From nobody Sat Apr 20 08:46:58 2024 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 Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1526469820054179.7883698577399; Wed, 16 May 2018 04:23:40 -0700 (PDT) Received: from localhost ([::1]:41217 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fIuWt-00051H-1M for importer@patchew.org; Wed, 16 May 2018 07:23:35 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53684) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fIuVZ-0004Ko-L0 for qemu-devel@nongnu.org; Wed, 16 May 2018 07:22:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fIuVW-0001Xf-9D for qemu-devel@nongnu.org; Wed, 16 May 2018 07:22:13 -0400 Received: from 10.mo1.mail-out.ovh.net ([178.32.96.102]:37103) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fIuVW-0001X4-1c for qemu-devel@nongnu.org; Wed, 16 May 2018 07:22:10 -0400 Received: from player696.ha.ovh.net (unknown [10.109.105.77]) by mo1.mail-out.ovh.net (Postfix) with ESMTP id 8F160F7812 for ; Wed, 16 May 2018 13:22:06 +0200 (CEST) Received: from bahia.lan (lns-bzn-46-82-253-208-248.adsl.proxad.net [82.253.208.248]) (Authenticated sender: groug@kaod.org) by player696.ha.ovh.net (Postfix) with ESMTPA id BF6FD3C00A1; Wed, 16 May 2018 13:22:00 +0200 (CEST) From: Greg Kurz To: qemu-devel@nongnu.org Date: Wed, 16 May 2018 13:21:54 +0200 Message-ID: <152646971421.34839.18198173866060880395.stgit@bahia.lan> User-Agent: StGit/0.17.1-46-g6855-dirty MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Ovh-Tracer-Id: 1467329056620714248 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: -100 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrgedthedrvdelgdefkecutefuodetggdotefrodftvfcurfhrohhfihhlvgemucfqggfjpdevjffgvefmvefgnecuuegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmd X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 178.32.96.102 Subject: [Qemu-devel] [PATCH] block: fix QEMU crash with scsi-hd and drive_del 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: Kevin Wolf , stable@vger.kernel.org, Stefan Hajnoczi , Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Removing a drive with drive_del while it is being used to run an I/O intensive workload can cause QEMU to crash. An AIO flush can yield at some point: blk_aio_flush_entry() blk_co_flush(blk) bdrv_co_flush(blk->root->bs) ... qemu_coroutine_yield() and let the HMP command to run, free blk->root and give control back to the AIO flush: hmp_drive_del() blk_remove_bs() bdrv_root_unref_child(blk->root) child_bs =3D blk->root->bs bdrv_detach_child(blk->root) bdrv_replace_child(blk->root, NULL) blk->root->bs =3D NULL g_free(blk->root) <=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D blk->= root becomes stale bdrv_unref(child_bs) bdrv_delete(child_bs) bdrv_close() bdrv_drained_begin() bdrv_do_drained_begin() bdrv_drain_recurse() aio_poll() ... qemu_coroutine_switch() and the AIO flush completion ends up dereferencing blk->root: blk_aio_complete() scsi_aio_complete() blk_get_aio_context(blk) bs =3D blk_bs(blk) ie, bs =3D blk->root ? blk->root->bs : NULL ^^^^^ stale The solution to this user-after-free situation is is to clear blk->root before calling bdrv_unref() in bdrv_detach_child(), and let blk_get_aio_context() fall back to the main loop context since the BDS has been removed. Signed-off-by: Greg Kurz --- The use-after-free condition is easy to reproduce with a stress-ng run in the guest: -device virtio-scsi-pci,id=3Dscsi1 \ -drive file=3D/home/greg/images/scratch.qcow2,format=3Dqcow2,if=3Dnone,id= =3Ddrive1 \ -device scsi-hd,bus=3Dscsi1.0,drive=3Ddrive1,id=3Dscsi-hd1 # stress-ng --hdd 0 --aggressive and doing drive_del from the QEMU monitor while stress-ng is still running: (qemu) drive_del drive1 The crash is less easy to hit though, as it depends on the bs field of the stale blk->root to have a non-NULL value that eventually breaks something when it gets dereferenced. The following patch simulates that, and allows to validate the fix: --- a/block.c +++ b/block.c @@ -2127,6 +2127,8 @@ BdrvChild *bdrv_attach_child(BlockDriverState *paren= t_bs, static void bdrv_detach_child(BdrvChild *child) { + BlockDriverState *bs =3D child->bs; + if (child->next.le_prev) { QLIST_REMOVE(child, next); child->next.le_prev =3D NULL; @@ -2135,7 +2137,15 @@ static void bdrv_detach_child(BdrvChild *child) bdrv_replace_child(child, NULL); g_free(child->name); - g_free(child); + /* Poison the BdrvChild instead of freeing it, in order to break blk_b= s() + * if the blk still has a pointer to this BdrvChild in blk->root. + */ + if (atomic_read(&bs->in_flight)) { + child->bs =3D (BlockDriverState *) -1; + fprintf(stderr, "\nPoisonned BdrvChild %p\n", child); + } else { + g_free(child); + } } void bdrv_root_unref_child(BdrvChild *child) --- block/block-backend.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/block/block-backend.c b/block/block-backend.c index 681b240b1268..ed9434e236b9 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -756,6 +756,7 @@ void blk_remove_bs(BlockBackend *blk) { ThrottleGroupMember *tgm =3D &blk->public.throttle_group_member; BlockDriverState *bs; + BdrvChild *root; =20 notifier_list_notify(&blk->remove_bs_notifiers, blk); if (tgm->throttle_state) { @@ -768,8 +769,9 @@ void blk_remove_bs(BlockBackend *blk) =20 blk_update_root_state(blk); =20 - bdrv_root_unref_child(blk->root); + root =3D blk->root; blk->root =3D NULL; + bdrv_root_unref_child(root); } =20 /*