(release/25.1) rootless: fix dangling screen pixmap on RootlessUpdateScreenPixmap() OOM - #3508
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On a calloc() failure while growing the rootless screen pixmap buffer, the old buffer was already freed and pixmap_data_size already bumped to the new (larger) size before the failure check -- leaving s->pixmap_data (and the screen pixmap pPix, still pointing at the freed block via its prior ModifyPixmapHeader() call) dangling. Worse, because pixmap_data_size was already bumped, a later same-or-smaller-size call sees pixmap_data_size < rowbytes as false and skips reallocating forever, permanently pinning the dangling state instead of retrying. Trigger: real host memory pressure during a rootless (Xquartz-style DDX) geometry change. Fix: allocate the replacement into a temporary first; only free the old buffer and update pixmap_data_size/pixmap_data together once the new allocation has succeeded. Found via a fleet-directed alloc-fail/UAF sweep of Xext/, mi/, and miext/, not from a live crash report. Verification note: miext/rootless/ only compiles into hw/xquartz, which this Linux build has disabled -- checked with a standalone `gcc -fsyntax-only` pass instead of the normal ninja+meson-test build/test cycle used for the other fixes in this sweep. Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net> (cherry picked from commit d536dde)
This was referenced Aug 7, 2026
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On a calloc() failure while growing the rootless screen pixmap buffer, the
old buffer was already freed and pixmap_data_size already bumped to the new
(larger) size before the failure check -- leaving s->pixmap_data (and the
screen pixmap pPix, still pointing at the freed block via its prior
ModifyPixmapHeader() call) dangling. Worse, because pixmap_data_size was
already bumped, a later same-or-smaller-size call sees pixmap_data_size <
rowbytes as false and skips reallocating forever, permanently pinning the
dangling state instead of retrying.
Trigger: real host memory pressure during a rootless (Xquartz-style DDX)
geometry change.
Fix: allocate the replacement into a temporary first; only free the old
buffer and update pixmap_data_size/pixmap_data together once the new
allocation has succeeded.
Found via a fleet-directed alloc-fail/UAF sweep of Xext/, mi/, and miext/,
not from a live crash report. Verification note: miext/rootless/ only
compiles into hw/xquartz, which this Linux build has disabled -- checked
with a standalone
gcc -fsyntax-onlypass instead of the normalninja+meson-test build/test cycle used for the other fixes in this sweep.
Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net
(cherry picked from commit d536dde)
Backport of master PR #3277 (rootless: fix dangling screen pixmap on RootlessUpdateScreenPixmap() OOM).
Backport series of master PR #3277 — same fix on every maintained release line:
This is the 25.1 copy (base
release/25.1). The others are linked above.