From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C123AC3F37A for ; Thu, 27 Feb 2020 14:15:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8C42824690 for ; Thu, 27 Feb 2020 14:15:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582812937; bh=ogiLIxHaBbWPMNutkARWHFbhFYk5LmlqhuZUM2LEdtI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=zgUP/FR+u0L2Emxk6uSkd9GHpX94d9BUuXVpxIvIOj2cHngtDh2bgNI9ZI6RkcZuF LUtFFb9KAucSRAhdVP2+E5pxfqC92xWs8oM4WEMMDtYDDNRbzTcVpHxdjMX43lynxU KCC5wuyBB0HPP7gkLO+smRXyL3I/XKYPIP0Q4MX4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388943AbgB0OPg (ORCPT ); Thu, 27 Feb 2020 09:15:36 -0500 Received: from mail.kernel.org ([198.145.29.99]:55156 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389319AbgB0OPe (ORCPT ); Thu, 27 Feb 2020 09:15:34 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 3413524690; Thu, 27 Feb 2020 14:15:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582812932; bh=ogiLIxHaBbWPMNutkARWHFbhFYk5LmlqhuZUM2LEdtI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=rhDl8LlKDfIS+qg85i1w+dMBNcAU163aO7v0VEQme/LJRaLgFhDxWEDsQH+BAWm6A KUKmFZwyjMn09/VA0EMyIi4PjfEEjzP14WwM510fQfInsfIFsJATGcndmdLTUumdn2 7AuLdwMvJg/9cvt+DM+Z5cBm3Z+aP0bn4qQS8o6s= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Ioanna Alifieraki , Manfred Spraul , "Herton R. Krzesinski" , Arnd Bergmann , Catalin Marinas , malat@debian.org, "Joel Fernandes (Google)" , Davidlohr Bueso , Jay Vosburgh , Andrew Morton , Linus Torvalds Subject: [PATCH 5.5 072/150] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" Date: Thu, 27 Feb 2020 14:36:49 +0100 Message-Id: <20200227132243.554032340@linuxfoundation.org> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20200227132232.815448360@linuxfoundation.org> References: <20200227132232.815448360@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Ioanna Alifieraki commit edf28f4061afe4c2d9eb1c3323d90e882c1d6800 upstream. This reverts commit a97955844807e327df11aa33869009d14d6b7de0. Commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") removes a lock that is needed. This leads to a process looping infinitely in exit_sem() and can also lead to a crash. There is a reproducer available in [1] and with the commit reverted the issue does not reproduce anymore. Using the reproducer found in [1] is fairly easy to reach a point where one of the child processes is looping infinitely in exit_sem between for(;;) and if (semid == -1) block, while it's trying to free its last sem_undo structure which has already been freed by freeary(). Each sem_undo struct is on two lists: one per semaphore set (list_id) and one per process (list_proc). The list_id list tracks undos by semaphore set, and the list_proc by process. Undo structures are removed either by freeary() or by exit_sem(). The freeary function is invoked when the user invokes a syscall to remove a semaphore set. During this operation freeary() traverses the list_id associated with the semaphore set and removes the undo structures from both the list_id and list_proc lists. For this case, exit_sem() is called at process exit. Each process contains a struct sem_undo_list (referred to as "ulp") which contains the head for the list_proc list. When the process exits, exit_sem() traverses this list to remove each sem_undo struct. As in freeary(), whenever a sem_undo struct is removed from list_proc, it is also removed from the list_id list. Removing elements from list_id is safe for both exit_sem() and freeary() due to sem_lock(). Removing elements from list_proc is not safe; freeary() locks &un->ulp->lock when it performs list_del_rcu(&un->list_proc) but exit_sem() does not (locking was removed by commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()"). This can result in the following situation while executing the reproducer [1] : Consider a child process in exit_sem() and the parent in freeary() (because of semctl(sid[i], NSEM, IPC_RMID)). - The list_proc for the child contains the last two undo structs A and B (the rest have been removed either by exit_sem() or freeary()). - The semid for A is 1 and semid for B is 2. - exit_sem() removes A and at the same time freeary() removes B. - Since A and B have different semid sem_lock() will acquire different locks for each process and both can proceed. The bug is that they remove A and B from the same list_proc at the same time because only freeary() acquires the ulp lock. When exit_sem() removes A it makes ulp->list_proc.next to point at B and at the same time freeary() removes B setting B->semid=-1. At the next iteration of for(;;) loop exit_sem() will try to remove B. The only way to break from for(;;) is for (&un->list_proc == &ulp->list_proc) to be true which is not. Then exit_sem() will check if B->semid=-1 which is and will continue looping in for(;;) until the memory for B is reallocated and the value at B->semid is changed. At that point, exit_sem() will crash attempting to unlink B from the lists (this can be easily triggered by running the reproducer [1] a second time). To prove this scenario instrumentation was added to keep information about each sem_undo (un) struct that is removed per process and per semaphore set (sma). CPU0 CPU1 [caller holds sem_lock(sma for A)] ... freeary() exit_sem() ... ... ... sem_lock(sma for B) spin_lock(A->ulp->lock) ... list_del_rcu(un_A->list_proc) list_del_rcu(un_B->list_proc) Undo structures A and B have different semid and sem_lock() operations proceed. However they belong to the same list_proc list and they are removed at the same time. This results into ulp->list_proc.next pointing to the address of B which is already removed. After reverting commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") the issue was no longer reproducible. [1] https://bugzilla.redhat.com/show_bug.cgi?id=1694779 Link: http://lkml.kernel.org/r/20191211191318.11860-1-ioanna-maria.alifieraki@canonical.com Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") Signed-off-by: Ioanna Alifieraki Acked-by: Manfred Spraul Acked-by: Herton R. Krzesinski Cc: Arnd Bergmann Cc: Catalin Marinas Cc: Cc: Joel Fernandes (Google) Cc: Davidlohr Bueso Cc: Jay Vosburgh Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- ipc/sem.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) --- a/ipc/sem.c +++ b/ipc/sem.c @@ -2368,11 +2368,9 @@ void exit_sem(struct task_struct *tsk) ipc_assert_locked_object(&sma->sem_perm); list_del(&un->list_id); - /* we are the last process using this ulp, acquiring ulp->lock - * isn't required. Besides that, we are also protected against - * IPC_RMID as we hold sma->sem_perm lock now - */ + spin_lock(&ulp->lock); list_del_rcu(&un->list_proc); + spin_unlock(&ulp->lock); /* perform adjustments registered in un */ for (i = 0; i < sma->sem_nsems; i++) {