CVE-2022-49554

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

zsmalloc: fix races between asynchronous zspage free and page migration

The asynchronous zspage free worker tries to lock a zspage's entire page list without defending against page migration. Since pages which haven't yet been locked can concurrently migrate off the zspage page list while lockzspage() churns away, lockzspage() can suffer from a few different lethal races.

It can lock a page which no longer belongs to the zspage and unsafely dereference pageprivate(), it can unsafely dereference a torn pointer to the next page (since there's a data race), and it can observe a spurious NULL pointer to the next page and thus not lock all of the zspage's pages (since a single page migration will reconstruct the entire page list, and createpage_chain() unconditionally zeroes out each list pointer in the process).

Fix the races by using migratereadlock() in lock_zspage() to synchronize with page migration.