mirror of
https://github.com/QubesOS/qubes-doc.git
synced 2024-12-18 04:04:39 -05:00
58 lines
2.5 KiB
ReStructuredText
58 lines
2.5 KiB
ReStructuredText
===================================
|
|
Inter-qube file copying (qfilecopy)
|
|
===================================
|
|
|
|
|
|
There are two cases when we need a mechanism to copy files between VMs:
|
|
|
|
- “regular” file copy - when user instructs file manager to copy a
|
|
given files/directories to a different VM
|
|
|
|
- DispVM copy - user selects “open in DispVM” on a file; this file must
|
|
be copied to a DisposableVM, edited by user, and possibly a modified
|
|
file copied back from DispVM to VM.
|
|
|
|
|
|
|
|
In the early days of Qubes OS, for both cases, a block device (backed by
|
|
a file in dom0 with a vfat filesystem on it) was attached to VM, file(s)
|
|
copied there, and then the device was detached and attached to target
|
|
VM. In the DispVM case, if a edited file has been modified, another
|
|
block device is passed to requester VM in order to update the source
|
|
file.
|
|
|
|
This has the following disadvantages:
|
|
|
|
- performance - dom0 has to prepare and attach/detach block devices,
|
|
which is slow because of hotplug scripts involvement.
|
|
|
|
- security - VM kernel parses partition table and filesystem metadata
|
|
from the block device; they are controlled by (potentially untrusted)
|
|
sender VM.
|
|
|
|
|
|
|
|
In modern Qubes OS releases, we have reimplemented interVM file copy
|
|
using qrexec, which addresses the above mentioned disadvantages.
|
|
Nowadays, even more generic solution (qubes rpc) is used. See the
|
|
developer docs on qrexec and qubes rpc. In a nutshell, the file sender
|
|
and the file receiver just read/write from stdin/stdout, and the qubes
|
|
rpc layer passes data properly - so, no block devices are used.
|
|
|
|
The rpc action for regular file copy is *qubes.Filecopy*, the rpc client
|
|
is named *qfile-agent*, the rpc server is named *qfile-unpacker*. For
|
|
DispVM copy, the rpc action is *qubes.OpenInVM*, the rpc client is named
|
|
*qopen-in-vm*, rpc server is named *vm-file-editor*. Note that the
|
|
qubes.OpenInVM action can be done on a normal AppVM, too.
|
|
|
|
Being a rpc server, *qfile-unpacker* must be coded securely, as it
|
|
processes potentially untrusted data format. Particularly, we do not
|
|
want to use external tar or cpio and be prone to all vulnerabilities in
|
|
them; we want a simplified, small utility, that handles only
|
|
directory/file/symlink file type, permissions, mtime/atime, and assume
|
|
user/user ownership. In the current implementation, the code that
|
|
actually parses the data from srcVM has ca 100 lines of code and
|
|
executes chrooted to the destination directory. The latter is hardcoded
|
|
to ``~user/QubesIncoming/srcVM``; because of chroot, there is no
|
|
possibility to alter files outside of this directory.
|