CVE-2026-68164: mm/damon/core: disallow overlapping input ranges for damon_set_regions()

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

mm/damon/core: disallow overlapping input ranges for damonsetregions()

damonsetregions() assumes the input ranges are sorted by the address and don't overlap each other. Hence the assumption was initially to be explicitly validated. But commit 97d482f4592f ("mm/damon/sysfs: reuse damonsetregions() for regions setting") has mistakenly removed the validation.

This can make DAMON behave in unexpected ways. At the best, the monitoring results snapshot will just look weird since there will be overlapping regions. DAMOS will also work weirdly, applying the same action multiple times for overlapping regions, and make DAMOS quota weird. More seriously, depending on the setup and regions updates sequence, negative size regions can be made. It will trigger WARNONCE() if the kernel is built with CONFIGDAMONDEBUGSANITY=y. Depending on the monitoring results, the negative size region can further trigger division by zero in damonmergetworegions().

Note that some of the consequences including the WARNONCE() and the divide by zero depend on commits that were introduced after the root cause commit 97d482f4592f ("mm/damon/sysfs: reuse damonsetregions() for regions setting").

Fix the problems by checking the assumption and returning an error if the input ranges don't meet the assumption.

The issue was discovered [1] by Sashiko.