CVE-2026-43404

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)