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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 12 Mar 2021 10:21:41 +0100
Message-ID: <CACT4Y+bLdCw+nWndwnv6W9=0EhNNxi=n5Zp032hZE1j0QBMkaA@mail.gmail.com>
Subject: Re: arm64 syzbot instances
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Mark Rutland <mark.rutland@arm.com>, Marc Zyngier <maz@kernel.org>,
        Will Deacon <will@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        syzkaller <syzkaller@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        John Garry <john.garry@huawei.com>,
        Peter Maydell <peter.maydell@linaro.org>,
        =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>
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On Fri, Mar 12, 2021 at 10:16 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Fri, Mar 12, 2021 at 9:46 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> > On Fri, Mar 12, 2021 at 9:40 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Thu, Mar 11, 2021 at 6:57 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> > > a) accessing a legacy ISA/LPC port should not result in an oops,
> > >     but should instead return values with all bits set. There could
> > >     be a ratelimited console warning about broken drivers, but we
> > >     can't assume that all drivers work correctly, as some ancient
> > >     PC style drivers still rely on this.
> > >     John Garry has recently worked on a related bugfix, so maybe
> > >     either this is the same bug he encountered (and hasn't merged
> > >     yet), or if his fix got merged there is still a remaining problem.
>
> > > b) It should not be possible to open /dev/ttyS3 if the device is
> > >     not initialized. What is the output of 'cat /proc/tty/driver/serial'
> > >     on this machine? Do you see any messages from the serial
> > >     driver in the boot log?
> > >     Unfortunately there are so many different ways to probe devices
> > >     in the 8250 driver that I don't know where this comes from.
> > >     Your config file has
> > >    CONFIG_SERIAL_8250_PNP=y
> > >    CONFIG_SERIAL_8250_NR_UARTS=32
> > >    CONFIG_SERIAL_8250_RUNTIME_UARTS=4
> > >    CONFIG_SERIAL_8250_EXTENDED=y
> > >    I guess it's probably the preconfigured uarts that somehow
> > >    become probed without initialization, but it could also be
> > >    an explicit device incorrectly described by qemu.
> >
> >
> > Here is fool boot log, /proc/tty/driver/serial and the crash:
> > https://gist.githubusercontent.com/dvyukov/084890d9b4aa7cd54f468e652a9b5881/raw/54c12248ff6a4885ba6c530d56b3adad59bc6187/gistfile1.txt
>
> Ok, so there are four 8250 ports, and none of them are initialized,
> while the console is on /dev/ttyAMA0 using a different driver.
>
> I'm fairly sure this is a bug in the kernel then, not in qemu.
>
>
> I also see that the PCI I/O space gets mapped to a physical address:
> [    3.974309][    T1] pci-host-generic 4010000000.pcie:       IO
> 0x003eff0000..0x003effffff -> 0x0000000000
>
> So it's probably qemu that triggers the 'synchronous external
> abort' when accessing the PCI I/O space, which in turn hints
> towards a bug in qemu. Presumably it only returns data from
> I/O ports that are actually mapped to a device when real hardware
> is supposed to return 0xffffffff when reading from unused I/O ports.
> This would be separate from the work that John did, which only
> fixed the kernel for accessing I/O port ranges that do not have
> a corresponding MMU mapping to hardware ports.

Will John's patch fix this crash w/o any changes in qemu? That would
be good enough for syzbot. Otherwise we need to report the issue to
qemu.