Previously, AllocateGlyph would return a new glyph with refcount=0 and a
re-used glyph would end up not changing the refcount at all. The
resulting glyph_new array would thus have multiple entries pointing to
the same non-refcounted glyphs.
AddGlyph may free a glyph, resulting in a UAF when the same glyph
pointer is then later used.
Fix this by returning a refcount of 1 for a new glyph and always
incrementing the refcount for a re-used glyph, followed by dropping that
refcount back down again when we're done with it.
CVE-2024-31083, ZDI-CAN-22880
This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1463>
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