CVE-2026-72164: ocfs2: avoid moving extents to occupied clusters

In the Linux kernel, the following vulnerability has been resolved:

ocfs2: avoid moving extents to occupied clusters

For non-auto OCFS2IOCMOVEEXT operations, userspace supplies a physical megoal. ocfs2moveextent() initializes newphyscpos from that goal and expects ocfs2probeallocgroup() to replace it with a free run in the target block group.

The probe currently leaves physcpos unchanged if the scan reaches the end of the group without finding a free run. An occupied goal at the last bit can therefore survive the probe and be passed to ocfs2moveextent(), which copies file data into a cluster still owned by another inode before the bitmap is updated.

When the probe does find a free run, it also subtracts movelen from the ending bit. The start of an N-bit run ending at i is i - N + 1, so the current calculation can report the bit immediately before the free run.

Clear physcpos before scanning and use the correct free-run start. Callers already treat a zero result as -ENOSPC, so failed probes no longer continue with an occupied caller-controlled goal.