From: André Almeida Date: Tue, 2 Jun 2026 09:10:16 +0000 (+0200) Subject: Documentation: futex: Add a note about robust list race condition X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=3f63e2545978abda58f2cf7ff0d7a2942965e8cb;p=thirdparty%2Fkernel%2Flinux.git Documentation: futex: Add a note about robust list race condition Add a note to the documentation giving a brief explanation why doing a robust futex release in userspace is racy, what should be done to avoid it and provide links to read more. [ tglx: Fixed a few typos ] Signed-off-by: André Almeida Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260329-tonyk-vdso_test-v2-1-b7db810e44a1@igalia.com Link: https://patch.msgid.link/20260602090535.936286833@kernel.org --- diff --git a/Documentation/locking/robust-futex-ABI.rst b/Documentation/locking/robust-futex-ABI.rst index f24904f1c16fb..5e6a0665b8ba8 100644 --- a/Documentation/locking/robust-futex-ABI.rst +++ b/Documentation/locking/robust-futex-ABI.rst @@ -153,6 +153,9 @@ On removal: 3) release the futex lock, and 4) clear the 'lock_op_pending' word. +Please note that the removal of a robust futex purely in userspace is +racy. Refer to the next chapter to learn more and how to avoid this. + On exit, the kernel will consider the address stored in 'list_op_pending' and the address of each 'lock word' found by walking the list starting at 'head'. For each such address, if the bottom 30 @@ -182,3 +185,44 @@ any point: When the kernel sees a list entry whose 'lock word' doesn't have the current threads TID in the lower 30 bits, it does nothing with that entry, and goes on to the next entry. + +Robust release is racy +---------------------- + +The removal of a robust futex from the list is racy when doing it solely in +userspace. Quoting Thomas Gleixner for the explanation: + + The robust futex unlock mechanism is racy in respect to the clearing of the + robust_list_head::list_op_pending pointer because unlock and clearing the + pointer are not atomic. The race window is between the unlock and clearing + the pending op pointer. If the task is forced to exit in this window, exit + will access a potentially invalid pending op pointer when cleaning up the + robust list. That happens if another task manages to unmap the object + containing the lock before the cleanup, which results in an UAF. In the + worst case this UAF can lead to memory corruption when unrelated content + has been mapped to the same address by the time the access happens. + +A full in-depth analysis can be read at +https://lore.kernel.org/lkml/20260316162316.356674433@kernel.org/ + +To overcome that, the kernel needs to participate in the lock release operation. +This ensures that the release happens "atomically" with regard to releasing +the lock and removing the address from ``list_op_pending``. If the release is +interrupted by a signal, the kernel will also verify if it interrupted the +release operation. + +For the contended unlock case, where other threads are waiting for the lock +release, there's the ``FUTEX_ROBUST_UNLOCK`` operation feature flag for the +``futex()`` system call, which must be used with one of the following +operations: ``FUTEX_WAKE``, ``FUTEX_WAKE_BITSET`` or ``FUTEX_UNLOCK_PI``. +The kernel will release the lock (set the futex word to zero), clean the +``list_op_pending`` field. Then, it will proceed with the normal wake path. + +For the non-contended path, there's still a race between checking the futex word +and clearing the ``list_op_pending`` field. To solve this without the need of a +complete system call, userspace should call the virtual syscall +``__vdso_futex_robust_listXX_try_unlock()`` (where XX is either 32 or 64, +depending on the size of the pointer). If the vDSO call succeeds, it means that +it released the lock and cleared ``list_op_pending``. If it fails, that means +that there are waiters for this lock and a call to ``futex()`` syscall with +``FUTEX_ROBUST_UNLOCK`` is needed.