Re: [PATCH 4/5] btrfs: raid56: Don't keep rbio for later steal

[Liu Bo <bo.li.liu@oracle.com>]

On Tue, Mar 21, 2017 at 10:23:56AM +0800, Qu Wenruo wrote:
> 
> 
> At 03/21/2017 10:08 AM, Liu Bo wrote:
> > On Tue, Mar 21, 2017 at 08:44:18AM +0800, Qu Wenruo wrote:
> > > 
> > > 
> > > At 03/21/2017 04:23 AM, Liu Bo wrote:
> > > > On Mon, Mar 20, 2017 at 02:21:48PM +0800, Qu Wenruo wrote:
> > > > > 
> > > > > 
> > > > > At 03/18/2017 10:03 AM, Liu Bo wrote:
> > > > > > On Fri, Mar 17, 2017 at 01:28:45PM +0800, Qu Wenruo wrote:
> > > > > > > 
> > > > > > > 
> > > > > > > At 03/17/2017 12:44 PM, Liu Bo wrote:
> > > > > > > > On Fri, Feb 03, 2017 at 04:20:22PM +0800, Qu Wenruo wrote:
> > > > > > > > > Before this patch, btrfs raid56 will keep raid56 rbio even all its IO is
> > > > > > > > > done.
> > > > > > > > > This may save some time allocating rbio, but it can cause deadly
> > > > > > > > > use-after-free bug, for the following case:
> > > > > > > > > 
> > > > > > > > > Original fs: 4 devices RAID5
> > > > > > > > > 
> > > > > > > > >        Process A                 |          Process B
> > > > > > > > > --------------------------------------------------------------------------
> > > > > > > > >                                  |  Start device replace
> > > > > > > > >                                  |    Now the fs has 5 devices
> > > > > > > > >                                  |    devid 0: replace device
> > > > > > > > >                                  |    devid 1~4: old devices
> > > > > > > > > btrfs_map_bio()                  |
> > > > > > > > > |- __btrfs_map_block()           |
> > > > > > > > > |    bbio has 5 stripes          |
> > > > > > > > > |    including devid 0           |
> > > > > > > > > |- raid56_parity_write()         |
> > > > > > > > >                                  |
> > > > > > > > > raid_write_end_io()              |
> > > > > > > > > |- rbio_orig_end_io()            |
> > > > > > > > >    |- unlock_stripe()            |
> > > > > > > > >        Keeps the old rbio for    |
> > > > > > > > >        later steal, which has    |
> > > > > > > > >        stripe for devid 0        |
> > > > > > > > >                                  |  Cancel device replace
> > > > > > > > >                                  |    Now the fs has 4 devices
> > > > > > > > >                                  |    devid 0 is freed
> > > > > > > > > Some IO happens                  |
> > > > > > > > > raid_write_end_io()              |
> > > > > > > > > |- rbio_orig_end_io()            |
> > > > > > > > >    |- unlock_stripe()            |
> > > > > > > > >       |- steal_rbio()            |
> > > > > > > > >            Use old rbio, whose   |
> > > > > > > > >            bbio has freed devid 0|
> > > > > > > > >            stripe                |
> > > > > > > > > Any access to rbio->bbio will    |
> > > > > > > > > cause general protection or NULL |
> > > > > > > > > pointer dereference              |
> > > > > > > > > 
> > > > > > > > > Such bug can already be triggered by fstests btrfs/069 for RAID5/6
> > > > > > > > > profiles.
> > > > > > > > > 
> > > > > > > > > Fix it by not keeping old rbio in unlock_stripe(), so we just free the
> > > > > > > > > finished rbio and rbio->bbio, so above problem wont' happen.
> > > > > > > > > 
> > > > > > > > 
> > > > > > > > I don't think this is acceptable, keeping a cache is important for
> > > > > > > > raid56 write performance, could you please fix it by checking if the
> > > > > > > > device is missing?
> > > > > > > 
> > > > > > > Not possible, as it's keeping the btrfs_device pointer and never release it,
> > > > > > > the stolen rbio can be kept forever until umount.
> > > > > > > 
> > > > > > 
> > > > > > steal_rbio() only takes pages from rbio->stripe_pages, so the cached
> > > > > > rbio->bbio is not going to the next IO's rbio because the cached one
> > > > > > got freed immediately in steal_rbio(), where could we dereference
> > > > > > rbio->bbio?
> > > > > 
> > > > > Did you mean in unlock_stripe(), we still keep the rbio in cache, while
> > > > > release its bbio?
> > > > > 
> > > > 
> > > > I thought it was lock_stripe_add(), OK, so unlock_stripe() just caches
> > > > the current rbio and doens't free it.  But the point about
> > > > steal_rbio() still stands, steal_rbio() is supposed to take uptodate
> > > > pages from the cached old rbio to the current processing rbio, but it
> > > > doesn't touch the cached old rbio's bbio, nor uses the cached old rbio
> > > > afterwards, instead it is the current processing rbio that would use
> > > > its bbio for writing into target devices, but it has increased its own
> > > > bio_counter.
> > > > 
> > > > > This sounds quite good but I'm afraid it may cause more problems.
> > > > > 
> > > > > Quite a lot of places are accessing rbio->bbio either for their btrfs
> > > > > logical address or stripes or even stripe->dev.
> > > > > 
> > > > 
> > > > I'm confused, could you please specify the call trace of general
> > > > protection you got in the commit log?
> > > 
> > > The 4th and 5th patches are designed to fix the same problem.
> > > 
> > > If one applies 5th patch only and run btrfs/069, it will cause hang when
> > > aborting replace, since btrfs_device of dev 0 is hold in bbio->stripes[] and
> > > never get released.
> > > 
> > > The 4th patch is used to solve such hang.
> > > 
> > 
> > OK, I see.
> > 
> > Firstly, the above commit log is misleading people a bit because it
> > says that steal_rbio() dereferences the device of the cached rbio and
> > that device has got free'd, but steal_rbio() actually doesn't.  Yes,
> > the cached rbio holds a reference on the free'd device, but I think
> > the below 'NULL pointer dereference' comes from writing back pages
> > into target devices when doing RMW with the current rbio instead of
> > the old cached one, right?
> 
> Yes, steal_bio() is not related to this problem, sorry for the confusion.
> 
> > 
> > Secondly, if it is the current rio that ends up this 'NULL pointer
> > dereference', it could hold a bio_counter and let the replace thread
> > canceled by scrub wait for this bio_counter to be zero.  Although
> > btrfs_dev_replace_finishing() has flushed delalloc IO and committed
> > transaction, seems like scrub is an exception because it can continued
> > after committing transaction.
> 
> If I understand it correctly, did you mean hold bio_counter when rbio holds
> bbio?
>

