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.
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