linux-kernel - Re: [PATCH v6 2/8] debugfs: prevent access to removed files' private data

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Date:	Fri, 20 May 2016 12:57:36 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Nicolai Stange <nicstange@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jonathan Corbet <corbet@....net>, Jan Kara <jack@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Julia Lawall <Julia.Lawall@...6.fr>,
	Gilles Muller <Gilles.Muller@...6.fr>,
	Nicolas Palix <nicolas.palix@...g.fr>,
	Michal Marek <mmarek@...e.com>, linux-kernel@...r.kernel.org,
	cocci@...teme.lip6.fr
Subject: Re: [PATCH v6 2/8] debugfs: prevent access to removed files' private
 data

On 05/18/2016 12:05 PM, Greg Kroah-Hartman wrote:
> On Wed, May 18, 2016 at 11:18:16AM -0400, Sasha Levin wrote:
>> On 05/18/2016 11:01 AM, Nicolai Stange wrote:
>>> Thanks a million for reporting!
>>>
>>> 1.) Do you have lockdep enabled?
>>
>> Yup, nothing there.
>>
>>> 2.) Does this happen before or after userspace init has been spawned,
>>>     i.e. does the lockup happen at debugfs file creation time or
>>>     possibly at usage time?
>>
>> So I looked closer, and it seems to happen after starting syzkaller, which
>> as far as I know tries to open many different debugfs files.
>>
>> Is there debug code I can add it that'll help us figure out what's up?
> 
> Trying to figure out _which_ debugfs file is causing this would be
> great, if at all possible.  strace?

What seems to be failing is syzkaller's attempt to mmap the coverage
debugfs file. So this isn't actually a kernel deadlock but syzkaller
misbehaves when that scenario happens.

Either way, it only fails to mmap with that commit that I've pointed
out.

	th->cover_fd = open("/sys/kernel/debug/kcov", O_RDWR);
	if (th->cover_fd == -1)
		fail("open of /sys/kernel/debug/kcov failed");
	if (ioctl(th->cover_fd, KCOV_INIT_TRACE, kCoverSize))
		fail("cover enable write failed");
	th->cover_data = (uintptr_t*)mmap(NULL, kCoverSize * sizeof(th->cover_data[0]), PROT_READ | PROT_WRITE, MAP_SHARED, th->cover_fd, 0);
	if ((void*)th->cover_data == MAP_FAILED)
		fail("cover mmap failed");

And it's the mmap() that fails with -ENODEV.


Thanks,
Sasha