Squashfs: handle corruption of directory structure

Handle the rare case where a directory metadata block is uncompressed and
corrupted, leading to a kernel oops in directory scanning (memcpy).
Normally corruption is detected at the decompression stage and dealt with
then, however, this will not happen if:

- metadata isn't compressed (users can optionally request no metadata
  compression), or
- the compressed metadata block was larger than the original, in which
  case the uncompressed version was used, or
- the data was corrupt after decompression

This patch fixes this by adding some sanity checks against known maximum
values.
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

fs/squashfs/dir.c

@@ -172,6 +172,11 @@ static int squashfs_readdir(struct file *file, void *dirent, filldir_t filldir)
 		length += sizeof(dirh);
 
 		dir_count = le32_to_cpu(dirh.count) + 1;
+
+		/* dir_count should never be larger than 256 */
+		if (dir_count > 256)
+			goto failed_read;
+
 		while (dir_count--) {
 			/*
 			 * Read directory entry.
@@ -183,6 +188,10 @@ static int squashfs_readdir(struct file *file, void *dirent, filldir_t filldir)
 
 			size = le16_to_cpu(dire->size) + 1;
 
+			/* size should never be larger than SQUASHFS_NAME_LEN */
+			if (size > SQUASHFS_NAME_LEN)
+				goto failed_read;
+
 			err = squashfs_read_metadata(inode->i_sb, dire->name,
 					&block, &offset, size);
 			if (err < 0)

fs/squashfs/namei.c

@@ -176,6 +176,11 @@ static struct dentry *squashfs_lookup(struct inode *dir, struct dentry *dentry,
 		length += sizeof(dirh);
 
 		dir_count = le32_to_cpu(dirh.count) + 1;
+
+		/* dir_count should never be larger than 256 */
+		if (dir_count > 256)
+			goto data_error;
+
 		while (dir_count--) {
 			/*
 			 * Read directory entry.
@@ -187,6 +192,10 @@ static struct dentry *squashfs_lookup(struct inode *dir, struct dentry *dentry,
 
 			size = le16_to_cpu(dire->size) + 1;
 
+			/* size should never be larger than SQUASHFS_NAME_LEN */
+			if (size > SQUASHFS_NAME_LEN)
+				goto data_error;
+
 			err = squashfs_read_metadata(dir->i_sb, dire->name,
 					&block, &offset, size);
 			if (err < 0)
@@ -228,6 +237,9 @@ static struct dentry *squashfs_lookup(struct inode *dir, struct dentry *dentry,
 	d_add(dentry, inode);
 	return ERR_PTR(0);
 
+data_error:
+	err = -EIO;
+
 read_failure:
 	ERROR("Unable to read directory block [%llx:%x]\n",
 		squashfs_i(dir)->start + msblk->directory_table,