ext4: fix potential use after free after remounting with noblock_validity

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@syzkaller.appspotmail.com
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

1 files changed, 9 insertions, 1 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 0664c43cc9dc..c35bb8d734df 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -185,6 +185,14 @@ struct ext4_map_blocks {
 };
 
 /*
+ * Block validity checking, system zone rbtree.
+ */
+struct ext4_system_blocks {
+        struct rb_root root;
+        struct rcu_head rcu;
+};
+
+/*
  * Flags for ext4_io_end->flags
  */
 #define EXT4_IO_END_UNWRITTEN   0x0001
@@ -1431,7 +1439,7 @@ struct ext4_sb_info {
         int s_jquota_fmt;                       /* Format of quota to use */
 #endif
         unsigned int s_want_extra_isize; /* New inodes should reserve # bytes */
-        struct rb_root system_blks;
+        struct ext4_system_blocks __rcu *system_blks;
 
 #ifdef EXTENTS_STATS
         /* ext4 extents stats */