glamorfontget() builds a per-font texture atlas by laying out every glyph in the font into a single backing buffer. It computes the slot dimensions from the font's declared maxbounds, but copies each per-glyph bitmap using the individual glyph's metrics (GLYPHHEIGHTPIXELS/GLYPHWIDTHBYTES macros). There is no check that maxbounds actually bounds the per-glyph values.

When the font is loaded from a malicious PCF file whose per-glyph metrics exceed the file's maxbounds, the per-glyph memcpy writes far beyond the heap-allocated slot, producing a heap buffer overflow with attacker-controlled extent and attacker-controlled content.

An authenticated X client can trigger this by using SetFontPath to add a directory containing a crafted PCF font, loading the font with OpenFont, and drawing text on a glamor-backed drawable. Only servers using the glamor acceleration backend (Xorg with modesetting driver, Xwayland) are affected.

A client that requests multiple DRI2BufferBackLeft attachments and one DRI2BufferFrontLeft can trigger an out-of-bounds heap write in DRIGetBuffers/DRIGetBuffersWithFormat.

Any local X client that can connect to the server can trigger this issue. This may be used to crash the server, or for privilege escalation if the X server runs as root.

Components affected: xorg-x11-server, xorg-x11-server-Xwayland Versions affected: xorg-x11-server <= 21.1.22, xorg-x11-server-Xwayland <= 24.1.9

Fixed upstream in xorg-server-21.1.23 and xwayland-24.1.12. Fix: https://gitlab.freedesktop.org/xorg/xserver/-/commit/339c279514326134b0878fc23ce6e9520440ce7f

Identified by Peter Hutterer, Red Hat. Tracking: PSIRTSUPT-16950.