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

BugLink: http://bugs.launchpad.net/bugs/1768429

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.

Backport notes:
3.18.y is missing bc98a42c ("VFS: Convert sb->s_flags & MS_RDONLY to sb_rdonly(sb)")
and e462ec50 ("VFS: Differentiate mount flags (MS_*) from internal superblock flags")
so we simply use the sb MS_RDONLY check from pre bc98a42c in place of the sb_rdonly
function used in the upstream variant of the patch.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Harsh Shandilya <harsh@prjkt.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/ext4/super.c

@@ -2154,6 +2154,8 @@ 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->s_flags & MS_RDONLY))
+				return 0;
 		}
 		if (block_bitmap < first_block || block_bitmap > last_block) {
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
@@ -2166,6 +2168,8 @@ 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->s_flags & MS_RDONLY))
+				return 0;
 		}
 		if (inode_bitmap < first_block || inode_bitmap > last_block) {
 			ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
@@ -2178,6 +2182,8 @@ 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->s_flags & MS_RDONLY))
+				return 0;
 		}
 		if (inode_table < first_block ||
 		    inode_table + sbi->s_itb_per_group - 1 > last_block) {