[PATCH] ext4: reject mount if clusters/inodes per group are not 8-aligned

[Baokun Li <libaokun@linux.alibaba.com>]

The block and inode bitmap checksums are computed over a whole number of
bytes: ext4_inode_bitmap_csum_*() use EXT4_INODES_PER_GROUP(sb) >> 3 and
ext4_block_bitmap_csum_*() use EXT4_CLUSTERS_PER_GROUP(sb) / 8 as the
length passed to ext4_chksum().

If s_inodes_per_group or s_clusters_per_group is not a multiple of 8, the
trailing fractional bits are excluded from the checksum.  Those bits are
then unprotected, and any incremental csum update path that assumes a
byte-aligned bitmap can compute a checksum inconsistent with the full
recalculation, corrupting the on-disk bitmap checksum.

Reject such filesystems at mount time by adding the missing " & 7"
alignment checks alongside the existing range validation.

Suggested-by: Theodore Ts'o <tytso@mit.edu>
Link: https://patch.msgid.link/h3n7jlfhyna64dn5o76qxcspnhxdddcs6crpxftmy7gnl7b3sx@jenszfpcsnit
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260508121539.4174601-1-libaokun%40linux.alibaba.com?part=10
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/ext4/super.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 6a77db4d3124..3daf4cdcf07e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4472,8 +4472,9 @@ static int ext4_handle_clustersize(struct super_block *sb)
 		sbi->s_cluster_bits = 0;
 	}
 	sbi->s_clusters_per_group = le32_to_cpu(es->s_clusters_per_group);
-	if (sbi->s_clusters_per_group > sb->s_blocksize * 8) {
-		ext4_msg(sb, KERN_ERR, "#clusters per group too big: %lu",
+	if (sbi->s_clusters_per_group > sb->s_blocksize * 8 ||
+	    sbi->s_clusters_per_group & 7) {
+		ext4_msg(sb, KERN_ERR, "invalid #clusters per group: %lu",
 			 sbi->s_clusters_per_group);
 		return -EINVAL;
 	}
@@ -5304,7 +5305,8 @@ static int ext4_block_group_meta_init(struct super_block *sb, int silent)
 		return -EINVAL;
 	}
 	if (sbi->s_inodes_per_group < sbi->s_inodes_per_block ||
-	    sbi->s_inodes_per_group > sb->s_blocksize * 8) {
+	    sbi->s_inodes_per_group > sb->s_blocksize * 8 ||
+	    sbi->s_inodes_per_group & 7) {
 		ext4_msg(sb, KERN_ERR, "invalid inodes per group: %lu\n",
 			 sbi->s_inodes_per_group);
 		return -EINVAL;
-- 
2.43.7