[SRU][Xenial][PATCH 1/1] ext4: fix false negatives *and* false positives in ext4_check_descriptors()

Joseph Salisbury joseph.salisbury at canonical.com
Fri Aug 31 14:14:59 UTC 2018

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

BugLink: https://bugs.launchpad.net/bugs/1789653

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

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
(backported from commit 44de022c4382541cebdd6de4465d1f4f465ff1dd)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
 fs/ext4/super.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index c507033..577a14e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2125,7 +2125,7 @@ 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;
@@ -3810,13 +3810,13 @@ 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!");
 		goto failed_mount2;
-	sbi->s_gdb_count = db_count;
 	get_random_bytes(&sbi->s_next_generation, sizeof(u32));

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