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Date:	Wed, 30 Jul 2014 17:52:11 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Josef Bacik <jbacik@...com>
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

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.


thanks,

Takashi
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