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Subject[PATCH 4.14 225/237] ext4: add reserved GDT blocks check
From: Zhang Yi <>

commit b55c3cd102a6f48b90e61c44f7f3dda8c290c694 upstream.

We capture a NULL pointer issue when resizing a corrupt ext4 image which
is freshly clear resize_inode feature (not run e2fsck). It could be
simply reproduced by following steps. The problem is because of the
resize_inode feature was cleared, and it will convert the filesystem to
meta_bg mode in ext4_resize_fs(), but the es->s_reserved_gdt_blocks was
not reduced to zero, so could we mistakenly call reserve_backup_gdb()
and passing an uninitialized resize_inode to it when adding new group

mkfs.ext4 /dev/sda 3G
tune2fs -O ^resize_inode /dev/sda #forget to run requested e2fsck
mount /dev/sda /mnt
resize2fs /dev/sda 8G

BUG: kernel NULL pointer dereference, address: 0000000000000028
CPU: 19 PID: 3243 Comm: resize2fs Not tainted 5.18.0-rc7-00001-gfde086c5ebfd #748
RIP: 0010:ext4_flex_group_add+0xe08/0x2570
Call Trace:
RIP: 0033:0x7f2dd739617b

The fix is simple, add a check in ext4_resize_begin() to make sure that
the es->s_reserved_gdt_blocks is zero when the resize_inode feature is

Signed-off-by: Zhang Yi <>
Reviewed-by: Ritesh Harjani <>
Reviewed-by: Jan Kara <>
Signed-off-by: Theodore Ts'o <>
Signed-off-by: Greg Kroah-Hartman <>
fs/ext4/resize.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -53,6 +53,16 @@ int ext4_resize_begin(struct super_block
return -EPERM;

+ * If the reserved GDT blocks is non-zero, the resize_inode feature
+ * should always be set.
+ */
+ if (EXT4_SB(sb)->s_es->s_reserved_gdt_blocks &&
+ !ext4_has_feature_resize_inode(sb)) {
+ ext4_error(sb, "resize_inode disabled but reserved GDT blocks non-zero");
+ }
+ /*
* If we are not using the primary superblock/GDT copy don't resize,
* because the user tools have no way of handling this. Probably a
* bad time to do it anyways.

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