rustdesk/DRM_CAPTURE_SECURITY.md

DRM/KMS capture — security model & threat model

The optional drm feature adds a Linux capture backend that reads the active scanout directly from DRM/KMS, bypassing the xdg-desktop-portal consent dialog. It exists for unattended / login-screen / Wayland scenarios where the portal prompt is not acceptable. Because it bypasses consent, treat it as a privileged, opt-in host-mode feature, not a normal Wayland capture backend.

How it works

Reading the active scanout needs CAP_SYS_ADMIN (to map other clients' framebuffers). RustDesk's root --service already runs with CAP_SYS_ADMIN, so the drm feature does the read in-process in that root service: it dlopens libdrmtap.so and calls it in direct mode — no privileged child, no setcap helper. On the default (split) path the root service does not touch pixels: it exports the active scanout as a DMA-BUF and passes just that read-only fd to the unprivileged user --server over a dedicated service-scoped IPC channel (_drm) via SCM_RIGHTS. The --server keeps an import-once EGLImage cache (keyed on the buffer, so a given scanout buffer is imported once and re-imports are elided), detiles/converts it to linear RGBA in its own unprivileged address space, and feeds the encoder — so the root service never loads libEGL/libGLESv2 and never copies scanout pixels. Only the CPU fallback path (used when the seat/driver cannot produce a transferable DMA-BUF, or the consumer has no render node of its own, see When the CPU fallback is chosen below) copies the scanout to packed BGRA inside the root service and streams those bytes over _drm. This mirrors the Windows portable_service split (a privileged process captures, an unprivileged one presents) but reuses RustDesk's own hardened IPC.

• libdrmtap.so is loaded through a small dlopen loader (drmtap_dl); if the library or one of its runtime deps is missing the load fails cleanly and the caller falls back to the PipeWire/portal path.
• The loader also refuses a library that cannot do the split: one reporting below 0.4.9, and one reporting a newer version without actually exporting drmtap_grab_desc / drmtap_open_render / drmtap_convert_dmabuf (a stale or pre-release build). The only way to capture with such a library is the in-process convert, which in the root service means loading the vendor GL stack there, so it is refused and the caller falls back to PipeWire/portal. The privileged process therefore never loads GL because of which file happened to be on the load path; the CPU fallback below is entered only for a fact about the seat or the consumer.
• The reader restricts the device it opens to a realpath under /dev/dri/ (drm_reader.rs); RustDesk always runs libdrmtap in direct in-process mode (helper_path is NULL), so no privileged child process is ever spawned and none is built, shipped, or installed. There is no drmtap-helper binary, no setcap, no capability-bearing file, and no capture group in this deployment.
• The _drm socket lives beside the hardened _service socket (/tmp/<app>-service/ipc_drm). It is 0666 so the unprivileged --server can connect, but every accepted peer is authorized in handle_drm_conn (authorize_service_scoped_ipc_connection: peer must be root or the active session uid, with a /proc/<pid>/exe identity match). Connectable is not authorized.

