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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 8 Jun 2022 10:20:04 +0200
Message-ID: <CACT4Y+anOkc=-A5=3EBrvmu+AJ+f7CcrOfWp85hEu+CHkS3BGw@mail.gmail.com>
Subject: Re: [syzbot] general protection fault in __device_attach
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        syzbot <syzbot+dd3c97de244683533381@syzkaller.appspotmail.com>,
        hdanton@sina.com, lenb@kernel.org, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org, rafael.j.wysocki@intel.com,
        rafael@kernel.org, rjw@rjwysocki.net,
        syzkaller-bugs@googlegroups.com, linux-usb@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>
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On Wed, 8 Jun 2022 at 05:25, Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Jun 07, 2022 at 09:15:09AM +0200, Dmitry Vyukov wrote:
> > On Mon, 6 Jun 2022 at 14:39, Dan Carpenter <dan.carpenter@oracle.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@linuxfoundation.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?
>
> I'm not the expert on page allocation, but ...
>
> I don't think GFP_ACCOUNT makes allocations fail.  It might make reclaim
> happen from within that cgroup, and it might cause an OOM kill for
> something in that cgroup.  But I don't think it makes a (low order)
> allocation more likely to fail.

Interesting.
I was thinking of some malicious specifically crafted configurations
with very low limit and particular pattern of allocations. Also what
if there is just 1 process (current)? Is it possible to kill and
reclaim the current process when a thread is stuck in the middle of
the kernel on a kmalloc?
Also I see e.g.:
        Tasks with the OOM protection (oom_score_adj set to -1000)
        are treated as an exception and are never killed.

I am not an expert on this either, but I think it may be hard to fight
with a specifically crafted attack.


> There's usually less memory avilable in DMA/DMA32 zones, but we have
> so few allocations from those zones, I question the utility of focusing
> testing on those allocations.
>
> GFP_ATOMIC allows access to emergency pools, so I would say _less_ likely
> to fail.  KSWAPD_RECLAIM has no effect on whether _this_ allocation
> succeeds or fails; it kicks kswapd to do reclaim, rather than doing
> reclaim directly.  DIRECT_RECLAIM definitely makes allocations more likely
> to succeed.  GFP_FS allows (direct) reclaim to happen from filesystems.
> GFP_IO allows IO to start (ie writeback can start) in order to clean
> dirty memory.
>
> Anyway, I hope somebody who knows the page allocator better than I do
> can say smarter things than this.  Even better if they can put it into
> Documentation/ somewhere ;-)

Even better to put this into code as a predicate function that fault
injection will use. It will also serve as precise up-to-date
documentation.

> https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html
> exists but isn't quite enough to answer this question.