Looking for guidance to reproduce a bug in drm/vkms reported by syzkaller

[Melissa Wen]

Hi,

I am trying to reproduce a syzkaller bug found in the vkms:
- WARNING in vkms_gem_free_object
- https://groups.google.com/forum/#!msg/syzkaller-bugs/_oARhriB1SA/PelnW1BqAwAJ

However, I was not very successful in this task. As I don't know how to deal
with syzkaller and I am not sure if I am following the right steps, it would be
great if someone can give me some tips and guidance.

First, looking at the bug history at:
- https://syzkaller.appspot.com/bug?extid=e7ad70d406e74d8fc9d0,
it seems like the bug still exists. Am I right?

Second, here is a report of what I tried to do:
Note: For testing, I use a VM (QEMU) with Debian 10 with a kernel compiled at
- https://cgit.freedesktop.org/drm/drm-misc (branch drm-misc-next)

1 - Using the usual .config for my VM, I compiled and installed the kernel and,
as root, ran the C program provided by syzkaller:
- https://syzkaller.appspot.com/x/repro.c?x=15e27c53600000
Nothing happened.

2 - Then, I checked the debug/panic/hacking/drm/i915 debugging/vkms settings on
the .config reported by syzkaller:
- https://syzkaller.appspot.com/x/.config?x=7cf4eed5fe42c31a
and enabled the same things in my .config.
I compiled and installed the kernel and ran the C program.
Nothing happened.

3 - So, I reverted my current branch to the commit that generated the bug
(as reported: 94e2ec3f7fef86506293a448273b2b4ee21e6195) and used the kernel on
that state.
Nothing happened.

4 - I decided to use the syzkaller .config without modifications and
adaptations for my VM (although I didn't think it felt right). I compiled,
installed... some boot problems happened, but the kernel worked. I ran the C
program and nothing.

So I gave up... for a while :)

Does anyone have any advice for me?
I have already searched for information on the Internet, but I am still stuck.

The last thing that I noted is that syzkaller stopped providing a C program of
this bug for crashes after the beginning of the year (I also don't know if it
is something important).

Thanks in advance,

Melissa Wen