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

Overview

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.

Affected

• Linux >= 56180dd20c19e5b0fa34822997a9ac66b517e7b3 < 5d5dc18b3a9ac011619b7d3af4514bf9e9b6a6af
• Linux >= 56180dd20c19e5b0fa34822997a9ac66b517e7b3 < f35e3b5784221654f9cdbd6222275a7fa203f6c3
• Linux 6.17

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.