[PATCH 3.2 011/152] ext4: validate s_reserved_gdt_blocks on mount

[Ben Hutchings]

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

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

From: Theodore Ts'o <tytso@xxxxxxx>

commit 5b9554dc5bf008ae7f68a52e3d7e76c0920938a2 upstream.

If s_reserved_gdt_blocks is extremely large, it's possible for
ext4_init_block_bitmap(), which is called when ext4 sets up an
uninitialized block bitmap, to corrupt random kernel memory.  Add the
same checks which e2fsck has --- it must never be larger than
blocksize / sizeof(__u32) --- and then add a backup check in
ext4_init_block_bitmap() in case the superblock gets modified after
the file system is mounted.

Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
[bwh: Backported to 3.2:
 - Drop the second check in ext4_init_block_bitmap() since it can't return
   an error code
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3429,6 +3429,13 @@ static int ext4_fill_super(struct super_
 		goto failed_mount;
 	}
 
+	if (le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) > (blocksize / 4)) {
+		ext4_msg(sb, KERN_ERR,
+			 "Number of reserved GDT blocks insanely large: %d",
+			 le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks));
+		goto failed_mount;
+	}
+
 	if (sb->s_blocksize != blocksize) {
 		/* Validate the filesystem blocksize */
 		if (!sb_set_blocksize(sb, blocksize)) {