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Date:   Wed, 31 Jul 2019 16:08:21 +0200
From:   Jan Kara <jack@suse.cz>
To:     "zhangyi (F)" <yi.zhang@huawei.com>
Cc:     linux-ext4@vger.kernel.org, tytso@mit.edu, jack@suse.cz,
        adilger.kernel@dilger.ca
Subject: Re: [PATCH] ext4: fix potential use after free in system zone via
 remount with noblock_validity
Message-ID: <20190731140821.GF15806@quack2.suse.cz>
References: <1563970268-33688-1-git-send-email-yi.zhang@huawei.com>
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On Wed 24-07-19 20:11:08, 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 whit 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 patch lock the system zone when accessing it to prevent it being
> released when doing a remount with "noblock_validity" mount option.
> 
> Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
> Cc: stable@vger.kernel.org

Thanks for the patch. It is a good catch. Some small comments below.

> diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
> index 8e83741..d9c4792 100644
> --- a/fs/ext4/block_validity.c
> +++ b/fs/ext4/block_validity.c
> @@ -191,7 +191,7 @@ int ext4_setup_system_zone(struct super_block *sb)
>  
>  	if (!test_opt(sb, BLOCK_VALIDITY)) {
>  		if (sbi->system_blks.rb_node)
> -			ext4_release_system_zone(sb);
> +			ext4_release_system_zone_lock(sb);
>  		return 0;
>  	}
>  	if (sbi->system_blks.rb_node)
> @@ -239,6 +239,14 @@ void ext4_release_system_zone(struct super_block *sb)
>  	EXT4_SB(sb)->system_blks = RB_ROOT;
>  }
>  
> +/* Called when (re)mounting the filesystem without BLOCK_VALIDITY */
> +void ext4_release_system_zone_lock(struct super_block *sb)
> +{
> +	spin_lock(&EXT4_SB(sb)->system_blks_lock);
> +	ext4_release_system_zone(sb);
> +	spin_unlock(&EXT4_SB(sb)->system_blks_lock);
> +}

Is there any reason why ext4_release_system_zone() should not always take
the system_blks_lock lock? I understand it may not be necessary in all the
cases but it won't hurt either...

Also ext4_setup_system_zone() should IMO use system_blks_lock to protect
modifications of the rbtree. It can get called during remount as well so
there can be racing ext4_data_block_valid() reading the rbtree at the same
time.

> @@ -256,6 +264,13 @@ int ext4_data_block_valid(struct ext4_sb_info *sbi, ext4_fsblk_t start_blk,
>  		sbi->s_es->s_last_error_block = cpu_to_le64(start_blk);
>  		return 0;
>  	}
> +
> +	/*
> +	 * Lock the system zone to prevent it being released concurrently
> +	 * when doing a remount with "noblock_validity" mount option.
> +	 */
> +	spin_lock(&sbi->system_blks_lock);
> +	n = sbi->system_blks.rb_node;
>  	while (n) {
>  		entry = rb_entry(n, struct ext4_system_zone, node);
>  		if (start_blk + count - 1 < entry->start_blk)
> @@ -264,9 +279,11 @@ int ext4_data_block_valid(struct ext4_sb_info *sbi, ext4_fsblk_t start_blk,
>  			n = n->rb_right;
>  		else {
>  			sbi->s_es->s_last_error_block = cpu_to_le64(start_blk);
> +			spin_unlock(&sbi->system_blks_lock);
>  			return 0;
>  		}
>  	}
> +	spin_unlock(&sbi->system_blks_lock);
>  	return 1;
>  }

So this will not only serialize ext4_data_block_valid() against remounts
but also against each other. So I suspect that a read-heavy workload on
fast storage could contend on your new fs-wide spinlock. So I think it
would be better to have some other synchronization scheme to avoid the
race.

If nothing else, rwlock_t would allow concurrent ext4_data_block_valid()
calls. It is still not ideal as the calls would be still bouncing around
the cacheline when updating the lock itself but better than nothing.

Ideal (performance-wise) would be to use RCU scheme for this -
ext4_data_block_valid() would be RCU protected when reading the RB-tree,
teardown of the block validity information would clear
sbi->system_blks.rb_node and then defer actual freeing of the tree nodes to
RCU callback. Setup would first construct the rbtree and then just set
sbi->system_blks.rb_node to the root of the constructed tree.

That being said I'm not *sure* this is going to be a performance issue
since ext4_map_blocks() are not that frequent and the lock hold times will
be very short (needs testing). So maybe rwlock_t is a reasonable compromise
between complexity and performance.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR