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 E89ECC71130 for ; Tue, 10 Jun 2025 17:04:39 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uP2DR-0006l5-BH; Tue, 10 Jun 2025 12:52:49 -0400 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 1uP2DL-0006bz-R2 for qemu-devel@nongnu.org; Tue, 10 Jun 2025 12:52:44 -0400 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 1uP2DJ-0000u3-QC for qemu-devel@nongnu.org; Tue, 10 Jun 2025 12:52:43 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749574359; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=IlqaPrpCrRhXVsFCVYho6swmBfXItq45l+TYY1Wcthw=; b=R8Mgmivfb+zxqtYjVhQY/lh3ZRCNIx6aMRDHrkSoHCutY+ITW3LxrBYsMzmSL+OejR1+p2 RwzpNNRtvp6li9a0BX0BX3EQHCyDQADXyeqjEeBhtLwzyeFBhkvfZaM9Dz0lh7ira4XwF8 6n+ufIr15ECJWQjTeXg/+c74I4DzpGE= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-477-Tt2CLkSpOsaVQsEEVfsJrA-1; Tue, 10 Jun 2025 12:52:37 -0400 X-MC-Unique: Tt2CLkSpOsaVQsEEVfsJrA-1 X-Mimecast-MFC-AGG-ID: Tt2CLkSpOsaVQsEEVfsJrA_1749574356 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 807AA195609F; Tue, 10 Jun 2025 16:52:36 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.87]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id C1DD719560A3; Tue, 10 Jun 2025 16:52:34 +0000 (UTC) Date: Tue, 10 Jun 2025 17:52:31 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: "Michael S. Tsirkin" Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, qemu-block@nongnu.org Subject: Re: [PATCH] virtio: avoid cost of -ftrivial-auto-var-init in hot path Message-ID: References: <20250604191843.399309-1-stefanha@redhat.com> <20250610123707-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250610123707-mutt-send-email-mst@kernel.org> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jun 10, 2025 at 12:41:00PM -0400, Michael S. Tsirkin wrote: > On Wed, Jun 04, 2025 at 03:18:43PM -0400, Stefan Hajnoczi wrote: > > Since commit 7ff9ff039380 ("meson: mitigate against use of uninitialize > > stack for exploits") the -ftrivial-auto-var-init=zero compiler option is > > used to zero local variables. While this reduces security risks > > associated with uninitialized stack data, it introduced a measurable > > bottleneck in the virtqueue_split_pop() and virtqueue_packed_pop() > > functions. > > > > These virtqueue functions are in the hot path. They are called for each > > element (request) that is popped from a VIRTIO device's virtqueue. Using > > __attribute__((uninitialized)) on large stack variables in these > > functions improves fio randread bs=4k iodepth=64 performance from 304k > > to 332k IOPS (+9%). > > I ask however whether it is always not worth it for all users. > It does reduce chances of leaking stack info, does it not? > > Maybe we can start with a tri-state Kconfig knob to select between > performance/balanced/paranoid for this and similar variables? I'd prefer this not to be configurable. With a fixed setup, when we analyse reported bugs, we can make a clear determination of whether it is actually an expoitable security bug or not, without having to concern ourselves with build time settings. Essentially the -ftrivial-auto-var-init=zero turned almost all 'uninitialized stack variable' bugs into plain bugs, or even non-bugs in most cases, instead of likely security issues. The QEMU_UNINITIALIZED annotations are targetted fixed to avoid the worst case perf hits, without compromising our security posture. > > > > This issue was found using perf-top(1). virtqueue_split_pop() was one of > > the top CPU consumers and the "annotate" feature showed that the memory > > zeroing instructions at the beginning of the functions were hot. > > > > Fixes: 7ff9ff039380 ("meson: mitigate against use of uninitialize stack for exploits") > > Cc: Daniel P. Berrangé > > Signed-off-by: Stefan Hajnoczi > > --- > > include/qemu/compiler.h | 12 ++++++++++++ > > hw/virtio/virtio.c | 8 ++++---- > > 2 files changed, 16 insertions(+), 4 deletions(-) > > > > diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h > > index 496dac5ac1..fabd540b02 100644 > > --- a/include/qemu/compiler.h > > +++ b/include/qemu/compiler.h > > @@ -207,6 +207,18 @@ > > # define QEMU_USED > > #endif > > > > +/* > > + * Disable -ftrivial-auto-var-init on a local variable. Use this in rare cases > > + * when the compiler zeroes a large on-stack variable and this causes a > > + * performance bottleneck. Only use it when performance data indicates this is > > + * necessary since security risks increase with uninitialized stack variables. > > + */ > > +#if __has_attribute(uninitialized) > > +# define QEMU_UNINITIALIZED __attribute__((uninitialized)) > > +#else > > +# define QEMU_UNINITIALIZED > > +#endif > > + > > /* > > * http://clang.llvm.org/docs/ThreadSafetyAnalysis.html > > * > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c > > index 5534251e01..82a285a31d 100644 > > --- a/hw/virtio/virtio.c > > +++ b/hw/virtio/virtio.c > > @@ -1689,8 +1689,8 @@ static void *virtqueue_split_pop(VirtQueue *vq, size_t sz) > > VirtIODevice *vdev = vq->vdev; > > VirtQueueElement *elem = NULL; > > unsigned out_num, in_num, elem_entries; > > - hwaddr addr[VIRTQUEUE_MAX_SIZE]; > > - struct iovec iov[VIRTQUEUE_MAX_SIZE]; > > + hwaddr QEMU_UNINITIALIZED addr[VIRTQUEUE_MAX_SIZE]; > > + struct iovec QEMU_UNINITIALIZED iov[VIRTQUEUE_MAX_SIZE]; > > VRingDesc desc; > > int rc; > > > > @@ -1836,8 +1836,8 @@ static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz) > > VirtIODevice *vdev = vq->vdev; > > VirtQueueElement *elem = NULL; > > unsigned out_num, in_num, elem_entries; > > - hwaddr addr[VIRTQUEUE_MAX_SIZE]; > > - struct iovec iov[VIRTQUEUE_MAX_SIZE]; > > + hwaddr QEMU_UNINITIALIZED addr[VIRTQUEUE_MAX_SIZE]; > > + struct iovec QEMU_UNINITIALIZED iov[VIRTQUEUE_MAX_SIZE]; > > VRingPackedDesc desc; > > uint16_t id; > > int rc; > > -- > > 2.49.0 > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|