Re: [syzbot] KFENCE: use-after-free in kvm_fastop_exception

Pavel Skripkin
2021-08-18T08:12:35Z

On 8/18/21 11:02 AM, Matthieu Baerts wrote:
> Hello,
>
> On 18/08/2021 00:21, syzbot wrote:
>> syzbot has bisected this issue to:
>>
>> commit c4512c63b1193c73b3f09c598a6d0a7f88da1dd8
>> Author: Matthieu Baerts <matthieu.baerts@tessares.net>
>> Date: Fri Jun 25 21:25:22 2021 +0000
>>
>> mptcp: fix 'masking a bool' warning
>>
>> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=122b0655300000
>> start commit: b9011c7e671d Add linux-next specific files for 20210816
>> git tree: linux-next
>> final oops: https://syzkaller.appspot.com/x/report.txt?x=112b0655300000
>> console output: https://syzkaller.appspot.com/x/log.txt?x=162b0655300000
>> kernel config: https://syzkaller.appspot.com/x/.config?x=a245d1aa4f055cc1
>> dashboard link: https://syzkaller.appspot.com/bug?extid=7b938780d5deeaaf938f
>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=157a41ee300000
>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14f78ff9300000
>
> I'm pretty sure the commit c4512c63b119 ("mptcp: fix 'masking a bool'
> warning") doesn't introduce the reported bug. This minor fix is specific
> to MPTCP which doesn't seem to be used here.
>
> I'm not sure how I can tell syzbot this is a false positive.
>

looks like it's fs/namei bug. Similar reports:

https://syzkaller.appspot.com/bug?id=517fa734b92b7db404c409b924cf5c997640e324

https://syzkaller.appspot.com/bug?id=484483daf3652b40dae18531923aa9175d392a4d

It's not false positive. I've suggested the fix here:
https://groups.google.com/g/syzkaller-bugs/c/HE3c2fP5nic/m/1Yk17GBeAwAJ
I am waiting for author comments about the fix :)

But, yes, syzbot bisection is often wrong, so don't rely on it much :)

With regards,
Pavel Skripkin