From patchwork Wed May 16 11:21:54 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kurz X-Patchwork-Id: 914556 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=2001:4830:134:3::11; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=none (p=none dis=none) header.from=kaod.org Received: from lists.gnu.org (lists.gnu.org [IPv6:2001:4830:134:3::11]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 40mBqt5WMwz9s0y for ; Wed, 16 May 2018 21:22:50 +1000 (AEST) Received: from localhost ([::1]:41191 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fIuW8-0004MJ-CZ for incoming@patchwork.ozlabs.org; Wed, 16 May 2018 07:22:48 -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 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+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" 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 = blk->root->bs bdrv_detach_child(blk->root) bdrv_replace_child(blk->root, NULL) blk->root->bs = NULL g_free(blk->root) <============== 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 = blk_bs(blk) ie, bs = 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=scsi1 \ -drive file=/home/greg/images/scratch.qcow2,format=qcow2,if=none,id=drive1 \ -device scsi-hd,bus=scsi1.0,drive=drive1,id=scsi-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 *parent_bs, static void bdrv_detach_child(BdrvChild *child) { + BlockDriverState *bs = child->bs; + if (child->next.le_prev) { QLIST_REMOVE(child, next); child->next.le_prev = 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_bs() + * if the blk still has a pointer to this BdrvChild in blk->root. + */ + if (atomic_read(&bs->in_flight)) { + child->bs = (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 = &blk->public.throttle_group_member; BlockDriverState *bs; + BdrvChild *root; notifier_list_notify(&blk->remove_bs_notifiers, blk); if (tgm->throttle_state) { @@ -768,8 +769,9 @@ void blk_remove_bs(BlockBackend *blk) blk_update_root_state(blk); - bdrv_root_unref_child(blk->root); + root = blk->root; blk->root = NULL; + bdrv_root_unref_child(root); } /*