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

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, Jan 17, 2020 at 10:20 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Jan 17, 2020 at 10:54:36AM +0100, Dmitry Vyukov wrote:
> > 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.
>
> This is duplicating the common panic-on-warn logic (see the generic bug
> code), so I'd like to just have the same behavior between the three
> implementations of panic-on-warn (generic bug, kasan, ubsan), and then
> work to merge them into a common handler, and then perhaps fix the
> details of the behavior. I think it's more correct to allow the panicing
> thread to complete than to care about what the other threads are doing.
> Right now, a WARN within the panic code will either a) hang the machine,
> or b) not panic, allowing the rest of the threads to continue, maybe
> then hitting other WARNs and hanging. The generic bug code does not
> suffer from this.

I see. Then:

Acked-by: Dmitry Vyukov <dvyukov@google.com>