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=-3.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 5F255C07E85 for ; Tue, 11 Dec 2018 08:05:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E898A2081B for ; Tue, 11 Dec 2018 08:04:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E898A2081B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726176AbeLKIE6 (ORCPT ); Tue, 11 Dec 2018 03:04:58 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:45334 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726036AbeLKIE6 (ORCPT ); Tue, 11 Dec 2018 03:04:58 -0500 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wBB84org089025 for ; Tue, 11 Dec 2018 03:04:57 -0500 Received: from e06smtp04.uk.ibm.com (e06smtp04.uk.ibm.com [195.75.94.100]) by mx0b-001b2d01.pphosted.com with ESMTP id 2pa4ta1yym-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 11 Dec 2018 03:04:56 -0500 Received: from localhost by e06smtp04.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 11 Dec 2018 08:04:54 -0000 Received: from b06cxnps4075.portsmouth.uk.ibm.com (9.149.109.197) by e06smtp04.uk.ibm.com (192.168.101.134) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 11 Dec 2018 08:04:51 -0000 Received: from d06av26.portsmouth.uk.ibm.com (d06av26.portsmouth.uk.ibm.com [9.149.105.62]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id wBB84ogi48693320 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 11 Dec 2018 08:04:50 GMT Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C41FDAE045; Tue, 11 Dec 2018 08:04:50 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7F63FAE04D; Tue, 11 Dec 2018 08:04:50 +0000 (GMT) Received: from oc4452167425.ibm.com (unknown [9.152.222.184]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 11 Dec 2018 08:04:50 +0000 (GMT) Subject: Re: [patch] futex: Cure exit race To: Thomas Gleixner , LKML Cc: Heiko Carstens , Peter Zijlstra , Darren Hart , Ingo Molnar References: <20181210152311.986181245@linutronix.de> From: Stefan Liebler Date: Tue, 11 Dec 2018 09:04:50 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181210152311.986181245@linutronix.de> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-IE Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 18121108-0016-0000-0000-00000234D28E X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18121108-0017-0000-0000-0000328CFE8E Message-Id: <06e7e269-6c87-f5f2-9a15-435b0a376105@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-12-11_03:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1011 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=982 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1812110077 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Thomas, does this also handle the ESRCH returned by attach_to_pi_owner(...) {... if (!pid) return -ESRCH; p = find_get_task_by_vpid(pid); if (!p) return -ESRCH; ... I think pid should never be zero when attach_to_pi_owner is called. But it can happen that p is null? At least I traced the "return -ESRCH" with the 4.17 kernel. Unfortunately both returns were done by the same instruction address. Bye Stefan On 12/10/2018 04:23 PM, Thomas Gleixner wrote: > Stefan reported, that the glibc tst-robustpi4 test case fails > occasionally. That case creates the following race between > sys_exit() and sys_futex(LOCK_PI): > > CPU0 CPU1 > > sys_exit() sys_futex() > do_exit() futex_lock_pi() > exit_signals(tsk) No waiters: > tsk->flags |= PF_EXITING; *uaddr == 0x00000PID > mm_release(tsk) Set waiter bit > exit_robust_list(tsk) { *uaddr = 0x80000PID; > Set owner died attach_to_pi_owner() { > *uaddr = 0xC0000000; tsk = get_task(PID); > } if (!tsk->flags & PF_EXITING) { > ... attach(); > tsk->flags |= PF_EXITPIDONE; } else { > if (!