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Marek Marczykowski-Górecki 2e99efd170

Minor update to GUI protocol documentation

2017-01-14 01:10:43 +01:00

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Qubes GUI protocol

qubes_gui and qubes_guid processes

All AppVM X applications connect to local (running in AppVM) Xorg server, that uses the following "hardware" drivers:

dummyqsb_drv - video driver, that paints onto a framebuffer located in RAM, not connected to real hardware
qubes_drv - it provides a virtual keyboard and mouse (in fact, more, see below)

For each AppVM, there is a pair of qubes_gui (running in AppVM) and qubes_guid (running in dom0) processes, connected over vchan. Main responsibilities of qubes_gui are:

call XCompositeRedirectSubwindows on the root window, so that each window has its own composition buffer
instruct the local Xorg server to notify it about window creation, configuration and damage events; pass information on these events to dom0
receive information about keyboard and mouse events from dom0, tell qubes_drv to fake appropriate events
receive information about window size/position change, apply them to the local window

Main responsibilities of qubes_guid are:

create a window in dom0 whenever an information on window creation in AppVM is received from qubes_gui
whenever the local window receives XEvent, pass information on it to AppVM (particularly, mouse and keyboard data)
whenever AppVM signals damage event, tell local Xorg server to repaint a given window fragment
receive information about window size/position change, apply them to the local window

Note that keyboard and mouse events are passed to AppVM only if a window belonging to this AppVM has focus. AppVM has no way to get information on keystrokes fed to other AppVMs (e.g. XTEST extension will report the status of local AppVM keyboard only), nor synthesize and pass events to other AppVMs.

Window content updates implementation

Typical remote desktop applications, like vnc, pass the information on all changed window content in-band (say, over tcp). As the channel has limited throughput, this impacts video performance. In case of Qubes, qubes_gui does not transfer all changed pixels via vchan. Instead, for each window, upon its creation or size change, qubes_gui

asks qubes_drv driver for the list of physical memory frames that hold the composition buffer of a window
pass this information via MFNDUMP message to qubes_guid in dom0

Now, qubes_guid has to tell dom0 Xorg server about the location of the buffer. There is no supported way (e.g. Xorg extension) to do this zero-copy style. The following method is used in Qubes:

in dom0, the Xorg server is started with LD_PRELOAD-ed library named shmoverride.so. This library hooks all function calls related to shared memory.
qubes_guid creates a shared memory segment, and then tells Xorg to attach it via MIT-SHM extension
when Xorg tries to attach the segment (via glibc shmat) shmoverride.so intercepts this call and instead maps AppVM memory via xc_map_foreign_pages
since then, we can use MIT-SHM functions, e.g. XShmPutImage to draw onto a dom0 window. XShmPutImage will paint with DRAM speed; actually, many drivers use DMA for this.

The important detail is that xc_map_foreign_pages verifies that a given mfn range actually belongs to a given domain id (and the latter is provided by trusted qubes_guid). Therefore, rogue AppVM cannot gain anything by passing crafted mnfs in the MFNDUMP message.

To sum up, this solution has the following benefits:

window updates at DRAM speed
no changes to Xorg code
minimal size of the supporting code

Security markers on dom0 windows

It is important that user knows which AppVM a given window belongs to. This prevents an attack when a rogue AppVM paints a window pretending to belong to other AppVM or dom0, and tries to steal e.g. passwords.

In Qubes, the custom window decorator is used, that paints a colourful frame (the colour is determined during AppVM creation) around decorated windows. Additionally, window title always starts with [name of the AppVM]. If a window has a override_redirect attribute, meaning that it should not be treated by a window manager (typical case is menu windows), qubes_guid draws a two-pixel colourful frame around it manually.

Certainly, it would be insecure to allow AppVM to read/write clipboard of other AppVMs unconditionally. Therefore, the following mechanism is used:

there is a "qubes clipboard" in dom0 - its contents is stored in a regular file in dom0.
if user wants to copy local AppVM clipboard to qubes clipboard, she must focus on any window belonging to this AppVM, and press Ctrl-Shift-C. This combination is trapped by qubes-guid, and CLIPBOARD_REQ message is sent to AppVM. qubes-gui responds with CLIPBOARD_DATA message followed by clipboard contents.
user focuses on other AppVM window, presses Ctrl-Shift-V. This combination is trapped by qubes-guid, and CLIPBOARD_DATA message followed by qubes clipboard contents is sent to AppVM; qubes_gui copies data to the local clipboard, and then user can paste its contents to local applications normally.

