ext4: don't allow r/w mounts if metadata blocks overlap the superblock

commit 18db4b4e upstream.

If some metadata block, such as an allocation bitmap, overlaps the
superblock, it's very likely that if the file system is mounted
read/write, the results will not be pretty.  So disallow r/w mounts
for file systems corrupted in this particular way.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ext4/super.c

@@ -2330,6 +2330,8 @@ static int ext4_check_descriptors(struct super_block *sb,
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
 				 "Block bitmap for group %u overlaps "
 				 "superblock", i);
+			if (!sb_rdonly(sb))
+				return 0;
 		}
 		if (block_bitmap < first_block || block_bitmap > last_block) {
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
@@ -2342,6 +2344,8 @@ static int ext4_check_descriptors(struct super_block *sb,
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
 				 "Inode bitmap for group %u overlaps "
 				 "superblock", i);
+			if (!sb_rdonly(sb))
+				return 0;
 		}
 		if (inode_bitmap < first_block || inode_bitmap > last_block) {
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
@@ -2354,6 +2358,8 @@ static int ext4_check_descriptors(struct super_block *sb,
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
 				 "Inode table for group %u overlaps "
 				 "superblock", i);
+			if (!sb_rdonly(sb))
+				return 0;
 		}
 		if (inode_table < first_block ||
 		    inode_table + sbi->s_itb_per_group - 1 > last_block) {