Re: [PATCH v4] ext4: fix potential use after free in system zone via remount with noblock_validity

["zhangyi (F)" <yi.zhang@huawei.com>]

On 2019/8/15 17:35, Jan Kara Wrote:
> On Thu 15-08-19 16:16:31, 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.
>>
>> Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
>> ---
>> Changes since v3:
>>  - add comments before ext4_setup_system_zone() and
>>    ext4_release_system_zone() to explain why we need to serializes update
>>    sbi->system_blks pointer.
>>  - Fix block validity checking logic changes in v3.
> 
> Thanks for the patch! The patch looks good. Just some language fixes in the
> new comments below.  You can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> 
>> +/*
>> + * Build system zone rbtree which is used for block validity checking.
>> + *
>> + * Note that system_blks pointer should be serializes updated at remount
>> + * time even under sb->s_umount semaphore protection, due to it can be
>> + * racing with ext4_data_block_valid() reading the system_blks rbtree at
>> + * the same time.
> 
> I'd rephrase this paragraph a bit to be easier to understand:
> 
> The update of system_blks pointer in this function is protected by
> sb->s_umount semaphore. However we have to be careful as we can be racing
> with ext4_data_block_valid() calls reading system_blks rbtree protected
> only by RCU. That's why we first build the rbtree and then swap it in place.
> 
>> -/* Called when the filesystem is unmounted */
>> +/*
>> + * Called when the filesystem is unmounted or when remounting it with
>> + * noblock_validity specified.
>> + *
>> + * Note that system_blks pointer should be serializes updated and do
>> + * the actual freeing after the RCU grace period at remount time even
>> + * under sb->s_umount semaphore protection, due to it can be racing with
>> + * ext4_data_block_valid() reading the system_blks rbtree at the same
>> + * time.
>> + */
> 
> Similarly here I'd phrase the last paragraph as:
> 
> The update of system_blks pointer in this function is protected by
> sb->s_umount semaphore. However we have to be careful as we can be racing
> with ext4_data_block_valid() calls reading system_blks rbtree protected
> only by RCU. So we first clear the system_blks pointer and then free the
> rbtree only after RCU grace period expires.
> 

Yes, it looks better, will do.

Thanks,
Yi.