render: fix refcounting of glyphs during ProcRenderAddGlyphs

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>

3 files changed, 15 insertions, 6 deletions

diff --git a/render/glyph.c b/render/glyph.c
index 850ea8440..13991f8a1 100644
--- a/render/glyph.c
+++ b/render/glyph.c
@@ -245,10 +245,11 @@ FreeGlyphPicture(GlyphPtr glyph)
     }
 }
 
-static void
+void
 FreeGlyph(GlyphPtr glyph, int format)
 {
     CheckDuplicates(&globalGlyphs[format], "FreeGlyph");
+    BUG_RETURN(glyph->refcnt == 0);
     if (--glyph->refcnt == 0) {
         GlyphRefPtr gr;
         int i;
@@ -354,7 +355,7 @@ AllocateGlyph(xGlyphInfo * gi, int fdepth)
     glyph = (GlyphPtr) malloc(size);
     if (!glyph)
         return 0;
-    glyph->refcnt = 0;
+    glyph->refcnt = 1;
     glyph->size = size + sizeof(xGlyphInfo);
     glyph->info = *gi;
     dixInitPrivates(glyph, (char *) glyph + head_size, PRIVATE_GLYPH);
diff --git a/render/glyphstr_priv.h b/render/glyphstr_priv.h
index 90a90fa4f..dd1bc5482 100644
--- a/render/glyphstr_priv.h
+++ b/render/glyphstr_priv.h
@@ -56,6 +56,7 @@ void AddGlyph(GlyphSetPtr glyphSet, GlyphPtr glyph, Glyph id);
 Bool DeleteGlyph(GlyphSetPtr glyphSet, Glyph id);
 GlyphPtr FindGlyph(GlyphSetPtr glyphSet, Glyph id);
 GlyphPtr AllocateGlyph(xGlyphInfo * gi, int format);
+void FreeGlyph(GlyphPtr glyph, int format);
 Bool ResizeGlyphSet(GlyphSetPtr glyphSet, CARD32 change);
 GlyphSetPtr AllocateGlyphSet(int fdepth, PictFormatPtr format);
 int FreeGlyphSet(void *value, XID gid);
diff --git a/render/render.c b/render/render.c
index 29c5055c6..fe5e37dd9 100644
--- a/render/render.c
+++ b/render/render.c
@@ -1076,6 +1076,7 @@ ProcRenderAddGlyphs(ClientPtr client)
 
         if (glyph_new->glyph && glyph_new->glyph != DeletedGlyph) {
             glyph_new->found = TRUE;
+            ++glyph_new->glyph->refcnt;
         }
         else {
             GlyphPtr glyph;
@@ -1168,8 +1169,10 @@ ProcRenderAddGlyphs(ClientPtr client)
         err = BadAlloc;
         goto bail;
     }
-    for (i = 0; i < nglyphs; i++)
+    for (i = 0; i < nglyphs; i++) {
         AddGlyph(glyphSet, glyphs[i].glyph, glyphs[i].id);
+        FreeGlyph(glyphs[i].glyph, glyphSet->fdepth);
+    }
 
     if (glyphsBase != glyphsLocal)
         free(glyphsBase);
@@ -1179,9 +1182,13 @@ ProcRenderAddGlyphs(ClientPtr client)
         FreePicture((void *) pSrc, 0);
     if (pSrcPix)
         FreeScratchPixmapHeader(pSrcPix);
-    for (i = 0; i < nglyphs; i++)
-        if (glyphs[i].glyph && !glyphs[i].found)
-            free(glyphs[i].glyph);
+    for (i = 0; i < nglyphs; i++) {
+        if (glyphs[i].glyph) {
+            --glyphs[i].glyph->refcnt;
+            if (!glyphs[i].found)
+                free(glyphs[i].glyph);
+        }
+    }
     if (glyphsBase != glyphsLocal)
         free(glyphsBase);
     return err;