linux-kernel - Re: ipc: BUG: sem_unlock unlocks non-locked lock

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Date:   Sun, 18 Dec 2016 19:32:52 +0100
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Davidlohr Bueso <dave@...olabs.net>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, fabf@...net.be,
        kernel@...p.com, LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: ipc: BUG: sem_unlock unlocks non-locked lock

On 12/18/2016 05:29 PM, Davidlohr Bueso wrote:
> On Sun, 18 Dec 2016, Bueso wrote:
>
>> On Fri, 16 Dec 2016, Dmitry Vyukov wrote:
>>
>>> [ BUG: bad unlock balance detected! ]
>>> 4.9.0+ #89 Not tainted
>>
>> Thanks for the report, I can reproduce the issue as of (which I 
>> obviously
>> should have tested with lockdep):
>>
>> 370b262c896 (ipc/sem: avoid idr tree lookup for interrupted semop)
>>
>> I need to think more about it this evening, but I believe the issue 
>> to be
>> the potentially bogus locknum in the unlock path, as we are calling 
>> sem_lock
>> without updating the variable. I'll send a patch after more testing. 
>> This
>> fixes it for me:
>>
>> diff --git a/ipc/sem.c b/ipc/sem.c
>> index e08b94851922..fba6139e7208 100644
>> --- a/ipc/sem.c
>> +++ b/ipc/sem.c
>> @@ -1977,7 +1977,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct 
>> sembuf __user *, tsops,
>>         }
>>
>>         rcu_read_lock();
>> -        sem_lock(sma, sops, nsops);
>> +            sem_lock(sma, sops, nsops);
>
> *sigh*, that would be:
>              locknum = sem_lock(sma, sops, nsops);

Yes, I can confirm that this fixes the issue.

Reproducing is simple:

- task A: single semop semop(), sleeps
- task B: multi semop semop(), sleeps
- task A woken up by signal/timeout

I'll send a patch.

--

     Manfred