We have bio_counter to prevent the race against a successful dev
replace (such as btrfs_map_bio and btrfs_discard_extent), but for a
canceled dev replace (either it's canceled by scrub or by 'btrfs
replace cancel'), bio_counter could also be utilized to avoid the
race.

Now that scrub, which does raid parity recover, is the only one
without increasing bio_counter while accessing devices and stripes.

> That's OK, but we still need the 4th patch, or it will block replace cancel
> forever.
>

It is not the current code but patch 5 that needs patch 4 to avoid
blocking replace threads.

> > 
> > Thirdly, it is possible that this canceled dev-replace could make
> > fstrim get a 'general protection' or 'NULL pointer dereference' since
> > it could access target devices and is not sychoronized by committing
> > transaction.
> 
> So I'm using the refcount for btrfs_device to do full protection for it.
> As long as btrfs_dev can only be freed when no on holds it, instead of no
> bio pending for it, it should be safer.
>

If increading bio_counter could fix the bug like other use-after-free dev-replace bugs, e.g.
4245215d6a8d Btrfs, raid56: fix use-after-free problem in the final device replace procedure on raid56
,
I'd prefer it, a fix with keeping the benefit of caching rbio.

> > 
> > Please correct me if I'm wrong since I failed to reproduce it.
> 
> The bug is harder to trigger in v4.11-rc2 now.
> 
> I modified btrfs/069 to make it easier to trigger, but it's still quite hard
> to reproduce it.
> 
> Even with modification, the possibility is still low, at about 10~20%.
> 
> Hopes the diff could help you to trigger it.

I'm using -n 2000 but no luck.

