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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 73477C19759 for ; Thu, 1 Aug 2019 11:15:25 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 42EFF2087E for ; Thu, 1 Aug 2019 11:15:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 42EFF2087E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:54660 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1ht93M-0000D0-EB for qemu-devel@archiver.kernel.org; Thu, 01 Aug 2019 07:15:24 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:58345) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1ht92t-0008C6-Aw for qemu-devel@nongnu.org; Thu, 01 Aug 2019 07:14:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ht92r-0001be-VU for qemu-devel@nongnu.org; Thu, 01 Aug 2019 07:14:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41664) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ht92r-0001Yh-Mq for qemu-devel@nongnu.org; Thu, 01 Aug 2019 07:14:53 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 604F1A3B65 for ; Thu, 1 Aug 2019 11:14:52 +0000 (UTC) Received: from work-vm (ovpn-117-184.ams2.redhat.com [10.36.117.184]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2CFAD1000335; Thu, 1 Aug 2019 11:14:46 +0000 (UTC) Date: Thu, 1 Aug 2019 12:14:44 +0100 From: "Dr. David Alan Gilbert" To: Stefan Hajnoczi Message-ID: <20190801111444.GB2773@work-vm> References: <20190726091103.23503-1-stefanha@redhat.com> <20190726091103.23503-6-stefanha@redhat.com> <20190731174452.GL3203@work-vm> <20190801091527.GB15179@stefanha-x1.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190801091527.GB15179@stefanha-x1.localdomain> User-Agent: Mutt/1.12.0 (2019-05-25) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Thu, 01 Aug 2019 11:14:52 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH 5/5] virtiofsd: prevent races with lo_dirp_put() X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: virtio-fs@redhat.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Stefan Hajnoczi (stefanha@redhat.com) wrote: > On Wed, Jul 31, 2019 at 06:44:52PM +0100, Dr. David Alan Gilbert wrote: > > * Stefan Hajnoczi (stefanha@redhat.com) wrote: > > > Introduce lo_dirp_put() so that FUSE_RELEASEDIR does not cause > > > use-after-free races with other threads that are accessing lo_dirp. > > > > > > Also make lo_releasedir() atomic to prevent FUSE_RELEASEDIR racing with > > > itself. This prevents double-frees. > > > > > > Signed-off-by: Stefan Hajnoczi > > > --- > > > contrib/virtiofsd/passthrough_ll.c | 42 +++++++++++++++++++++++++----- > > > 1 file changed, 36 insertions(+), 6 deletions(-) > > > > > > diff --git a/contrib/virtiofsd/passthrough_ll.c b/contrib/virtiofsd/passthrough_ll.c > > > index ad3abdd532..f74e7d2d21 100644 > > > --- a/contrib/virtiofsd/passthrough_ll.c > > > +++ b/contrib/virtiofsd/passthrough_ll.c > > > @@ -1293,11 +1293,28 @@ static void lo_readlink(fuse_req_t req, fuse_ino_t ino) > > > } > > > > > > struct lo_dirp { > > > + gint refcount; > > > DIR *dp; > > > struct dirent *entry; > > > off_t offset; > > > }; > > > > > > +static void lo_dirp_put(struct lo_dirp **dp) > > > +{ > > > + struct lo_dirp *d = *dp; > > > + > > > + if (!d) { > > > + return; > > > + } > > > + *dp = NULL; > > > + > > > + if (g_atomic_int_dec_and_test(&d->refcount)) { > > > + closedir(d->dp); > > > + free(d); > > > + } > > > +} > > > + > > > +/* Call lo_dirp_put() on the return value when no longer needed */ > > > static struct lo_dirp *lo_dirp(fuse_req_t req, struct fuse_file_info *fi) > > > { > > > struct lo_data *lo = lo_data(req); > > > @@ -1305,6 +1322,9 @@ static struct lo_dirp *lo_dirp(fuse_req_t req, struct fuse_file_info *fi) > > > > > > pthread_mutex_lock(&lo->mutex); > > > elem = lo_map_get(&lo->dirp_map, fi->fh); > > > + if (elem) { > > > + g_atomic_int_inc(&elem->dirp->refcount); > > > > I don't understand what protects against reading the elem->dirp > > here at the same time it's free'd by lo_releasedir's call to lo_dirp_put > > It is lo->mutex and not the refcount that prevents the race with > lo_releasedir(). Two cases: > > 1. lo_releasedir() runs before lo_dirp(). lo_map_get() returns NULL and > lo_dirp() fails. Ah that's what I was missing; it's that the lo_releasedir doesn't need to have completed before lo_dirp runs, it's just that it's lo_map_remove has happened. > 2. lo_releasedir() runs after lo_dirp(). lo_map_get() succeeds and the > lo_dirp() caller keeps the object alive until lo_dirp_put(), when we > finally free it. > > There is no third case since lo->mutex ensures that lo_releasedir() and > lo_dirp() are serialized in the dirp_map lookup. > > > + } > > > pthread_mutex_unlock(&lo->mutex); > > > if (!elem) > > > return NULL; > > > @@ -1335,6 +1355,8 @@ static void lo_opendir(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi > > > d->offset = 0; > > > d->entry = NULL; > > > > > > + g_atomic_int_set(&d->refcount, 1); /* paired with lo_releasedir() */ > > > + > > > fh = lo_add_dirp_mapping(req, d); > > > if (fh == -1) > > > goto out_err; > > > @@ -1363,7 +1385,7 @@ static void lo_do_readdir(fuse_req_t req, fuse_ino_t ino, size_t size, > > > off_t offset, struct fuse_file_info *fi, int plus) > > > { > > > struct lo_data *lo = lo_data(req); > > > - struct lo_dirp *d; > > > + struct lo_dirp *d = NULL; > > > struct lo_inode *dinode; > > > char *buf = NULL; > > > char *p; > > > @@ -1451,6 +1473,8 @@ static void lo_do_readdir(fuse_req_t req, fuse_ino_t ino, size_t size, > > > > > > err = 0; > > > error: > > > + lo_dirp_put(&d); > > > + > > > // If there's an error, we can only signal it if we haven't stored > > > // any entries yet - otherwise we'd end up with wrong lookup > > > // counts for the entries that are already in the buffer. So we > > > @@ -1477,22 +1501,25 @@ static void lo_readdirplus(fuse_req_t req, fuse_ino_t ino, size_t size, > > > static void lo_releasedir(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) > > > { > > > struct lo_data *lo = lo_data(req); > > > + struct lo_map_elem *elem; > > > struct lo_dirp *d; > > > > > > (void) ino; > > > > > > - d = lo_dirp(req, fi); > > > - if (!d) { > > > + pthread_mutex_lock(&lo->mutex); > > > + elem = lo_map_get(&lo->dirp_map, fi->fh); > > > + if (!elem) { > > > + pthread_mutex_unlock(&lo->mutex); > > > fuse_reply_err(req, EBADF); > > > return; > > > } > > > > > > - pthread_mutex_lock(&lo->mutex); > > > + d = elem->dirp; > > > lo_map_remove(&lo->dirp_map, fi->fh); > > > pthread_mutex_unlock(&lo->mutex); > > > > > > - closedir(d->dp); > > > - free(d); > > > + lo_dirp_put(&d); /* paired with lo_opendir() */ > > > > Is the &d really what's intended? That's the local stack variable, so > > lo_dirp_put will set that local to NULL rather than the elem->dirp wont > > it? > > Yes, the put(&ptr) pattern prevents user-after-free in the caller. It's > slightly safer than put(ptr) since that leaves ptr initialized and the > caller might access it later by accident. > > elem has already been returned to the freelist by lo_map_remove() and we > must not touch it anymore. OK, thanks. > Stefan -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK