From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from outgoing2021.csail.mit.edu (outgoing2021.csail.mit.edu [128.30.2.78]) (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 AA72B3E5A0E for ; Sun, 11 Oct 2026 12:34:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=128.30.2.78 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791722093; cv=none; b=F8BFoI4Jsh0CmF3/W/KtMRa1fN39ioqx5csgnSYyWxd6ugyRiWfiLOqfufwhB3UGdQUrlCeWUjshl3u2WYlKvB7ina4jXWiOQeN5HJTiWiHea27UDiyfBktHQYJdlPtL8elzlo3rXXjpnwbny1M1m7QGEqZ+0LYaSDqfHgAl/v0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791722093; c=relaxed/simple; bh=r7xmvUgbULUKWloNTRMRe90TObNszbwY+sbmuKveLBg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=BpxtgLXW/tjbc+IeSstl0DKzH3xqtLmpf1ujMGBknBTj4EvbOSwZc/2OeDcFcvvVgVHoljxFsRz6pgqVC6CXG95R2YWm1EEd0pYk65QOKt1FBuKNbhykCVXE+OXSBWUykgaLKnrG2vStY05iuJuO2R8s0Oazt83uw0WBT/QR7dA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=csail.mit.edu; spf=pass smtp.mailfrom=csail.mit.edu; arc=none smtp.client-ip=128.30.2.78 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=csail.mit.edu Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=csail.mit.edu Received: from escalante.csail.mit.edu ([128.52.128.198]) by outgoing2021.csail.mit.edu with esmtp (Exim 4.95) (envelope-from ) id 1xFslE-00ACEu-AE; Sun, 11 Oct 2026 08:34:40 -0400 From: Nickolai Zeldovich To: Oleg Nesterov , Christian Brauner Cc: Andrew Morton , linux-kernel@vger.kernel.org, Nickolai Zeldovich Subject: [PATCH v2] exit: fix lost-update race on the child_subreaper bitfields Date: Sun, 11 Oct 2026 08:34:39 -0400 Message-ID: <20261011123440.768054-1-nickolai@csail.mit.edu> X-Mailer: git-send-email 2.53.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit is_child_subreaper, has_child_subreaper, autoreap and, with CONFIG_POSIX_TIMERS=y, timer_create_restore_ids are 1-bit fields that share one memory location in struct signal_struct. A store to any of them is a read-modify-write of the whole word, and three of the stores run without a common lock: - PR_SET_CHILD_SUBREAPER writes me->signal->is_child_subreaper with no lock held. - propagate_has_child_subreaper(), run by walk_process_tree() from the same prctl in an ancestor, writes p->signal->has_child_subreaper of every descendant p under read_lock(&tasklist_lock), which does not exclude the other writers. - PR_TIMER_CREATE_RESTORE_IDS writes current->signal->timer_create_restore_ids with no lock held. When a process calls prctl(PR_SET_CHILD_SUBREAPER, 1) while an ancestor's PR_SET_CHILD_SUBREAPER walk visits it, or while another thread of the same process calls prctl(PR_TIMER_CREATE_RESTORE_IDS), the read-modify-write sequences interleave and one of the bits is silently lost: - If has_child_subreaper is lost, the process's orphaned descendants are reparented straight to init when it exits: find_new_reaper() tests father->signal->has_child_subreaper first and never looks for the ancestor subreaper. The service manager that registered as the subreaper never gets SIGCHLD for them and cannot wait() on them. - If is_child_subreaper is lost, the prctl returned 0 but the process is not a subreaper: orphans of its descendants skip it and are reparented to the next subreaper up or to init. PR_GET_CHILD_SUBREAPER reads back 0. - If timer_create_restore_ids is lost, the prctl returned 0 but timer_create() does not restore the requested timer ids (or keeps restoring them after _OFF), and _GET reads back the old value. Documentation/memory-barriers.txt requires all fields of one bitfield to be protected by one lock for exactly this reason. No lock is taken or documented for these flags, and the prctls need no privilege, so any process can hit the race against its own ancestors or descendants, or between two of its own threads. Make is_child_subreaper and has_child_subreaper separate bool members. Each store then has its own memory location and no read-modify-write of a neighbour. The readers and their locking are unchanged. autoreap stays a bit-field and so shares a memory location only with timer_create_restore_ids. That pair is race-free: autoreap is stored exactly once, in copy_process() before the new signal_struct is reachable by any other task, and timer_create_restore_ids is only stored to the signal_struct of an already running process. Fixes: 749860ce2427 ("prctl: propagate has_child_subreaper flag to every descendant") Assisted-by: LLM Signed-off-by: Nickolai Zeldovich --- Changes in v2: - keep autoreap as a bit-field (Oleg); it is only stored before the signal_struct is published, so sharing a memory location with timer_create_restore_ids is fine. - mention timer_create_restore_ids, which shares the memory location with the subreaper bits today and is a third unlocked writer. v1: https://lore.kernel.org/lkml/20261009222404.760787-1-nickolai@csail.mit.edu/ include/linux/sched/signal.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index d45a5476b97de..efa1e2d6c7a6b 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -130,8 +130,8 @@ struct signal_struct { * process will inherit a flag if they should look for a * child_subreaper process at exit. */ - unsigned int is_child_subreaper:1; - unsigned int has_child_subreaper:1; + bool is_child_subreaper; + bool has_child_subreaper; unsigned int autoreap:1; #ifdef CONFIG_POSIX_TIMERS base-commit: fc1c25014ff8e416d75e8a5ba3b9c0ba5eb8e877 -- 2.55.0