(tsk->flags & PF_EXITPIDONE)) > return -EAGAIN; > return -ESRCH; <--- FAIL > } > > ESRCH is returned all the way to user space, which triggers the glibc test > case assert. Returning ESRCH unconditionally is wrong here because the user > space value has been changed by the exiting task to 0xC0000000, i.e. the > FUTEX_OWNER_DIED bit is set and the futex PID value has been cleared. This > is a valid state and the kernel has to handle it, i.e. taking the futex. > > Cure it by rereading the user space value when PF_EXITING and PF_EXITPIDONE > is set in the task which owns the futex. If the value has changed, let > the kernel retry the operation, which includes all regular sanity checks > and correctly handles the FUTEX_OWNER_DIED case. > > If it hasn't changed, then return ESRCH as there is no way to distinguish > this case from malfunctioning user space. This happens when the exiting > task did not have a robust list, the robust list was corrupted or the user > space value in the futex was simply bogus. > > Reported-by: Stefan Liebler > Signed-off-by: Thomas Gleixner > Cc: Heiko Carstens > Cc: Peter Zijlstra > Cc: Darren Hart > Cc: Ingo Molnar > Cc: stable@vger.kernel.org > Link: https://bugzilla.kernel.org/show_bug.cgi?id=200467 > --- > kernel/futex.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 53 insertions(+), 4 deletions(-) > > --- a/kernel/futex.c > +++ b/kernel/futex.c > @@ -1148,11 +1148,60 @@ static int attach_to_pi_state(u32 __user > return ret; > } > > +static int handle_exit_race(u32 __user *uaddr, u32 uval, struct task_struct *tsk) > +{ > + u32 uval2; > + > + /* > + * If PF_EXITPIDONE is not yet set try again. > + */ > + if (!(tsk->flags & PF_EXITPIDONE)) > + return -EAGAIN; > + > + /* > + * Reread the user space value to handle the following situation: > + * > + * CPU0 CPU1 > + * > + * sys_exit() sys_futex() > + * do_exit() futex_lock_pi() > + * exit_signals(tsk) No waiters: > + * tsk->flags |= PF_EXITING; *uaddr == 0x00000PID > + * mm_release(tsk) Set waiter bit > + * exit_robust_list(tsk) { *uaddr = 0x80000PID; > + * Set owner died attach_to_pi_owner() { > + * *uaddr = 0xC0000000; tsk = get_task(PID); > + * } if (!tsk->flags & PF_EXITING) { > + * ... attach(); > + * tsk->flags |= PF_EXITPIDONE; } else { > + * if (!(tsk->flags & PF_EXITPIDONE)) > + * return -EAGAIN; > + * return -ESRCH; <--- FAIL > + * } > + * > + * Returning ESRCH unconditionally is wrong here because the > + * user space value has been changed by the exiting task. > + */ > + if (get_futex_value_locked(&uval2, uaddr)) > + return -EFAULT; > + > + /* If the user space value has changed, try again. */ > + if (uval2 != uval) > + return -EAGAIN; > + > + /* > + * The exiting task did not have a robust list, the robust list was > + * corrupted or the user space value in *uaddr is simply bogus. > + * Give up and tell user space. > + */ > + return -ESRCH; > +} > + > /* > * Lookup the task for the TID provided from user space and attach to > * it after doing proper sanity checks. > */ > -static int attach_to_pi_owner(u32 uval, union futex_key *key, > +static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key, > struct futex_pi_state **ps) > { > pid_t pid = uval & FUTEX_TID_MASK; > @@ -1187,7 +1236,7 @@ static int attach_to_pi_owner(u32 uval, > * set, we know that the task has finished the > * cleanup: > */ > - int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN; > + int ret = handle_exit_race(uaddr, uval, p); > > raw_spin_unlock_irq(&p->pi_lock); > put_task_struct(p); > @@ -1244,7 +1293,7 @@ static int lookup_pi_state(u32 __user *u > * We are the first waiter - try to look up the owner based on > * @uval and attach to it. > */ > - return attach_to_pi_owner(uval, key, ps); > + return attach_to_pi_owner(uaddr, uval, key, ps); > } > > static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) > @@ -1352,7 +1401,7 @@ static int futex_lock_pi_atomic(u32 __us > * attach to the owner. If that fails, no harm done, we only > * set the FUTEX_WAITERS bit in the user space variable. > */ > - return attach_to_pi_owner(uval, key, ps); > + return attach_to_pi_owner(uaddr, uval, key, ps); > } > > /** > >