linux-kernel - Re: [syzbot] KFENCE: use-after-free in kvm_fastop

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Date:   Wed, 18 Aug 2021 11:12:14 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Matthieu Baerts <matthieu.baerts@...sares.net>,
        syzbot <syzbot+7b938780d5deeaaf938f@...kaller.appspotmail.com>,
        davem@...emloft.net, johan.hedberg@...il.com, kuba@...nel.org,
        linux-bluetooth@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, luiz.dentz@...il.com,
        marcel@...tmann.org, mathew.j.martineau@...ux.intel.com,
        netdev@...r.kernel.org, pabeni@...hat.com,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: [syzbot] KFENCE: use-after-free in kvm_fastop_exception

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@...sares.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