linux-kernel - Re: WARNING in __do_kernel

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Date:   Wed, 27 Jan 2021 20:56:24 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>, Will Deacon <will@...nel.org>
Cc:     syzbot <syzbot+45b6fce29ff97069e2c5@...kaller.appspotmail.com>,
        Dave Martin <Dave.Martin@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING in __do_kernel_fault

On Wed, Jan 27, 2021 at 8:43 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > > > > > > This happens on arm64 instance with mte enabled.
> > > > > > > There is a GPF in reiserfs_xattr_init on x86_64 reported:
> > > > > > > https://syzkaller.appspot.com/bug?id=8abaedbdeb32c861dc5340544284167dd0e46cde
> > > > > > > so I would assume it's just a plain NULL deref. Is this WARNING not
> > > > > > > indicative of a kernel bug? Or there is something special about this
> > > > > > > particular NULL deref?
> > > > > >
> > > > > > Congratulations, you're the first person to trigger this warning!
> > > > > >
> > > > > > This fires if we take an unexpected data abort in the kernel but when we
> > > > > > get into the fault handler the page-table looks ok (according to the CPU via
> > > > > > an 'AT' instruction). Are you using QEMU system emulation? Perhaps its
> > > > > > handling of AT isn't quite right.
> > > > >
> > > > > Hi Will,
> > > > >
> > > > > Yes, it's qemu-system-aarch64 5.2 with -machine virt,mte=on -cpu max.
> > > > > Do you see any way forward for this issue? Can somehow prove/disprove
> > > > > it's qemu at fault?
> > > >
> > > > I've reproduced this crash (by taking [1] and changing
> > > > sys_memfd_create to 279), but it manifests as a normal null-ptr-deref
> > > > for me. I'm using the latest QEMU master. Which QEMU does syzbot use
> > > > exactly?
> > >
> > > qemu-system-aarch64 5.2 from this container:
> > > https://github.com/google/syzkaller/blob/master/tools/docker/syzbot/Dockerfile
> > > you can get a prebuilt version with:
> > > docker pull gcr.io/syzkaller/syzbot
> >
> > Reproduced with this QEMU, still a normal null-ptr-deref. Where do I
> > find the full list of arguments that are passed to QEMU on syzbot?
>
> I am yet to document all details of these new instances, but the
> syzkaller config contains:
> "qemu_args": "-machine
> virt,virtualization=on,mte=on,graphics=on,usb=on -cpu max"
> the rest are in vm/qemu/qemu.go

OK, the virtualization=on part is what causes this. Bug in QEMU?