[PATCH 3.16 28/63] ext4: don't allow r/w mounts if metadata blocks overlap the superblock

[Ben Hutchings]

3.16.58-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@xxxxxxx>

commit 18db4b4e6fc31eda838dd1c1296d67dbcb3dc957 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 bc98a42c1f7d ("VFS: Convert sb->s_flags & MS_RDONLY
to sb_rdonly(sb)") and e462ec50cb5f ("VFS: Differentiate mount flags
(MS_*) from internal superblock flags") so we simply use the sb
MS_RDONLY check from pre bc98a42c1f7d in place of the sb_rdonly
function used in the upstream variant of the patch.

Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Harsh Shandilya <harsh@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ext4/super.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2115,6 +2115,8 @@ static int ext4_check_descriptors(struct
 			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: "
@@ -2127,6 +2129,8 @@ static int ext4_check_descriptors(struct
 			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: "
@@ -2139,6 +2143,8 @@ static int ext4_check_descriptors(struct
 			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) {