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Date:	Wed, 30 Jul 2014 12:01:31 -0400
From:	Josef Bacik <jbacik@...com>
To:	Takashi Iwai <tiwai@...e.de>
CC:	Chris Mason <clm@...com>, <linux-btrfs@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Btrfs: Fix memory corruption by ulist_add_merge() on
 32bit arch

On 07/30/2014 11:52 AM, Takashi Iwai wrote:
> At Wed, 30 Jul 2014 11:40:14 -0400,
> Josef Bacik wrote:
>>
>> On 07/30/2014 11:05 AM, Takashi Iwai wrote:
>>> At Wed, 30 Jul 2014 17:01:52 +0200,
>>> Takashi Iwai wrote:
>>>>
>>>> At Wed, 30 Jul 2014 10:29:46 -0400,
>>>> Josef Bacik wrote:
>>>>>
>>>>> On 07/30/2014 05:57 AM, Takashi Iwai wrote:
>>>>>> At Mon, 28 Jul 2014 16:01:55 +0200,
>>>>>> Takashi Iwai wrote:
>>>>>>>
>>>>>>> At Mon, 28 Jul 2014 15:48:41 +0200,
>>>>>>> Takashi Iwai wrote:
>>>>>>>>
>>>>>>>> At Mon, 28 Jul 2014 09:16:48 -0400,
>>>>>>>> Josef Bacik wrote:
>>>>>>>>>
>>>>>>>>> On 07/28/2014 04:57 AM, Takashi Iwai wrote:
>>>>>>>>>> We've got bug reports that btrfs crashes when quota is enabled on
>>>>>>>>>> 32bit kernel, typically with the Oops like below:
>>>>>>>>>>      BUG: unable to handle kernel NULL pointer dereference at 00000004
>>>>>>>>>>      IP: [<f9234590>] find_parent_nodes+0x360/0x1380 [btrfs]
>>>>>>>>>>      *pde = 00000000
>>>>>>>>>>      Oops: 0000 [#1] SMP
>>>>>>>>>>      CPU: 0 PID: 151 Comm: kworker/u8:2 Tainted: G S      W 3.15.2-1.gd43d97e-default #1
>>>>>>>>>>      Workqueue: btrfs-qgroup-rescan normal_work_helper [btrfs]
>>>>>>>>>>      task: f1478130 ti: f147c000 task.ti: f147c000
>>>>>>>>>>      EIP: 0060:[<f9234590>] EFLAGS: 00010213 CPU: 0
>>>>>>>>>>      EIP is at find_parent_nodes+0x360/0x1380 [btrfs]
>>>>>>>>>>      EAX: f147dda8 EBX: f147ddb0 ECX: 00000011 EDX: 00000000
>>>>>>>>>>      ESI: 00000000 EDI: f147dda4 EBP: f147ddf8 ESP: f147dd38
>>>>>>>>>>       DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
>>>>>>>>>>      CR0: 8005003b CR2: 00000004 CR3: 00bf3000 CR4: 00000690
>>>>>>>>>>      Stack:
>>>>>>>>>>       00000000 00000000 f147dda4 00000050 00000001 00000000 00000001 00000050
>>>>>>>>>>       00000001 00000000 d3059000 00000001 00000022 000000a8 00000000 00000000
>>>>>>>>>>       00000000 000000a1 00000000 00000000 00000001 00000000 00000000 11800000
>>>>>>>>>>      Call Trace:
>>>>>>>>>>       [<f923564d>] __btrfs_find_all_roots+0x9d/0xf0 [btrfs]
>>>>>>>>>>       [<f9237bb1>] btrfs_qgroup_rescan_worker+0x401/0x760 [btrfs]
>>>>>>>>>>       [<f9206148>] normal_work_helper+0xc8/0x270 [btrfs]
>>>>>>>>>>       [<c025e38b>] process_one_work+0x11b/0x390
>>>>>>>>>>       [<c025eea1>] worker_thread+0x101/0x340
>>>>>>>>>>       [<c026432b>] kthread+0x9b/0xb0
>>>>>>>>>>       [<c0712a71>] ret_from_kernel_thread+0x21/0x30
>>>>>>>>>>       [<c0264290>] kthread_create_on_node+0x110/0x110
>>>>>>>>>>
>>>>>>>>>> This indicates a NULL corruption in prefs_delayed list.  The further
>>>>>>>>>> investigation and bisection pointed that the call of ulist_add_merge()
>>>>>>>>>> results in the corruption.
>>>>>>>>>>
>>>>>>>>>> ulist_add_merge() takes u64 as aux and writes a 64bit value into
>>>>>>>>>> old_aux.  The callers of this function in backref.c, however, pass a
>>>>>>>>>> pointer of a pointer to old_aux.  That is, the function overwrites
>>>>>>>>>> 64bit value on 32bit pointer.  This caused a NULL in the adjacent
>>>>>>>>>> variable, in this case, prefs_delayed.
>>>>>>>>>>
>>>>>>>>>> Here is a quick attempt to band-aid over this: a new function,
>>>>>>>>>> ulist_add_merge_ptr() is introduced to pass/store properly a pointer
>>>>>>>>>> value instead of u64.  There are still ugly void ** cast remaining
>>>>>>>>>> in the callers because void ** cannot be taken implicitly.  But, it's
>>>>>>>>>> safer than explicit cast to u64, anyway.
>>>>>>>>>>
