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Date: Wed, 30 Jul 2014 13:00:24 -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 12:35 PM, Takashi Iwai wrote: > At Wed, 30 Jul 2014 12:01:31 -0400, > Josef Bacik wrote: >> >> 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, > > Yes, I do care because I know of this kind of horror very well. > > OK, I'm going to send the new patch(es). > I meant that I don't care how you do it, if you want to do it the simple way first and then send the cleanup later thats fine by me. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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