ext4: Make the extent validity check more paranoid

Instead of just checking that the extent block number is greater or
equal than s_first_data_block, make sure it it is not pointing into
the block group descriptors, since that is clearly wrong.  This helps
prevent filesystem from getting very badly corrupted in case an extent
block is corrupted.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -326,10 +326,13 @@ ext4_ext_max_entries(struct inode *inode, int depth)
 
 static int ext4_valid_extent(struct inode *inode, struct ext4_extent *ext)
 {
-	ext4_fsblk_t block = ext_pblock(ext);
+	ext4_fsblk_t block = ext_pblock(ext), valid_block;
 	int len = ext4_ext_get_actual_len(ext);
 	struct ext4_super_block *es = EXT4_SB(inode->i_sb)->s_es;
-	if (unlikely(block < le32_to_cpu(es->s_first_data_block) ||
+
+	valid_block = le32_to_cpu(es->s_first_data_block) +
+		EXT4_SB(inode->i_sb)->s_gdb_count;
+	if (unlikely(block <= valid_block ||
 		     ((block + len) > ext4_blocks_count(es))))
 		return 0;
 	else
@@ -339,9 +342,12 @@ static int ext4_valid_extent(struct inode *inode, struct ext4_extent *ext)
 static int ext4_valid_extent_idx(struct inode *inode,
 				struct ext4_extent_idx *ext_idx)
 {
-	ext4_fsblk_t block = idx_pblock(ext_idx);
+	ext4_fsblk_t block = idx_pblock(ext_idx), valid_block;
 	struct ext4_super_block *es = EXT4_SB(inode->i_sb)->s_es;
-	if (unlikely(block < le32_to_cpu(es->s_first_data_block) ||
+
+	valid_block = le32_to_cpu(es->s_first_data_block) +
+		EXT4_SB(inode->i_sb)->s_gdb_count;
+	if (unlikely(block <= valid_block ||
 		     (block >= ext4_blocks_count(es))))
 		return 0;
 	else