In the Linux kernel, the following vulnerability has been resolved: futex: Fix private hash use-after-free on resize poll_state_synchroniz

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

futex: Fix private hash use-after-free on resize

poll_state_synchronize_rcu(mm->futex.phash.batches) is used by
futex_ref_drop() to check that a grace period has passed since the
current hash was published. This relies on batches referencing a grace
period which started after the hash pointer was assigned.

__futex_pivot_hash() sets mmph->batches before it replaces mmph->hash:

 1scoped_guard(rcu) {
 2        mmph->batches = get_state_synchronize_rcu();
 3        rcu_assign_pointer(mmph->hash, new);
 4}

The scoped_guard(rcu) doesn't stop new grace periods from starting, and
if one starts between those two assignments, futex_ref_drop() can move
forward while a reader still holds a pointer to the old hash.

Fix things by setting mmph->batches after assigning mmph->hash. The
scoped_guard(rcu) isn't needed, so let's drop that as well.