From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793211755; b=SPX83ZIP7QX4sFdX7CocCxGBMtRrw8JcA8D8DGmY/WVhIWkwgieTaOx6oIVndfxuaRq+ snQa5o+KEJaXSJtbP69jF5UjwclMW0tpsGI2JDK1w/ljjysK5wiq9FEE6T33qjXBMaVqD +pbhddnjP348i4MfURlbEjg7hZxJx3ujQN2we1A7o4D6gviEU918vNBTsx3Ymxii/qKQ+ p1apGxxwwcEYftdsYCtKmZOxVMDb+FbzVieintrXtXyfs+p3eLMS2BQeelubbpZPTfv7B n0CAlWrG1VRgLasRQtx44iIZGIxqNIm9i7iivzN/bNHVaZAnmFDNrrUbxM0ImgdOLHA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793211755; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:User-Agent:X-stable:X-Patchwork-Hint: MIME-Version:Content-Transfer-Encoding; bh=r53YmDKbMfDFSIWx5nySnReCwOmGBIPz/owHg4B92js=; b=4zPpZSaAZSWd3iyVnXtDrTutQUnvm0hdfvhP/jfnniB0+dQpQdu+EIlIe8rg3AaZa/vk WlfvzLXRJmBlYqXwTXQiDrSJMprPuM8I5ufdv1fNtbU/os/FKZOP5ngMAsuEHOFpSsWMi 0asAFwgKYQ6ZjvYSxxNNYrt6jcVzaOLZ08r0nByGW+LIHijEBlknqSH6OwW82c44RptcU yWtkCThnVriW3wpscjbryChzY2SxZ1Ga2RwES92fq4aRBqWi33ekUCjunnkeb3umDj0rt B89nbJCAGgY/LIT7KHvBERCGPtCdV2vcbEAM6qZF4sV6/pWA0J5kX2srUbjr3CC/ZIw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=ZIVgCyav; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 50BA160A2623; Wed, 28 Oct 2026 18:22:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=ZIVgCyav Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9FC0E1F01825; Wed, 28 Oct 2026 18:22:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1793211755; bh=r53YmDKbMfDFSIWx5nySnReCwOmGBIPz/owHg4B92js=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=ZIVgCyavhXu+FrWnM7Bu3Ekmgzh9F8tCrRBwvWxWXC3HYUADpa7yzqJdjhEELkcgj g0Tn0KOrladt6sOryBPz41WCEZzy/aKMm2r7tEcuxKolAoGJvefFqNrjf4B0oJzvCi cskv8Pp2N4LrrWDiCA8LAQ7HRsLy1pgSGd3Pzi4c= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Nickolai Zeldovich , Oleg Nesterov , "Christian Brauner (Amutable)" , Sasha Levin Subject: [PATCH 5.15 489/571] exit: fix lost-update race on the child_subreaper bitfields Date: Wed, 28 Oct 2026 18:00:36 +0100 Message-ID: <20261028170019.631754331@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261028170008.009143722@linuxfoundation.org> References: <20261028170008.009143722@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: patches@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Nickolai Zeldovich [ Upstream commit 28d2905d77bc2e7c20260a6bc6b4179bdf95cb06 ] 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") Signed-off-by: Nickolai Zeldovich Link: https://patch.msgid.link/20261011123440.768054-1-nickolai@csail.mit.edu Acked-by: Oleg Nesterov Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Sasha Levin --- 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 7135324f8fb3c..324a3fd0f3c3e 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -119,8 +119,8 @@ 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 -- 2.53.0