linux-kernel - Re: KASAN: slab-out-of-bounds Read in vhci_hub

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Message-ID: <CACT4Y+bycsbrkeT8PHYxfooTkw2VyCAONY_BH2dTuZKtvCj3pg@mail.gmail.com>
Date:   Tue, 2 Oct 2018 18:42:09 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Shuah Khan <shuah@...nel.org>
Cc:     syzbot <syzbot+bccc1fe10b70fadc78d0@...kaller.appspotmail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Valentina Manea <valentina.manea.m@...il.com>
Subject: Re: KASAN: slab-out-of-bounds Read in vhci_hub_control

On Tue, Oct 2, 2018 at 6:04 PM, Shuah Khan <shuah@...nel.org> wrote:
> On 09/04/2018 12:52 PM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    420f51f4ab6b Merge tag 'arm64-fixes' of git://git.kernel.o..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=126a6f0e400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=531a917630d2a492
>> dashboard link: https://syzkaller.appspot.com/bug?extid=bccc1fe10b70fadc78d0
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=121caa46400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14ed8ab6400000
>
> C producer doesn't reproduce the problem on 4.19-rc5. Does this C producer
> depend on state of the machine? i.e what is the status of vhci_hcd - are
> there any devices attached?

Hi Shuah,

syzbot always runs tests reproducers on a clean machine. There is some
state are running a Debian wheezy init, but no test/fuzz/stress
workload is run before the reproducer.
syzbot also uses VMs, so there are no real devices attached. And it's
GCE VMs (not qemu), and I think GCE does not even emulate any USB
devices.

An obvious thing to try would be to use the exact commit and config
syzbot gave (rather than 4.19-rc5).
You can also take the image syzbot uses here:
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#crash-does-not-reproduce

> I can see the problem looking at the code and fix is easy. However, I would
> like be able to reproduce it and verify the fix works. Also this would be a
> good regression for the driver I could consider adding to selftests.

syzbot can test fixes for bugs with reproducers:
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches
So it can test your fix. But this obviously won't help with a test.