Improper Resource Locking Affecting kernel-kdump package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-kdump package and not the kernel-kdump package as distributed by Centos. See How to fix? for Centos:6 relevant fixed versions and status.

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

mm: Fix a hmm_range_fault() livelock / starvation problem

If hmm_range_fault() fails a folio_trylock() in do_swap_page, trying to acquire the lock of a device-private folio for migration, to ram, the function will spin until it succeeds grabbing the lock.

However, if the process holding the lock is depending on a work item to be completed, which is scheduled on the same CPU as the spinning hmm_range_fault(), that work item might be starved and we end up in a livelock / starvation situation which is never resolved.

This can happen, for example if the process holding the device-private folio lock is stuck in migrate_device_unmap()->lru_add_drain_all() sinc lru_add_drain_all() requires a short work-item to be run on all online cpus to complete.

A prerequisite for this to happen is: a) Both zone device and system memory folios are considered in migrate_device_unmap(), so that there is a reason to call lru_add_drain_all() for a system memory folio while a folio lock is held on a zone device folio. b) The zone device folio has an initial mapcount > 1 which causes at least one migration PTE entry insertion to be deferred to try_to_migrate(), which can happen after the call to lru_add_drain_all(). c) No or voluntary only preemption.

This all seems pretty unlikely to happen, but indeed is hit by the "xe_exec_system_allocator" igt test.

Resolve this by waiting for the folio to be unlocked if the folio_trylock() fails in do_swap_page().

Rename migration_entry_wait_on_locked() to softleaf_entry_wait_unlock() and update its documentation to indicate the new use-case.

Future code improvements might consider moving the lru_add_drain_all() call in migrate_device_unmap() to be called after all pages have migration entries inserted. That would eliminate also b) above.

v2:

• Instead of a cond_resched() in hmm_range_fault(), eliminate the problem by waiting for the folio to be unlocked in do_swap_page() (Alistair Popple, Andrew Morton) v3:
• Add a stub migration_entry_wait_on_locked() for the !CONFIG_MIGRATION case. (Kernel Test Robot) v4:
• Rename migrate_entry_wait_on_locked() to softleaf_entry_wait_on_locked() and update docs (Alistair Popple) v5:
• Add a WARN_ON_ONCE() for the !CONFIG_MIGRATION version of softleaf_entry_wait_on_locked().
• Modify wording around function names in the commit message (Andrew Morton)

(cherry picked from commit a69d1ab971a624c6f112cea61536569d579c3215)

• https://access.redhat.com/security/cve/CVE-2026-43404
• https://git.kernel.org/stable/c/7e6e2fc91d4b9b12ec6e137019532568ebcf2680
• https://git.kernel.org/stable/c/94b6d0ba4b640ba23bb6c708a59316e74e5ede63
• https://git.kernel.org/stable/c/b570f37a2ce480be26c665345c5514686a8a0274