This way, user can quickly copy clipboards between AppVMs. This action is fully controlled by the user, it cannot be triggered/forced by any AppVM.

qubes_gui and qubes_guid code notes

Both applications are structures similarly. They use select function to wait for any of the two event sources

messages from the local X server
messages from the vchan connecting to the remote party

The XEvents are handled by handle_xevent_eventname function, messages are handled by handle_messagename function. One should be very careful when altering the actual select loop - e.g. both XEvents and vchan messages are buffered, meaning that select will not wake for each message.

If one changes the number/order/signature of messages, one should increase the QUBES_GUID_PROTOCOL_VERSION constant in messages.h include file.

qubes_guid writes debugging information to /var/log/qubes/qubes.domain_id.log file; qubes_gui writes debugging information to /var/log/qubes/gui_agent.log. Include these files when reporting a bug.

AppVM -> dom0 messages

Proper handling of the below messages is security-critical. Observe that beside two messages (CLIPBOARD and MFNDUMP) the rest have fixed size, so the parsing code can be small.

The override_redirect window attribute is explained at Override Redirect Flag. The transient_for attribute is explained at Transient_for attribute.

Window manager hints and flags are described at http://standards.freedesktop.org/wm-spec/latest/, especially part about _NET_WM_STATE.

Each message starts with the following header

struct msghdr {   
        uint32_t type;
        uint32_t window;
        /* This field is intended for use by gui_agents to skip unknown
         * messages from the (trusted) guid. Guid, on the other hand,
         * should never rely on this field to calculate the actual len of
         * message to be read, as the (untrusted) agent can put here
         * whatever it wants! */
        uint32_t untrusted_len;
};

The header is followed by message-specific data.

Message name	Structure after header	Action
MSG_CLIPBOARD_DATA	amorphic blob (length determined by the "window" field)	Store the received clipboard content (not parsing in any way)
MSG_CREATE	struct msg_create { uint32_t x; uint32_t y; uint32_t width; uint32_t height; uint32_t parent; uint32_t override_redirect; };	Create a window with given parameters
MSG_DESTROY	None	Destroy a window
MSG_MAP	struct msg_map_info { uint32_t transient_for; uint32_t override_redirect; };	Map a window with given parameters
MSG_UNMAP	None	Unmap a window
MSG_CONFIGURE	struct msg_configure { uint32_t x; uint32_t y; uint32_t width; uint32_t height; uint32_t override_redirect; };	Change window position/size/type
MSG_MFNDUMP	struct shm_cmd { uint32_t shmid; uint32_t width; uint32_t height; uint32_t bpp; uint32_t off; uint32_t num_mfn; uint32_t domid; uint32_t mfns[0]; };	Retrieve the array of mfns that constitute the composition buffer of a remote window. The "num_mfn" 32bit integers follow the shm_cmd structure; "off" is the offset of the composite buffer start in the first frame; "shmid" and "domid" parameters are just placeholders (to be filled by qubes_guid), so that we can use the same structure when talking to shmoverride.so\|
MSG_SHMIMAGE	struct msg_shmimage { uint32_t x; uint32_t y; uint32_t width; uint32_t height; };	Repaint the given window fragment
MSG_WMNAME	struct msg_wmname { char data[128]; };	Set the window name; only printable characters are allowed
MSG_DOCK	None	Dock the window in the tray
MSG_WINDOW_HINTS	struct msg_window_hints { uint32_t flags; uint32_t min_width; uint32_t min_height; uint32_t max_width; uint32_t max_height; uint32_t width_inc; uint32_t height_inc; uint32_t base_width; uint32_t base_height; };	Size hints for window manager
MSG_WINDOW_FLAGS	struct msg_window_flags { uint32_t flags_set; uint32_t flags_unset; };	Change window state request; fields contains bitmask which flags request to be set and which unset

Dom0 -> AppVM messages

Proper handling of the below messages is NOT security-critical.