Thanks,

-liubo

> 
> Thanks,
> Qu
> 
> diff --git a/tests/btrfs/069 b/tests/btrfs/069
> index d939cd8..f9ff945 100755
> --- a/tests/btrfs/069
> +++ b/tests/btrfs/069
> @@ -74,7 +74,7 @@ run_test()
>         _scratch_mount >>$seqres.full 2>&1
>         SCRATCH_DEV_POOL=$saved_scratch_dev_pool
> 
> -       args=`_scale_fsstress_args -p 20 -n 100 $FSSTRESS_AVOID -d
> $SCRATCH_MNT/stressdir`
> +       args=`_scale_fsstress_args -p 20 -n 1000 $FSSTRESS_AVOID -d
> $SCRATCH_MNT/stressdir`
>         echo "Run fsstress $args" >>$seqres.full
>         $FSSTRESS_PROG $args >/dev/null 2>&1 &
>         fsstress_pid=$!
> @@ -115,9 +115,10 @@ run_test()
>  }
> 
>  echo "Silence is golden"
> -for t in "${_btrfs_profile_configs[@]}"; do
> -       run_test "$t"
> -done
> +#for t in "${_btrfs_profile_configs[@]}"; do
> +#      run_test "$t"
> +#done
> +run_test "-d raid5 -m raid5"
> 
>  status=0
>  exit
> 
> > 
> > Thanks,
> > 
> > -liubo
> > 
> > > And the kernel NULL pointer access is like this when running modified
> > > btrfs/069 (only run on RAID5, and improve the duration of fsstress):
> > > 
> > > [  884.877421] BUG: unable to handle kernel NULL pointer dereference at
> > > 00000000000005e0
> > > [  884.878206] IP: generic_make_request_checks+0x4d/0x610
> > > [  884.878541] PGD 2d45a067
> > > [  884.878542] PUD 3ba0e067
> > > [  884.878857] PMD 0
> > > [  884.879189]
> > > [  884.879899] Oops: 0000 [#1] SMP
> > > [  884.880207] Modules linked in: btrfs(O) ext4 jbd2 mbcache xor raid6_pq
> > > netconsole xfs [last unloaded: btrfs]
> > > [  884.880845] CPU: 1 PID: 11676 Comm: kworker/u4:14 Tainted: G  O
> > > 4.11.0-rc2 #72
> > > [  884.881455] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
> > > 1.10.2-20170228_101828-anatol 04/01/2014
> > > [  884.882119] Workqueue: btrfs-endio-raid56 btrfs_endio_raid56_helper
> > > [btrfs]
> > > [  884.883089] task: ffff88002875b4c0 task.stack: ffffc90001334000
> > > [  884.883527] RIP: 0010:generic_make_request_checks+0x4d/0x610
> > > [  884.883855] RSP: 0018:ffffc90001337bb8 EFLAGS: 00010283
> > > [  884.884186] RAX: 0000000000000000 RBX: ffff8800126503e8 RCX:
> > > 0000000000218800
> > > [  884.884539] RDX: 0000000000000040 RSI: 0000000000000001 RDI:
> > > ffff88003d8116c0
> > > [  884.884897] RBP: ffffc90001337c18 R08: 0000000000000001 R09:
> > > 0000000000000001
> > > [  884.885778] R10: 0000000000000000 R11: 00000000000162b9 R12:
> > > 0000000000000040
> > > [  884.886346] R13: ffff8800126503e8 R14: 00000000ffffffff R15:
> > > 0000000000000010
> > > [  884.887146] FS:  0000000000000000(0000) GS:ffff88003e400000(0000)
> > > knlGS:0000000000000000
> > > [  884.888457] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > [  884.888792] CR2: 00000000000005e0 CR3: 000000003ad30000 CR4:
> > > 00000000000006e0
> > > [  884.889212] Call Trace:
> > > [  884.889526]  ? generic_make_request+0xc7/0x360
> > > [  884.889841]  generic_make_request+0x24/0x360
> > > [  884.890163]  ? generic_make_request+0xc7/0x360
> > > [  884.890486]  submit_bio+0x64/0x120
> > > [  884.890828]  ? page_in_rbio+0x4d/0x80 [btrfs]
> > > [  884.891206]  ? rbio_orig_end_io+0x80/0x80 [btrfs]
> > > [  884.891543]  finish_rmw+0x3f4/0x540 [btrfs]
> > > [  884.891875]  validate_rbio_for_rmw+0x36/0x40 [btrfs]
> > > [  884.892213]  raid_rmw_end_io+0x7a/0x90 [btrfs]
> > > [  884.892565]  bio_endio+0x56/0x60
> > > [  884.892891]  end_workqueue_fn+0x3c/0x40 [btrfs]
> > > [  884.893265]  btrfs_scrubparity_helper+0xef/0x620 [btrfs]
> > > [  884.893698]  btrfs_endio_raid56_helper+0xe/0x10 [btrfs]
> > > [  884.894101]  process_one_work+0x2af/0x720
> > > [  884.894837]  ? process_one_work+0x22b/0x720
> > > [  884.895278]  worker_thread+0x4b/0x4f0
> > > [  884.895760]  kthread+0x10f/0x150
> > > [  884.896106]  ? process_one_work+0x720/0x720
> > > [  884.896448]  ? kthread_create_on_node+0x40/0x40
> > > [  884.896803]  ret_from_fork+0x2e/0x40
