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render: fix refcounting of glyphs during ProcRenderAddGlyphs

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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>
This commit is contained in:
Peter Hutterer
2024-01-30 13:13:35 +10:00
committed by Alan Coopersmith
parent 6c684d035c
commit bdca6c3d1f
3 changed files with 15 additions and 6 deletions
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15
render/render.c
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@@ -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;