qubes-doc/Qfileexchgd.md

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wiki	Qfileexchgd	/wiki/Qfileexchgd/

This mechanism is obsolete as of Qubes Beta 1!

Please see this page instead.

qfilexchgd, the Qubes file exchange daemon

Overview

qfilexchgd is a dom0 daemon responsible for managing exchange of block devices ("virtual pendrives") between VMs. It is used for

• copying files between AppVMs
• copying a single file between an AppVM and a DisposableVM

qfilexchgd is started after first qubes_guid has been started, so that it has access to X display in dom0 to present dialog messages.

qfilexchgd is event driven. The sources of events are:

• trigger of xenstore watch for the changes in /local/domain xenstore hierarchy - to detect start/stop of VMs, and maintain vmname->vm_xid dictionary
• trigger of xenstore watch for a change in /local/domain/domid/device/qpen key - VMs write to this key to request service from qfilexchgd

Copying files between AppVMs

1. AppVM1 user runs qvm-copy-to-vm script (accessible from Dolphin file manager by right click on a "file(s)->Actions->Send to VM" menu). It calls /usr/lib/qubes/qubes_penctl new, and it writes "new" request to its device/qpen xenstore key. qfilexchgd creates a new 1G file, makes vfat fs on it, and does block-attach so that this file is attached as /dev/xvdg in AppVM1.
2. AppVM1 mounts /dev/xvdg on /mnt/outgoing and copies requested files there, then unmounts it.
3. AppVM1 writes "send DestVM" request to its device/qpen xenstore key (calling /usr/lib/qubes/qubes_penctl send DestVM). After getting confirmation by displaying a dialog box in dom0 display, qfilexchgd detaches /dev/xvdg from AppVM1, attaches it as /dev/xvdh to DestVM.
4. In DestVM, udev script for /dev/xvdh named qubes_add_pendrive_script (see /etc/udev/rules.d/qubes.rules) mounts /dev/xvdh on /mnt/incoming, and then waits for /mnt/incoming to become unmounted. A file manager running in DestVM shows a new volume, and user in DestVM may copy files from it. When user in DestVM is done, then user unmounts /mnt/incoming. qubes_add_pendrive_script then tells qfilexchgd to detach /dev/xvdh and terminates.

Copying a single file between AppVM and a DisposableVM

In order to minimize attack surface presented by necessity to process virtual pendrive metadata sent by (potentially compromised and malicious) DisposableVM, AppVM<->DisposableVM file exchange protocol does not use any filesystem.

1. User in AppVM1 runs qvm-open-in-dvm (accessible from Dolphin file manager by right click on a "file->Actions->Open in DisposableVM" menu). qvm-open-in-dvm

   1. gets a new /dev/xvdg (just as described in previous paragraph)
   2. computes a new unique transaction seq SEQ by incrementing /home/user/.dvm/seq contents,
   3. writes the requested file name (say, /home/user/document.txt) to /home/user/.dvm/SEQ
   4. creates a dvm_header (see core.git/appvm/dvm.h) on /dev/xvdg, followed by file contents
   5. writes the "send disposable SEQ" command to its device/qpen xenstore key.

2. qfilexchgd sees that "send" argument=="disposable", and creates a new DisposableVM by calling /usr/lib/qubes/qubes_restore. It adds the new DisposableVM to qubesDB via qvm_collection.add_new_disposablevm. Then it attaches the virtual pendrive (previously attached as /dev/xvdg at AppVM1) as /dev/xvdh in DisposableVM.

3. In DisposableVM, qubes_add_pendrive_script sees non-zero qubes_transaction_seq key in xenstore, and instead processing the virtual pendrive as in the case of normal copy, treats it as DVM transaction (a request, because we run in DisposableVM). It retrieves the body of the file passed in /dev/xvdh, copies to /tmp, and runs mime-open utility to open appropriate executable to edit it. When mime-open returns, if the file was modified, it is sent back to AppVM1 (by writing "send AppVM1 SEQ" to device/qpen xenstore key). Then DisposableVM destroys itself.

4. In AppVM1, a new /dev/xvdh appears (because DisposableVM has sent it). qubes_add_pendrive_script sees non-zero qubes_transaction_seq key, and treats it as DVM transaction (a response, because we run in AppVM, not DisposableVM). It retrieves the filename from /home/user/.dvm/SEQ, and copies data from /dev/xvdh to it.