From nobody Wed Nov 5 20:00:14 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1536843391603614.1414788385213; Thu, 13 Sep 2018 05:56:31 -0700 (PDT) Received: from localhost ([::1]:42284 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1g0RAc-0004lh-He for importer@patchew.org; Thu, 13 Sep 2018 08:56:30 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46870) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1g0R7K-0002PE-F3 for qemu-devel@nongnu.org; Thu, 13 Sep 2018 08:53:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1g0R7J-0001xZ-Jb for qemu-devel@nongnu.org; Thu, 13 Sep 2018 08:53:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46576) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1g0R7F-0001u4-Lr; Thu, 13 Sep 2018 08:53:01 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0BA92308338E; Thu, 13 Sep 2018 12:53:01 +0000 (UTC) Received: from localhost.localdomain.com (ovpn-117-67.ams2.redhat.com [10.36.117.67]) by smtp.corp.redhat.com (Postfix) with ESMTP id 34B8760BE0; Thu, 13 Sep 2018 12:52:59 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Date: Thu, 13 Sep 2018 14:52:10 +0200 Message-Id: <20180913125217.23173-11-kwolf@redhat.com> In-Reply-To: <20180913125217.23173-1-kwolf@redhat.com> References: <20180913125217.23173-1-kwolf@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Thu, 13 Sep 2018 12:53:01 +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: [Qemu-devel] [PATCH v2 10/17] block-backend: Fix potential double blk_delete() X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, famz@redhat.com, slp@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, pbonzini@redhat.com, jsnow@redhat.com Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RDMRC_1 RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" blk_unref() first decreases the refcount of the BlockBackend and calls blk_delete() if the refcount reaches zero. Requests can still be in flight at this point, they are only drained during blk_delete(): At this point, arbitrary callbacks can run. If any callback takes a temporary BlockBackend reference, it will first increase the refcount to 1 and then decrease it to 0 again, triggering another blk_delete(). This will cause a use-after-free crash in the outer blk_delete(). Fix it by draining the BlockBackend before decreasing to refcount to 0. Assert in blk_ref() that it never takes the first refcount (which would mean that the BlockBackend is already being deleted). Signed-off-by: Kevin Wolf Reviewed-by: Fam Zheng Reviewed-by: Max Reitz --- block/block-backend.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/block/block-backend.c b/block/block-backend.c index efa8d8011c..1b2d7a6ff5 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -435,6 +435,7 @@ int blk_get_refcnt(BlockBackend *blk) */ void blk_ref(BlockBackend *blk) { + assert(blk->refcnt > 0); blk->refcnt++; } =20 @@ -447,7 +448,11 @@ void blk_unref(BlockBackend *blk) { if (blk) { assert(blk->refcnt > 0); - if (!--blk->refcnt) { + if (blk->refcnt > 1) { + blk->refcnt--; + } else { + blk_drain(blk); + blk->refcnt =3D 0; blk_delete(blk); } } --=20 2.13.6