linux-kernel - Re: GPF in shm

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Date:	Sat, 2 Jan 2016 16:58:52 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	syzkaller <syzkaller@...glegroups.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, Joe Perches <joe@...ches.com>,
	sds@...ho.nsa.gov, Oleg Nesterov <oleg@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, mhocko@...e.cz,
	gang.chen.5i5j@...il.com, Peter Feiner <pfeiner@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: GPF in shm_lock ipc

Hi Dmitry,

On 01/02/2016 01:19 PM, Dmitry Vyukov wrote:
> On Sat, Jan 2, 2016 at 12:33 PM, Manfred Spraul
> <manfred@...orfullife.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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