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

Pavel Skripkin
2021-08-18T08:55:28Z

On 8/18/21 11:21 AM, Matthieu Baerts wrote:
> Hi Pavel,
>
[snip]
>>>
>>> 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
>
> Thank you for having checked!
> Should we mark them as "#syz dup" if you think they have the same root
> cause?
>

I think, yes, but I want to receive feedback from fs people about this
bug. There were huge updates last month, and, maybe, I am missing some
details. Alloc/free calltrace is the same, but anyway, I want some
confirmation to not close different bugs by mistake :)

If these bugs really have same root case I will close them manually
after fix posted.

>> 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 :)
>
> Yes sorry, I wanted to say the bisection picked a wrong commit :)
>
> All good then if syzbot often blames the wrong modification :)
>
With regards,
Pavel Skripkin