[PATCH] ext4: add reserved GDT blocks check

[Zhang Yi <yi.zhang@huawei.com>]

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
descriptors.

 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:
  <TASK>
  ext4_resize_fs+0xbec/0x1660
  __ext4_ioctl+0x1749/0x24e0
  ext4_ioctl+0x12/0x20
  __x64_sys_ioctl+0xa6/0x110
  do_syscall_64+0x3b/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 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
disabled.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 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) {
-- 
2.31.1