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

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

[syzbot]

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

Reported-by: syzbot+7b938780d5deeaaf938f@syzkaller.appspotmail.com
Fixes: c4512c63b119 ("mptcp: fix 'masking a bool' warning")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

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

[Matthieu Baerts]

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.

Cheers,
Matt
-- 
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net

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

[Pavel Skripkin]

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

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

[Matthieu Baerts]

Hi Pavel,

On 18/08/2021 10:12, Pavel Skripkin wrote:
> 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

Thank you for having checked!
Should we mark them as "#syz dup" if you think they have the same root
cause?

> 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 :)

Cheers,
Matt
-- 
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net

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

[Pavel Skripkin]

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

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

[Matthieu Baerts]

On 18/08/2021 10:55, Pavel Skripkin wrote:
> 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.

Thank you for the explanation. Sounds good to me!

Cheers,
Matt
-- 
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net