Re: [syzbot] [mm?] general protection fault in mas_store_prealloc

[Aleksandr Nogikh]

Hi Liam,

On Mon, Nov 18, 2024 at 3:49 AM 'Liam R. Howlett' via syzkaller-bugs
<syzkaller-bugs@xxxxxxxxxxxxxxxx> wrote:
>
> * syzbot <syzbot+bc6bfc25a68b7a020ee1@xxxxxxxxxxxxxxxxxxxxxxxxx> [241117 05:42]:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    8e9a54d7181b Merge remote-tracking branch 'iommu/arm/smmu'..
> > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17b0ace8580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=a1eb85a42cb8ccec
> > dashboard link: https://syzkaller.appspot.com/bug?extid=bc6bfc25a68b7a020ee1
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > userspace arch: arm64
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=147521a7980000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=102e14c0580000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/ad658fb4d0a2/disk-8e9a54d7.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/1b7754fa8c67/vmlinux-8e9a54d7.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/50315382fefb/Image-8e9a54d7.gz.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+bc6bfc25a68b7a020ee1@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> I was unable to get this reproducer to work on my own image, even using
> the config and compiler specified in the report.  The injection was not
> happening at the same location as the crash reports.
>
> After using the provided disk (which was tricky), I was able to get it
> to work.

Just in case: did you follow the official syzbot instructions on
reproducing the bugs from the attached assets? [1] If yes, what extra
information could have made the process simpler for you?

[1] https://github.com/google/syzkaller/blob/master/docs/syzbot_assets.md

> Booting was painfully slow, and makes me wonder if there is a
> better way for reproducing issues in the future.

I guess that unless you run it on an arm64 device with a nested
virtualization support, this bug's reproduction will be slow anyway :(

If you recompile the kernel image without CONFIG_KASAN and
CONFIG_KCOV, that should speed it up to some degree.

>
> I've been debating just abusing the bot to debug, but it will spam the
> entire list, but I have it reproducing now.

Syzbot only makes sure that linux-kernel@xxxxxxxxxxxxxxx and
syzkaller-bugs@xxxxxxxxxxxxxxxx are included, both are not really
assumed to be closely followed by human beings. So I think it should
be totally fine to debug with syzbot if you just Cc those two lists.

-- 
Aleksandr

>
> Thanks,
> Liam
>