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Date:   Wed, 28 Aug 2019 11:15:00 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     "zhangyi (F)" <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, jack@...e.cz, adilger.kernel@...ger.ca
Subject: Re: [PATCH v2] ext4: fix potential use after free after remounting
 with noblock_validity

On Tue, Aug 27, 2019 at 08:08:39PM +0800, zhangyi (F) wrote:
> Remount process will release system zone which was allocated before if
> "noblock_validity" is specified. If we mount an ext4 file system to two
> mountpoints with default mount options, and then remount one of them
> with "noblock_validity", it may trigger a use after free problem when
> someone accessing the other one.
> 
>  # mount /dev/sda foo
>  # mount /dev/sda bar
> 
> User access mountpoint "foo"   |   Remount mountpoint "bar"
>                                |
> ext4_map_blocks()              |   ext4_remount()
> check_block_validity()         |   ext4_setup_system_zone()
> ext4_data_block_valid()        |   ext4_release_system_zone()
>                                |   free system_blks rb nodes
> access system_blks rb nodes    |
> trigger use after free         |
> 
> This problem can also be reproduced by one mountpint, At the same time,
> add_system_zone() can get called during remount as well so there can be
> racing ext4_data_block_valid() reading the rbtree at the same time.
> 
> This patch add RCU to protect system zone from releasing or building
> when doing a remount which inverse current "noblock_validity" mount
> option. It assign the rbtree after the whole tree was complete and
> do actual freeing after rcu grace period, avoid any intermediate state.
> 
> Reported-by: syzbot+1e470567330b7ad711d5@...kaller.appspotmail.com
> Signed-off-by: zhangyi (F) <yi.zhang@...wei.com>
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> Reviewed-by: Jan Kara <jack@...e.cz>

Applied, thanks!

						- Ted

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