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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
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@xxxxxxxx>
SUSE Labs, CR
