Re: [PATCH v2] kernel/printk: prevent deadlock at unexpected call kmsg_dump in NMI context

[Konstantin Khlebnikov]

On 16.07.2019 10:41, Petr Mladek wrote:
> On Mon 2019-07-15 11:04:55, Konstantin Khlebnikov wrote:
> > Kernel message dumper - function kmsg_dump() is called on various oops or
> > panic paths which could happen in unpredictable context including NMI.
> >
> > Panic in NMI is handled especially by stopping all other cpus with
> > smp_send_stop() and busting locks in printk_safe_flush_on_panic().
> >
> > Other less-fatal cases shouldn't happen in NMI and cannot be handled.
> > But this might happen for example on oops in nmi context. In this case
> > dumper could deadlock on lockbuf_lock or break internal structures.
>
> If I get it correctly than this patch could really prevent a deadlock
> in at least:
>
>    + oops_end()
>      + oops_exit()
>        + kmsg_dump(KMSG_DUMP_OOPS)
>
> If it is called in NMI, it should end up with panic(). Then the dump
> will be called later after stopping CPUs...
>
> Or am I wrong?

Yep. Under 'oops in nmi context' I mean exactly that case.

> Otherwise, the patch looks good to me. I would just mention
> the above scenario if it is correct.
>
> Best Regards,
> Petr
>
> > This patch catches kmsg_dump() called in NMI context except panic and
> > prints warning once.
> >
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/lkml/156294329676.1745.2620297516210526183.stgit@buzz/ (v1)
> > ---
> >   kernel/printk/printk.c |    7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 1888f6a3b694..e711f64a1843 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -3104,6 +3104,13 @@ void kmsg_dump(enum kmsg_dump_reason reason)
> >   	struct kmsg_dumper *dumper;
> >   	unsigned long flags;
> >   
> > +	/*
> > +	 * In NMI context only panic could be handled safely:
> > +	 * it stops other cpus and busts logbuf lock.
> > +	 */
> > +	if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi()))
> > +		return;
> > +
> >   	if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
> >   		return;
> >