AZL-89607

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

lib: test_hmm: evict device pages on file close to avoid use-after-free

Patch series "Minor hmm_test fixes and cleanups".

Two bugfixes a cleanup for the HMM kernel selftests. These were mostly reported by Zenghui Yu with special thanks to Lorenzo for analysing and pointing out the problems.

This patch (of 3):

When dmirrorfopsrelease() is called it frees the dmirror struct but doesn't migrate device private pages back to system memory first. This leaves those pages with a dangling zonedevicedata pointer to the freed dmirror.

If a subsequent fault occurs on those pages (eg. during coredump) the dmirrordevmemfault() callback dereferences the stale pointer causing a kernel panic. This was reported [1] when running mm/ksft_hmm.sh on arm64, where a test failure triggered SIGABRT and the resulting coredump walked the VMAs faulting in the stale device private pages.

Fix this by calling dmirrordeviceevictchunk() for each devmem chunk in dmirrorfops_release() to migrate all device private pages back to system memory before freeing the dmirror struct. The function is moved earlier in the file to avoid a forward declaration.