Re: [PATCH] Btrfs: Fix memory corruption by ulist_add_merge() on 32bit arch

[Takashi Iwai <tiwai@suse.de>]

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@vger.kernel.org> [v3.11+]
> > > Signed-off-by: Takashi Iwai <tiwai@suse.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.


thanks,

Takashi