Re: GPF in shm_lock ipc

From	Manfred Spraul <manfred@colorfullife.com>
Newsgroups	linux.kernel
Subject	Re: GPF in shm_lock ipc
Date	2016-01-02 17:00 +0100
Message-ID	<qMwL0-1wI-9@gated-at.bofh.it> (permalink)
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Hi Dmitry,

On 01/02/2016 01:19 PM, Dmitry Vyukov wrote:
> On Sat, Jan 2, 2016 at 12:33 PM, Manfred Spraul
> <manfred@colorfullife.com> wrote:
>> Hi Dmitry,
>>
>> shm locking differs too much from msg/sem locking, I never looked at it in
>> depth, so I'm not able to perform a proper review.
>>
>> Except for the obvious: Races that can be triggered from user space are
>> inacceptable.
>> Regardless if there is a BUG_ON, a WARN_ON or nothing at all.
>>
>> On 12/21/2015 04:44 PM, Dmitry Vyukov wrote:
>>>> +
>>>> +/* This is called by fork, once for every shm attach. */
>>>> +static void shm_open(struct vm_area_struct *vma)
>>>> +{
>>>> +       int err = __shm_open(vma);
>>>> +       /*
>>>> +        * We raced in the idr lookup or with shm_destroy().
>>>> +        * Either way, the ID is busted.
>>>> +        */
>>>> +       WARN_ON_ONCE(err);
>>>>    }
>> Is it possible to trigger this race? Parallel IPC_RMID & fork()?
> Hi Manfred,
>
> As far as I see my reproducer triggers exactly this warning (and later a crash).
Do I understand it right, shm_open() is also called by remap()?
Then please update the comment above shm_open().

And: If this is something that userspace can trigger, why a WARN_ON_ONCE()?
If the WARN_ON doesn't indicate a bug, then I would remove it entirely.

--
     Manfred
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Re: GPF in shm_lock ipc Dmitry Vyukov <dvyukov@google.com> - 2015-12-21 16:50 +0100
  Re: GPF in shm_lock ipc Manfred Spraul <manfred@colorfullife.com> - 2016-01-02 12:40 +0100
    Re: GPF in shm_lock ipc Dmitry Vyukov <dvyukov@google.com> - 2016-01-02 13:30 +0100
      Re: GPF in shm_lock ipc Manfred Spraul <manfred@colorfullife.com> - 2016-01-02 17:00 +0100

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