linux-kernel - Re: [syzbot] general protection fault in __device

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Date:   Tue, 7 Jun 2022 09:15:09 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        syzbot <syzbot+dd3c97de244683533381@...kaller.appspotmail.com>,
        hdanton@...a.com, lenb@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
        rafael@...nel.org, rjw@...ysocki.net,
        syzkaller-bugs@...glegroups.com, linux-usb@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [syzbot] general protection fault in __device_attach

On Mon, 6 Jun 2022 at 14:39, Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> On Sat, Jun 04, 2022 at 10:32:46AM +0200, 'Dmitry Vyukov' via syzkaller-bugs wrote:
> > On Fri, 3 Jun 2022 at 18:12, Greg KH <gregkh@...uxfoundation.org> wrote:
> > >
> > > But again, is this a "real and able to be triggered from userspace"
> > > problem, or just fault-injection-induced?
> >
> > Then this is something to fix in the fault injection subsystem.
> > Testing systems shouldn't be reporting false positives.
> > What allocations cannot fail in real life? Is it <=page_size?
> >
>
> Apparently in 2014, anything less than *EIGHT?!!* pages succeeded!
>
> https://lwn.net/Articles/627419/
>
> I have been on the look out since that article and never seen anyone
> mention it changing.  I think we should ignore that and say that
> anything over PAGE_SIZE can fail.  Possibly we could go smaller than
> PAGE_SIZE...

+linux-mm for GFP expertise re what allocations cannot possibly fail
and should be excluded from fault injection.

Interesting, thanks for the link.

PAGE_SIZE looks like a good start. Once we have the predicate in
place, we can refine it later when/if we have more inputs.

But I wonder about GFP flags. They definitely have some impact on allocations.
If GFP_ACCOUNT is set, all allocations can fail, right?
If GFP_DMA/DMA32 is set, allocations can fail, right? What about other zones?
If GFP_NORETRY is set, allocations can fail?
What about GFP_NOMEMALLOC and GFP_ATOMIC?
What about GFP_IO/GFP_FS/GFP_DIRECT_RECLAIM/GFP_KSWAPD_RECLAIM? At
least some of these need to be set for allocations to not fail? Which
ones?
Any other flags are required to be set/unset for allocations to not fail?

FTR here is quick link to flags list:
https://elixir.bootlin.com/linux/v5.19-rc1/source/include/linux/gfp.h#L32