From mboxrd@z Thu Jan 1 00:00:00 1970 From: Halil Pasic Subject: Re: [BUG/RFC] vhost: net: big endian viring access despite virtio 1 Date: Fri, 27 Jan 2017 13:24:13 +0100 Message-ID: References: <20170126211511-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, netdev@vger.kernel.org, Greg Kurz , "virtualization@lists.linux-foundation.org" To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20170126211511-mutt-send-email-mst@kernel.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: netdev.vger.kernel.org On 01/26/2017 08:20 PM, Michael S. Tsirkin wrote: > On Thu, Jan 26, 2017 at 06:39:14PM +0100, Halil Pasic wrote: >> >> Hi! >> >> Recently I have been investigating some strange migration problems on >> s390x. >> >> It turned out under certain circumstances vhost_net corrupts avail.idx by >> using wrong endianness. [..] >> -------------------------8<-------------- >> >From b26e2bbdc03832a0204ee2b42967a1b49a277dc8 Mon Sep 17 00:00:00 2001 >> From: Halil Pasic >> Date: Thu, 26 Jan 2017 00:06:15 +0100 >> Subject: [PATCH] vhost: remove useless/dangerous reset of is_le >> >> The reset of is_le does no good, but it contributes its fair share to a >> bug in vhost_net, which occurs if we have some oldubufs when stopping and >> setting a fd = -1 as a backend. Instead of doing something convoluted in >> vhost_net, let's just get rid of the reset. >> >> Signed-off-by: Halil Pasic >> Fixes: commit 2751c9882b94 >> --- >> drivers/vhost/vhost.c | 4 +--- >> 1 file changed, 1 insertion(+), 3 deletions(-) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index d643260..08072a2 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -1714,10 +1714,8 @@ int vhost_vq_init_access(struct vhost_virtqueue *vq) >> int r; >> bool is_le = vq->is_le; >> >> - if (!vq->private_data) { >> - vhost_reset_is_le(vq); >> + if (!vq->private_data) >> return 0; >> - } >> >> vhost_init_is_le(vq); > > > I think you do need to reset it, just maybe within vhost_init_is_le. > > if (vhost_has_feature(vq, VIRTIO_F_VERSION_1)) > vq->is_le = true; > else > vhost_reset_is_le(vq); > > That is a very good point! I have overlooked that while the CONFIG_VHOST_CROSS_ENDIAN_LEGACY variant static void vhost_init_is_le(struct vhost_virtqueue *vq) { /* Note for legacy virtio: user_be is initialized at reset time * according to the host endianness. If userspace does not set an * explicit endianness, the default behavior is native endian, as * expected by legacy virtio. */ vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be; } is fine the other variant static void vhost_init_is_le(struct vhost_virtqueue *vq) { if (vhost_has_feature(vq, VIRTIO_F_VERSION_1)) vq->is_le = true; } is a very strange initializer (makes assumptions about the state to be initialized). I agree, setting native endianness there sounds very reasonable. I have a question regarding readability. IMHO the relationship of reset_is_le and int_is_le is a bit confusing, and I'm afraid it could become even more confusing with using reset in one of the init_is_le's. How about we do the following? static void vhost_init_is_le(struct vhost_virtqueue *vq) { if (vhost_has_feature(vq, VIRTIO_F_VERSION_1)) vq->is_le = true; + else + vq->is_le = virtio_legacy_is_little_endian(); } static void vhost_reset_is_le(struct vhost_virtqueue *vq) { - vq->is_le = virtio_legacy_is_little_endian(); + vhost_init_is_le(vq); } That way we would have correct endianness both after reset and after init, I think :). Thank you very much! Halil