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Date:   Wed, 1 Jun 2022 17:27:17 +0800
From:   Zhang Yi <>
To:     <>
CC:     <>, <>, <>,
        <>, <>,
Subject: [PATCH v2] 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_begin() to make sure that
the es->s_reserved_gdt_blocks is zero when the resize_inode feature is

Signed-off-by: Zhang Yi <>
 - move check from ext4_resize_fs() to ext4_resize_begin().

 fs/ext4/resize.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index 90a941d20dff..8b70a4701293 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -53,6 +53,16 @@ int ext4_resize_begin(struct super_block *sb)
 	if (!capable(CAP_SYS_RESOURCE))
 		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");
+		return -EFSCORRUPTED;
+	}
 	 * 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

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