Threat model

• Consent bypass. This mode does not show the portal "select what to share" prompt. On a misconfigured install it could expose the login screen, the lock screen, or another local user's graphical session.
• The scanout parse runs in the root service. Moving the read in-process removes the old setcap helper and its world-exec attack surface. On the default (split) path the root service does only a metadata-only parse of the scanout descriptor and exports the DMA-BUF fd; the untrusted-framebuffer detile / pixel-format conversion runs in the unprivileged --server, outside CAP_SYS_ADMIN. Export-side validation is therefore metadata-only — geometry bounded to <= MAX_DIM (16384) and num_planes in 1..=4 (drm_reader.rs grab_desc); there is no fourcc gate on the export side, because the format check is delegated to the unprivileged converter, which handles every format libdrmtap supports (XRGB/ARGB8888, 10-bit XR30/AR30, HDR, CCS-compressed). The exported fd is read-only: libdrmtap exports the DMA-BUF via drmPrimeHandleToFD with DRM_RDWR dropped (O_RDONLY), and drm_reader dup()s it — which shares the same open file description and so preserves that access mode — so the unprivileged consumer can map the scanout for reading but never write into the live framebuffer. On the CPU fallback path the pixel-format conversion / detile instead runs inside the CAP_SYS_ADMIN service without a seccomp cage; there the frame copy has format / stride / geometry and integer-overflow guards (drm_reader.rs grab), and non-32bpp scanouts are rejected before the copy. The device is realpath-gated to /dev/dri/ on both paths.
• _drm is a screen-content channel. It is authorized per connection (see above); without that authz any local process could read the screen. On the default (split) path the channel carries the scanout DMA-BUF fd, passed to the unprivileged --server over SCM_RIGHTS as a read-only descriptor (the --server holds an import-once EGLImage cache, so a given scanout buffer is imported once and re-imports are elided); the peer can map the scanout for reading but cannot write it. The CPU fallback path instead carries plain packed-BGRA bytes over the same authorized socket (no fd passing, no shared memory).
• When the CPU fallback is chosen. The split path is the default; the consumer asks the service for the CPU-converted frame in two cases: no render node can be opened for this seat, or a previous convert on this display already failed. A third case is a multi-GPU safety fallback: if the service could not name the render node of the GPU that exports the scanout (an older libdrmtap without drmtap_render_node) and the host has more than one render node, the consumer refuses to guess one, because importing a scanout on a device that did not export it can succeed and return corrupted pixels rather than fail. The conversion then happens in the service, on the device it already has open, so it is correct by construction. Hosts with a single render node have nothing to pick wrong and keep the DMA-BUF fast path.

Deployment

• Off by default. The drm feature is not in the default feature set and is not enabled in standard release packages; the drm-off build is byte-identical to upstream. Build it explicitly with python3 build.py --flutter --drm (Linux only).

• Separate opt-in package. A --drm build ships as a distinctly named rustdesk-unattended-wayland package (Conflicts/Replaces rustdesk), so enabling consent-free capture is an explicit install choice.

• Bundled library, no capabilities. The package installs the versioned libdrmtap.so.0.<minor>.<patch> plus a libdrmtap.so.0 soname symlink under /usr/lib/rustdesk/, and the in-process dlopen names that absolute path (/usr/lib/rustdesk/libdrmtap.so.0). The package deliberately does not register the directory with the dynamic linker: no /etc/ld.so.conf.d/ drop-in and no ldconfig trigger are shipped, so a private library cannot shadow a system one for unrelated binaries (Debian Policy 10.2). The bare-soname lookups remain only as a fallback for a development build reached through LD_LIBRARY_PATH.

  There is no setcap, no rustdesk-capture group, and no privileged binary: the capture runs inside the root --service, which already holds the capability it needs. Hosts without /dev/dri access (or where the library fails to load) transparently fall back to the PipeWire/portal path.

• Minimum OS: Ubuntu 18.04 (or equivalent, libdrm ≥ 2.4.95). libdrmtap needs the DRM GetFB2 framebuffer API (libdrm 2.4.95); Ubuntu 18.04 ships 2.4.101, so 18.04 is the floor. The rustdesk-unattended-wayland deb is built and packaged on an ubuntu18.04 container in CI (a build-time compatibility check only — DRM capture itself is not installed or exercised there), so it is built against the 18.04 toolchain and libraries and is compatible with 18.04 and newer. Capture also requires an active KMS scanout (a Wayland/KMS session with a display on); on hosts where the compositor drives the display outside DRM/KMS (e.g. the proprietary NVIDIA X11 stack) there is no capturable CRTC and the path falls back to PipeWire/portal.

• Recommended for single-user, physically-controlled, or unattended hosts.

Auditing

# the bundled capture library and its soname symlink — no capabilities are set on either
ls -l /usr/lib/rustdesk/libdrmtap.so.0*
# the dlopen names the symlink by absolute path, so what matters is where the symlink points:
readlink /usr/lib/rustdesk/libdrmtap.so.0   # expect: the versioned object shipped by the package
# and there should be no other object left beside it (a leftover is not loaded on its own, but it
# is what a stray ldconfig over this directory would repoint the symlink to)
ls /etc/ld.so.conf.d/ | grep -i rustdesk               # expect: no output (none is shipped)
# confirm no privileged helper is present (there should be none)
getcap -r /usr/lib/rustdesk 2>/dev/null                  # expect: no output