ext4: recover from filesystem corruption when reading

Some fixes when reading EXT files and directory entries were identified
after using e2fuzz to corrupt an EXT3 filesystem:

 - Stop reading directory entries if the offset becomes badly aligned.

 - Avoid overwriting memory by clamping the length used to zero the buffer
   in ext4fs_read_file.  Also sanity check blocksize.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Reviewed-by: Stefano Babic <sbabic@denx.de>

fs/ext4/ext4_common.c

@@ -660,6 +660,11 @@ static int search_dir(struct ext2_inode *parent_inode, char *dirname)
 
 		offset = 0;
 		do {
+			if (offset & 3) {
+				printf("Badly aligned ext2_dirent\n");
+				break;
+			}
+
 			dir = (struct ext2_dirent *)(block_buffer + offset);
 			direntname = (char*)(dir) + sizeof(struct ext2_dirent);
 
@@ -880,6 +885,11 @@ static int unlink_filename(char *filename, unsigned int blknr)
 
 	offset = 0;
 	do {
+		if (offset & 3) {
+			printf("Badly aligned ext2_dirent\n");
+			break;
+		}
+
 		previous_dir = dir;
 		dir = (struct ext2_dirent *)(block_buffer + offset);
 		direntname = (char *)(dir) + sizeof(struct ext2_dirent);

fs/ext4/ext4fs.c

@@ -64,6 +64,9 @@ int ext4fs_read_file(struct ext2fs_node *node, loff_t pos,
 	char *delayed_buf = NULL;
 	short status;
 
+	if (blocksize <= 0)
+		return -1;
+
 	/* Adjust len so it we can't read past the end of the file. */
 	if (len + pos > filesize)
 		len = (filesize - pos);
@@ -127,6 +130,7 @@ int ext4fs_read_file(struct ext2fs_node *node, loff_t pos,
 					(blockend >> log2blksz);
 			}
 		} else {
+			int n;
 			if (previous_block_number != -1) {
 				/* spill */
 				status = ext4fs_devread(delayed_start,
@@ -137,7 +141,11 @@ int ext4fs_read_file(struct ext2fs_node *node, loff_t pos,
 					return -1;
 				previous_block_number = -1;
 			}
-			memset(buf, 0, blocksize - skipfirst);
+			/* Zero no more than `len' bytes. */
+			n = blocksize - skipfirst;
+			if (n > len)
+				n = len;
+			memset(buf, 0, n);
 		}
 		buf += blocksize - skipfirst;
 	}