linux-kernel - Re: arm64 syzbot instances

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Message-ID: <CAK8P3a2NEcHG+nOUCc6-DPeFKkc-GF-LEOkynhNdgxiXBHdQaw@mail.gmail.com>
Date:   Fri, 12 Mar 2021 10:16:06 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>, Marc Zyngier <maz@...nel.org>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        John Garry <john.garry@...wei.com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>
Subject: Re: arm64 syzbot instances

On Fri, Mar 12, 2021 at 9:46 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Fri, Mar 12, 2021 at 9:40 AM Arnd Bergmann <arnd@...db.de> wrote:
> > On Thu, Mar 11, 2021 at 6:57 PM Dmitry Vyukov <dvyukov@...gle.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.

       Arnd