on_warn before calling panic()

From: Dmitry Vyukov <dvyukov@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Elena Petrova <lenaptr@google.com>,
	Alexander Potapenko <glider@google.com>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-hardening@lists.openwall.com,
	syzkaller <syzkaller@googlegroups.com>
Subject: Re: [PATCH v3 5/6] kasan: Unset panic_on_warn before calling panic()
Date: Fri, 17 Jan 2020 10:54:36 +0100	[thread overview]
Message-ID: <CACT4Y+Z9o4B37-sNU2582FBv_2+evgyKVbVo-OAufLrsney=wA@mail.gmail.com> (raw)
In-Reply-To: <202001161548.9E126B774F@keescook>

On Fri, Jan 17, 2020 at 12:49 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Jan 16, 2020 at 06:23:01AM +0100, Dmitry Vyukov wrote:
> > On Thu, Jan 16, 2020 at 2:24 AM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > As done in the full WARN() handler, panic_on_warn needs to be cleared
> > > before calling panic() to avoid recursive panics.
> > >
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > ---
> > >  mm/kasan/report.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > > index 621782100eaa..844554e78893 100644
> > > --- a/mm/kasan/report.c
> > > +++ b/mm/kasan/report.c
> > > @@ -92,8 +92,16 @@ static void end_report(unsigned long *flags)
> > >         pr_err("==================================================================\n");
> > >         add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> > >         spin_unlock_irqrestore(&report_lock, *flags);
> > > -       if (panic_on_warn)
> > > +       if (panic_on_warn) {
> > > +               /*
> > > +                * This thread may hit another WARN() in the panic path.
> > > +                * Resetting this prevents additional WARN() from panicking the
> > > +                * system on this thread.  Other threads are blocked by the
> > > +                * panic_mutex in panic().
> >
> > I don't understand part about other threads.
> > Other threads are not necessary inside of panic(). And in fact since
> > we reset panic_on_warn, they will not get there even if they should.
> > If I am reading this correctly, once one thread prints a warning and
> > is going to panic, other threads may now print infinite amounts of
> > warning and proceed past them freely. Why is this the behavior we
> > want?
>
> AIUI, the issue is the current thread hitting another WARN and blocking
> on trying to call panic again. WARNs encountered during the execution of
> panic() need to not attempt to call panic() again.

Yes, but the variable is global and affects other threads and the
comment talks about other threads, and that's the part I am confused
about (for both comment wording and the actual behavior). For the
"same thread hitting another warning" case we need a per-task flag or
something.

> -Kees
>
> >
> > > +                */
> > > +               panic_on_warn = 0;
> > >                 panic("panic_on_warn set ...\n");
> > > +       }
> > >         kasan_enable_current();
> > >  }
>
> --
> Kees Cook