From nobody Thu Apr 2 22:04:18 2026 Received: from fanzine2.igalia.com (fanzine2.igalia.com [213.97.179.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 868972C08AB for ; Thu, 26 Mar 2026 14:32:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.97.179.56 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774535555; cv=none; b=I1Jkdc9Uw7OcrTJzhnfb3N6K7PBPxXYRtqOmr6ODjUvMmOMl+3FJ6MCe7STLfPPuV6lAcnObuVCAmc0r1BxEtaf5+Y3qCf8wC/5vSa4+mLrMuMTFGAjKL5tQy1lXuu9+ys7wUFuQBXhbQ+9oGCg3Szc0GB7PpW+ergNco0IbEDg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774535555; c=relaxed/simple; bh=KGggsxKWWJDvxaZ4e4xHAmrO3udLwXZkKDv0ovsn68Q=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=humhrlZDDhy8q1lD3q2IQu/rBojvroZUZyXpjWeXcGXmf1u2IfqH0/uzmAjCLTERZTa55bkVOmmQ5uKUQltQMIiG/0Z9kx6je1T35V+euR0bcKloecbWtKCVeC2Dg+JDqk3Awn8pdZItNvC3WI8icQnYwFL5EDkc5IT1SxgIndY= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=igalia.com; spf=pass smtp.mailfrom=igalia.com; dkim=pass (2048-bit key) header.d=igalia.com header.i=@igalia.com header.b=mzhWLpHN; arc=none smtp.client-ip=213.97.179.56 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=igalia.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=igalia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=igalia.com header.i=@igalia.com header.b="mzhWLpHN" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=igalia.com; s=20170329; h=Cc:To:In-Reply-To:References:Message-Id: Content-Transfer-Encoding:Content-Type:MIME-Version:Subject:Date:From:Sender: Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender :Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=qmFOeZD7OODiOJY74657oUnTl2CWU008CdCSDu/nIPo=; b=mzhWLpHNzOFu92ZSI4R3D7v+O9 IpTr9UPvxvekkXfTmyrpQRCMMt4xp5H0BlH0ZDr4Q0dgMqGCUAO8ZUy9xGUIU1TqjHJD0zTVQxM7k l1UyI61hhx9Tf4GsSPIFwyAQ3CIMlKlWIV3zMhAVse8xiD+Oi9ROPJRhZer4PiPz7tnm/uBMsOj/0 bXA2kkNvc9cxsEf94cmG23qna9BJqoFpZVunJlnMAO+T+/OlQfrv9BJHSt+frHC2zb58EHLwS9Fqm niu134kLXkQPq+KuuZvo1ZylEe0OVrCf/rkAT7KG0Vjn5xRv2eC78zTa+FjRdgj5SfW0csplZhhw2 OgaE9hZg==; Received: from [179.118.189.200] (helo=[192.168.15.100]) by fanzine2.igalia.com with esmtpsa (Cipher TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim) id 1w5lkt-006MiO-Mw; Thu, 26 Mar 2026 15:32:15 +0100 From: =?utf-8?q?Andr=C3=A9_Almeida?= Date: Thu, 26 Mar 2026 11:31:50 -0300 Subject: [PATCH 1/2] Documentation: futex: Add a note about robust list race condition Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260326-tonyk-vdso_test-v1-1-30a6f78c8bc3@igalia.com> References: <20260326-tonyk-vdso_test-v1-0-30a6f78c8bc3@igalia.com> In-Reply-To: <20260326-tonyk-vdso_test-v1-0-30a6f78c8bc3@igalia.com> To: Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Darren Hart , Davidlohr Bueso , Mathieu Desnoyers , Sebastian Andrzej Siewior , Carlos O'Donell , Florian Weimer , Darren Hart , Arnd Bergmann , =?utf-8?q?Thomas_Wei=C3=9Fschuh?= Cc: linux-kernel@vger.kernel.org, kernel-dev@igalia.com, =?utf-8?q?Andr=C3=A9_Almeida?= X-Mailer: b4 0.15.0 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. Signed-off-by: Andr=C3=A9 Almeida --- Documentation/locking/robust-futex-ABI.rst | 44 ++++++++++++++++++++++++++= ++++ 1 file changed, 44 insertions(+) diff --git a/Documentation/locking/robust-futex-ABI.rst b/Documentation/loc= king/robust-futex-ABI.rst index f24904f1c16f..1808b108a58e 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. =20 +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 solely in +userspace. Quoting Thomas Gleixer 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 cleari= ng + the pending op pointer. If the task is forced to exit in this window, ex= it + 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 dept analysis can be read at +https://lore.kernel.org/lkml/20260316162316.356674433@kernel.org/ + +To overcome that, the kernel needs to participate of the lock release oper= ation. +This ensures that the release happens "atomically" in the regard of releas= ing +the lock and removing the address from ``lock_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 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 w= ill +release the lock (set the futex word to zero), clean the ``lock_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 fute= x word +and clearing the ``lock_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 ``lock_op_pending``. If it fails, that me= ans +that there are waiters for this lock and a call to ``futex()`` syscall with +``FUTEX_ROBUST_UNLOCK`` is needed. --=20 2.53.0