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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 681DBC433FE for ; Tue, 15 Nov 2022 15:46:59 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ouxyy-00066v-Ri; Tue, 15 Nov 2022 10:36:16 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ouxyw-00063C-0Z for qemu-devel@nongnu.org; Tue, 15 Nov 2022 10:36:14 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ouxys-0008Ov-2N for qemu-devel@nongnu.org; Tue, 15 Nov 2022 10:36:13 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1668526569; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=lv4vZEI7OQUfHArHUOc7UL/dhPGExU7enAwFsaFQkm8=; b=QgO7J6D+/TqzRiWAu3gOgjYGqyu/cTedweN7WlN12jAVlK55Ew9qjqFxKzIHT2LLXEeo3c xe9nlRCYiUgqLyRJozZokbC6NZstegNCzB5q1pJSrnVYfWawqMjDVlxO24kpqluWGaOy+Q pV5A0nQQSoPzWSJ3wVh5I/yWWbpP1MQ= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-379-rRnEhYIOP4ePxd5R-PsBpQ-1; Tue, 15 Nov 2022 10:36:02 -0500 X-MC-Unique: rRnEhYIOP4ePxd5R-PsBpQ-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 3BB843C0F7EF; Tue, 15 Nov 2022 15:35:57 +0000 (UTC) Received: from secure.mitica (unknown [10.39.195.11]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8462F2166B2B; Tue, 15 Nov 2022 15:35:54 +0000 (UTC) From: Juan Quintela To: qemu-devel@nongnu.org Cc: Michael Tokarev , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , David Hildenbrand , Laurent Vivier , Juan Quintela , Paolo Bonzini , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Peter Xu , Stefan Hajnoczi , "Dr. David Alan Gilbert" , Thomas Huth , qemu-block@nongnu.org, qemu-trivial@nongnu.org, =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , Fam Zheng Subject: [PULL 11/30] migration: Fix race on qemu_file_shutdown() Date: Tue, 15 Nov 2022 16:34:55 +0100 Message-Id: <20221115153514.28003-12-quintela@redhat.com> In-Reply-To: <20221115153514.28003-1-quintela@redhat.com> References: <20221115153514.28003-1-quintela@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass client-ip=170.10.129.124; envelope-from=quintela@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org From: Peter Xu In qemu_file_shutdown(), there's a possible race if with current order of operation. There're two major things to do: (1) Do real shutdown() (e.g. shutdown() syscall on socket) (2) Update qemufile's last_error We must do (2) before (1) otherwise there can be a race condition like: page receiver other thread ------------- ------------ qemu_get_buffer() do shutdown() returns 0 (buffer all zero) (meanwhile we didn't check this retcode) try to detect IO error last_error==NULL, IO okay install ALL-ZERO page set last_error --> guest crash! To fix this, we can also check retval of qemu_get_buffer(), but not all APIs can be properly checked and ultimately we still need to go back to qemu_file_get_error(). E.g. qemu_get_byte() doesn't return error. Maybe some day a rework of qemufile API is really needed, but for now keep using qemu_file_get_error() and fix it by not allowing that race condition to happen. Here shutdown() is indeed special because the last_error was emulated. For real -EIO errors it'll always be set when e.g. sendmsg() error triggers so we won't miss those ones, only shutdown() is a bit tricky here. Cc: Daniel P. Berrange Reviewed-by: Dr. David Alan Gilbert Signed-off-by: Peter Xu Reviewed-by: Juan Quintela Signed-off-by: Juan Quintela --- migration/qemu-file.c | 27 ++++++++++++++++++++++++--- 1 file changed, 24 insertions(+), 3 deletions(-) diff --git a/migration/qemu-file.c b/migration/qemu-file.c index 4f400c2e52..2d5f74ffc2 100644 --- a/migration/qemu-file.c +++ b/migration/qemu-file.c @@ -79,6 +79,30 @@ int qemu_file_shutdown(QEMUFile *f) int ret = 0; f->shutdown = true; + + /* + * We must set qemufile error before the real shutdown(), otherwise + * there can be a race window where we thought IO all went though + * (because last_error==NULL) but actually IO has already stopped. + * + * If without correct ordering, the race can happen like this: + * + * page receiver other thread + * ------------- ------------ + * qemu_get_buffer() + * do shutdown() + * returns 0 (buffer all zero) + * (we didn't check this retcode) + * try to detect IO error + * last_error==NULL, IO okay + * install ALL-ZERO page + * set last_error + * --> guest crash! + */ + if (!f->last_error) { + qemu_file_set_error(f, -EIO); + } + if (!qio_channel_has_feature(f->ioc, QIO_CHANNEL_FEATURE_SHUTDOWN)) { return -ENOSYS; @@ -88,9 +112,6 @@ int qemu_file_shutdown(QEMUFile *f) ret = -EIO; } - if (!f->last_error) { - qemu_file_set_error(f, -EIO); - } return ret; } -- 2.38.1