CVE-2026-80778

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

futex/pi: Reject cross-mm private futex owners

A private futex key borrows the waiter's mm without taking an mmusers reference. Nevertheless, attachtopiowner() currently accepts an owner from a different address space and copies the private key into the owner's PI state.

When that owner exits, exitpistatelist() uses the saved key to find the hash bucket and acquires a reference to the waiter's private hash. If the last user of the waiter's mm exits concurrently, futexhash_free() frees the hash while the owner still uses its bucket and reference.

Prevent this by validating in attachtopiowner() that, for private futexes, the owner mm and waiter mm are the same. Perform the check with the owner's pilock held and after validating owner::futex::state to serialize against a concurrent PI-state exit cleanup.

[ tglx: Amended comment ]