Squashfs: fix uninit-value in squashfs_get_parent

Syzkaller reports a "KMSAN: uninit-value in squashfs_get_parent" bug.

This is caused by open_by_handle_at() being called with a file handle
containing an invalid parent inode number. In particular the inode number
is that of a symbolic link, rather than a directory.

Squashfs_get_parent() gets called with that symbolic link inode, and
accesses the parent member field.

unsigned int parent_ino = squashfs_i(inode)->parent;

Because non-directory inodes in Squashfs do not have a parent value, this
is uninitialised, and this causes an uninitialised value access.

The fix is to initialise parent with the invalid inode 0, which will cause
an EINVAL error to be returned.

Regular inodes used to share the parent field with the block_list_start
field. This is removed in this commit to enable the parent field to
contain the invalid inode number 0.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 122601408d20 ("Squashfs: export operations")
Signed-off-by: Phillip Lougher <[e-mail]>
Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]/
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/squashfs/inode.c

···
 		squashfs_i(inode)->start = le32_to_cpu(sqsh_ino->start_block);
 		squashfs_i(inode)->block_list_start = block;
 		squashfs_i(inode)->offset = offset;
+		squashfs_i(inode)->parent = 0;
 		inode->i_data.a_ops = &squashfs_aops;
 
 		TRACE("File inode %x:%x, start_block %llx, block_list_start "
···
 		squashfs_i(inode)->start = le64_to_cpu(sqsh_ino->start_block);
 		squashfs_i(inode)->block_list_start = block;
 		squashfs_i(inode)->offset = offset;
+		squashfs_i(inode)->parent = 0;
 		inode->i_data.a_ops = &squashfs_aops;
 
 		TRACE("File inode %x:%x, start_block %llx, block_list_start "
···
 		inode->i_mode |= S_IFLNK;
 		squashfs_i(inode)->start = block;
 		squashfs_i(inode)->offset = offset;
+		squashfs_i(inode)->parent = 0;
 
 		if (type == SQUASHFS_LSYMLINK_TYPE) {
 			__le32 xattr;
···
 		set_nlink(inode, le32_to_cpu(sqsh_ino->nlink));
 		rdev = le32_to_cpu(sqsh_ino->rdev);
 		init_special_inode(inode, inode->i_mode, new_decode_dev(rdev));
+		squashfs_i(inode)->parent = 0;
 
 		TRACE("Device inode %x:%x, rdev %x\n",
 				SQUASHFS_INODE_BLK(ino), offset, rdev);
···
 		set_nlink(inode, le32_to_cpu(sqsh_ino->nlink));
 		rdev = le32_to_cpu(sqsh_ino->rdev);
 		init_special_inode(inode, inode->i_mode, new_decode_dev(rdev));
+		squashfs_i(inode)->parent = 0;
 
 		TRACE("Device inode %x:%x, rdev %x\n",
 				SQUASHFS_INODE_BLK(ino), offset, rdev);
···
 			inode->i_mode |= S_IFSOCK;
 		set_nlink(inode, le32_to_cpu(sqsh_ino->nlink));
 		init_special_inode(inode, inode->i_mode, 0);
+		squashfs_i(inode)->parent = 0;
 		break;
 	}
 	case SQUASHFS_LFIFO_TYPE:
···
 		inode->i_op = &squashfs_inode_ops;
 		set_nlink(inode, le32_to_cpu(sqsh_ino->nlink));
 		init_special_inode(inode, inode->i_mode, 0);
+		squashfs_i(inode)->parent = 0;
 		break;
 	}
 	default:

fs/squashfs/squashfs_fs_i.h

···
 	u64		xattr;
 	unsigned int	xattr_size;
 	int		xattr_count;
+	int		parent;
 	union {
 		struct {
 			u64		fragment_block;
···
 			u64		dir_idx_start;
 			int		dir_idx_offset;
 			int		dir_idx_cnt;
-			int		parent;
 		};
 	};
 	struct inode	vfs_inode;