CVE-2026-98788

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:

scoped guard(rcu) { mmph->batches = get state synchronize rcu(); rcu assign pointer(mmph->hash, new); }

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.