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Date:   Thu, 26 May 2022 15:32:22 +0800
From:   Zhang Yi <>
To:     <>
CC:     <>, <>, <>,
        <>, <>,
        <>, <>
Subject: [PATCH] ext4: add reserved GDT blocks check

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_fs() to make sure that the
es->s_reserved_gdt_blocks is zero when the resize_inode feature is

Signed-off-by: Zhang Yi <>
 fs/ext4/resize.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index 90a941d20dff..5791eb7c0761 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -2031,6 +2031,9 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 			ext4_warning(sb, "Error opening resize inode");
 			return PTR_ERR(resize_inode);
+	} else if (es->s_reserved_gdt_blocks) {
+		ext4_error(sb, "resize_inode disabled but reserved GDT blocks non-zero");
+		return -EFSCORRUPTED;
 	if ((!resize_inode && !meta_bg) || n_blocks_count == o_blocks_count) {

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