[REGRESSION/bionic] ext4: fix false negatives *and* false positives in ext4_check_descriptors()

[Stefan Bader]

From: Theodore Ts'o <tytso at mit.edu>

BugLink: https://bugs.launchpad.net/bugs/1813727

Ext4_check_descriptors() was getting called before s_gdb_count was
initialized.  So for file systems w/o the meta_bg feature, allocation
bitmaps could overlap the block group descriptors and ext4 wouldn't
notice.

For file systems with the meta_bg feature enabled, there was a
fencepost error which would cause the ext4_check_descriptors() to
incorrectly believe that the block allocation bitmap overlaps with the
block group descriptor blocks, and it would reject the mount.

Fix both of these problems.

Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Cc: stable at vger.kernel.org

(cherry picked from commit 44de022c4382541cebdd6de4465d1f4f465ff1dd)
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---

This likely was introduced via upstream stable 2018-12-12
e1d8355dd9508466024730813a84c8ee386d7b65.

 fs/ext4/super.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 6503bbac60cb..4288dede4c69 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2328,7 +2328,7 @@ static int ext4_check_descriptors(struct super_block *sb,
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	ext4_fsblk_t first_block = le32_to_cpu(sbi->s_es->s_first_data_block);
 	ext4_fsblk_t last_block;
-	ext4_fsblk_t last_bg_block = sb_block + ext4_bg_num_gdb(sb, 0) + 1;
+	ext4_fsblk_t last_bg_block = sb_block + ext4_bg_num_gdb(sb, 0);
 	ext4_fsblk_t block_bitmap;
 	ext4_fsblk_t inode_bitmap;
 	ext4_fsblk_t inode_table;
@@ -4083,14 +4083,13 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 			goto failed_mount2;
 		}
 	}
+	sbi->s_gdb_count = db_count;
 	if (!ext4_check_descriptors(sb, logical_sb_block, &first_not_zeroed)) {
 		ext4_msg(sb, KERN_ERR, "group descriptors corrupted!");
 		ret = -EFSCORRUPTED;
 		goto failed_mount2;
 	}
 
-	sbi->s_gdb_count = db_count;
-
 	timer_setup(&sbi->s_err_report, print_daily_error_info, 0);
 
 	/* Register extent status tree shrinker */
-- 
2.17.1