>>>>>>>>>> Bugzilla: https://urldefense.proofpoint.com/v1/url?u=https://bugzilla.novell.com/show_bug.cgi?id%3D887046&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=cKCbChRKsMpTX8ybrSkonQ%3D%3D%0A&m=m3qrbo6ngjqKO%2B7ofuwRfQflb9Cx%2FXrF8TKejkPjxfA%3D%0A&s=199a5b6f0ed181925e9ba2c1060fe20d1c8ad2831dd1d96cc7eddd2a343fa72b
>>>>>>>>>> Cc: <stable@...r.kernel.org> [v3.11+]
>>>>>>>>>> Signed-off-by: Takashi Iwai <tiwai@...e.de>
>>>>>>>>>> ---
>>>>>>>>>>
>>>>>>>>>> Alternatively, we can change the argument of aux and old_aux to a
>>>>>>>>>> pointer from u64, as backref.c is the only user of ulist_add_merge()
>>>>>>>>>> function.  I'll cook up another patch if it's the preferred way.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Yeah lets just use a pointer and see how that works out.  Thanks,
>>>>>>>>
>>>>>>>> Oops, I forgot that ulist_add() takes aux as u64 and it calling
>>>>>>>> ulist_add_merge() internally.  So, we can't change the type blindly
>>>>>>>> there, unfortunately.
>>>>>>>
>>>>>>> Looking back at the code, it seems that all aux arguments passed to
>>>>>>> ulist_add() in qgroup.c are pointers, too.  So, indeed, all aux values
>>>>>>> are pointers, so far, and it'd be even cleaner to replace all these
>>>>>>> from u64 to void *.
>>>>>>>
>>>>>>> But, such a replacement patch will become difficult for backporting to
>>>>>>> stable kernels (the bug existed since 3.11, at least).  So IMO, we
>>>>>>> should put a smaller fix like my previous one, let it backported to
>>>>>>> stable kernels, and do more comprehensive replacements to pointer on
>>>>>>> its top.
>>>>>>
>>>>>> Ping.  Could you guys take my original patch as is, or do you prefer
>>>>>> changing in a different way?  If so, how?
>>>>>>
>>>>>
>>>>> I don't care how hard it is to backport to stable,
>>>>
>>>> You must do care as a maintainer.  It's a long-standing and serious
>>>> bug since 3.11.  The kernel hangs up immediately when you enable quota
>>>> on 32bit kernel.  And it's really hard to revert it when the rootfs is
>>>> btrfs.  (The mount follows the immediate hang up after reboot.)
>>>>
>>
>> What I mean is that we want the right fix first, not something that is easier to
>> pull back to stable and then the right fix later.  Do it right first and then
>> backport it to the stable kernels, it's perfectly acceptable to adjust patches
>> when sending them to the stable team.  "But it's hard" is not a valid excuse for
>> not doing it right the first time.
>
> Well, I guess this underestimates the burden of backports.  Currently,
> there is stable kernel for each kernel release.  Anyone has to
> backport for each version, and you'll be asked.  I, as a long-time
> subsystem maintainer, wouldn't go in that way :)
>
> And, speaking of "rightness" -- replacing the callers with a wrapper
> is also a right fix.  It's even a safer fix.  That's basically why I
> posted it as the primary patch.
>
> The merit of replacing all callers is that you can eliminate nasty
> casts by that.  This is however rather a cleanup, which is a different
> bonus from what we need to fix.
>
>>>>> since we're using pointers
>>>>> everywhere just change it to void * and be done with it.  Thanks,
>>>>
>>>> Fix it quickly, then do cleanup.  This is the golden rule for
>>>> regression :)
>>>
>>> Also, another question is whether you guys are OK to change the type
>>> to a pointer.  Through a glance, the ulist code was intended to handle
>>> any generic data, thus it uses u64, right?  Using void pointer breaks
>>> this concept.
>>>
>>
>> It's fine, ulist today resembles very little from what it was originally.  The
>> current users all shove pointers into there, so we might as well just make it a
>> pointer.  Thanks,
>
> OK, good to know.
>
> If this post still doesn't convince you, I'll prepare the patch to do
> all replacements.
>
>

I don't care that much, do it however you want.  Thanks,

Josef
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