> > > [  884.897148] Code: 67 28 48 c7 c7 27 7f c9 81 e8 90 6c d4 ff e8 0b bb 54
> > > 00 41 c1 ec 09 48 8b 7b 08 45 85 e4 0f 85 be 00 00 00 48 8b 87 00 01 00 00
> > > <4c> 8b b0 e0 05 00 00 4d 85 f6 0f 84 86 01 00 00 4c 8b af f0 00
> > > [  884.898449] RIP: generic_make_request_checks+0x4d/0x610 RSP:
> > > ffffc90001337bb8
> > > [  884.899223] CR2: 00000000000005e0
> > > [  884.900223] ---[ end trace 307e118b57a9995e ]---
> > > 
> > > Thanks,
> > > Qu
> > > 
> > > > 
> > > > I wonder if patch 4 and 5 are fixing the same use-after-free problem?
> > > > 
> > > > Thanks,
> > > > 
> > > > -liubo
> > > > 
> > > > > Thanks,
> > > > > Qu
> > > > > 
> > > > > 
> > > > > > 
> > > > > > Thanks,
> > > > > > 
> > > > > > -liubo
> > > > > > 
> > > > > > > And I think the logical is very strange, if RAID5/6 is unstable, there is no
> > > > > > > meaning to keep it fast.
> > > > > > > 
> > > > > > > Keep it stable first, and then consider the performance.
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > Qu
> > > > > > > 
> > > > > > > > 
> > > > > > > > Thanks,
> > > > > > > > 
> > > > > > > > -liubo
> > > > > > > > 
> > > > > > > > > Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> > > > > > > > > ---
> > > > > > > > >  fs/btrfs/raid56.c | 18 +-----------------
> > > > > > > > >  1 file changed, 1 insertion(+), 17 deletions(-)
> > > > > > > > > 
> > > > > > > > > diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> > > > > > > > > index 453eefdcb591..aba82b95ec73 100644
> > > > > > > > > --- a/fs/btrfs/raid56.c
> > > > > > > > > +++ b/fs/btrfs/raid56.c
> > > > > > > > > @@ -776,7 +776,6 @@ static noinline void unlock_stripe(struct btrfs_raid_bio *rbio)
> > > > > > > > >  	int bucket;
> > > > > > > > >  	struct btrfs_stripe_hash *h;
> > > > > > > > >  	unsigned long flags;
> > > > > > > > > -	int keep_cache = 0;
> > > > > > > > > 
> > > > > > > > >  	bucket = rbio_bucket(rbio);
> > > > > > > > >  	h = rbio->fs_info->stripe_hash_table->table + bucket;
> > > > > > > > > @@ -788,19 +787,6 @@ static noinline void unlock_stripe(struct btrfs_raid_bio *rbio)
> > > > > > > > >  	spin_lock(&rbio->bio_list_lock);
> > > > > > > > > 
> > > > > > > > >  	if (!list_empty(&rbio->hash_list)) {
> > > > > > > > > -		/*
> > > > > > > > > -		 * if we're still cached and there is no other IO
> > > > > > > > > -		 * to perform, just leave this rbio here for others
> > > > > > > > > -		 * to steal from later
> > > > > > > > > -		 */
> > > > > > > > > -		if (list_empty(&rbio->plug_list) &&
> > > > > > > > > -		    test_bit(RBIO_CACHE_BIT, &rbio->flags)) {
> > > > > > > > > -			keep_cache = 1;
> > > > > > > > > -			clear_bit(RBIO_RMW_LOCKED_BIT, &rbio->flags);
> > > > > > > > > -			BUG_ON(!bio_list_empty(&rbio->bio_list));
> > > > > > > > > -			goto done;
> > > > > > > > > -		}
> > > > > > > > > -
> > > > > > > > >  		list_del_init(&rbio->hash_list);
> > > > > > > > >  		atomic_dec(&rbio->refs);
> > > > > > > > > 
> > > > > > > > > @@ -848,13 +834,11 @@ static noinline void unlock_stripe(struct btrfs_raid_bio *rbio)
> > > > > > > > >  			goto done_nolock;
> > > > > > > > >  		}
> > > > > > > > >  	}
> > > > > > > > > -done:
> > > > > > > > >  	spin_unlock(&rbio->bio_list_lock);
> > > > > > > > >  	spin_unlock_irqrestore(&h->lock, flags);
> > > > > > > > > 
> > > > > > > > >  done_nolock:
> > > > > > > > > -	if (!keep_cache)
> > > > > > > > > -		remove_rbio_from_cache(rbio);
> > > > > > > > > +	remove_rbio_from_cache(rbio);
> > > > > > > > >  }
> > > > > > > > > 
> > > > > > > > >  static void __free_raid_bio(struct btrfs_raid_bio *rbio)
> > > > > > > > > --
> > > > > > > > > 2.11.0
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > --
> > > > > > > > > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> > > > > > > > > the body of a message to majordomo@vger.kernel.org
> > > > > > > > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
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