linux-kernel - Re: namespace: deadlock in dec_pid

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Message-ID: <87lgu5w92e.fsf@xmission.com>
Date:   Sat, 21 Jan 2017 13:28:09 +1300
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Nikolay Borisov <n.borisov.lkml@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, avagin <avagin@...nvz.org>,
        serge@...lyn.com, Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: namespace: deadlock in dec_pid_namespaces

Nikolay Borisov <n.borisov.lkml@...il.com> writes:

> On 20.01.2017 20:05, Eric W. Biederman wrote:
>> Nikolay Borisov <n.borisov.lkml@...il.com> writes:
>> 
>>> On 20.01.2017 15:07, Dmitry Vyukov wrote:
>>>> Hello,
>>>>
>>>> I've got the following deadlock report while running syzkaller fuzzer
>>>> on eec0d3d065bfcdf9cd5f56dd2a36b94d12d32297 of linux-next (on odroid
>>>> device if it matters):
>> 
>> I am puzzled I thought we had fixed this with:
>>   add7c65ca426 ("pid: fix lockdep deadlock warning due to ucount_lock")
>> But apparently not.  We  just moved it from hardirq to softirq context.  Bah.
>> 
>> Thank you very much for the report.
>> 
>> Nikolay can you make your change use spinlock_irq?  And have put_ucounts
>> do spin_lock_irqsave?  That way we just don't care where we call this.
>
> Like the one attached?

Exactly thank you.  Dmitry if you have time to test that patch and
verify it fixes your issue I would appreciate it.

> I haven't really taken careful look as to whether
> the function where _irq versions do fiddle with irq state, since this
> might cause a problem if we unconditionally enable them.

In code paths where we can sleep irqs must come in enabled or it's a
bug.

spin_lock_irq which unconditionally disables irqs is thus safe on the
allocation path.

Similary spin_unlock_irq which unconditionally enables irqs is also safe
on the allocation path.

Eric