futex/pi: Reject cross-mm private futex owners

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

When that owner exits, exit_pi_state_list() 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, futex_hash_free() frees
the hash while the owner still uses its bucket and reference.

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

[ tglx: Amended comment ]

Fixes: 80367ad01d ("futex: Add basic infrastructure for local task local hash")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Assisted-by: Codex:gpt-5.6-sol
Cc: stable@vger.kernel.org

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Kyle Zeng 2026-08-07 17:07:03 +02:00 • committed by Thomas Gleixner

parent 8e7ff730dd

commit 59b3732f95

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kernel/futex/pi.c

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