futex/pi: Reject cross-mm private futex owners

Summary

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 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 ]

Timeline

1. Sep 4, 2026, 3:12 PM UTCCVE published
2. Oct 19, 2026, 8:43 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 80367ad01d93ac781b0e1df246edaf006928002f · fixed in 2b92e5562653b5293529f63b0300837d9dcedbd7; From 80367ad01d93ac781b0e1df246edaf006928002f · fixed in f7fb3e07752688842cbe0b85cf0d98c2fbf76b68; From 80367ad01d93ac781b0e1df246edaf006928002f · fixed in 43b148d796aa338858792d0167cebdc12b8cb4b9; From 80367ad01d93ac781b0e1df246edaf006928002f · fixed in 59b3732f95dda1fbd2234514d35f4fb6b5bb6d85

• Linux Linux

  6.16

• KernelLinux

  From 6.16.0 · fixed in 6.18.47; From 6.19.0 · fixed in 7.1.11; From 7.2.0 · fixed in 7.2.1

References 4