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[79.183.51.3]) by smtp.gmail.com with ESMTPSA id 5sm3306952wmg.34.2020.04.21.06.25.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Apr 2020 06:25:17 -0700 (PDT) Date: Tue, 21 Apr 2020 09:25:15 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org Subject: Re: [PATCH v3] virtio: force spec specified alignment on types Message-ID: <20200421092418-mutt-send-email-mst@kernel.org> References: <20200420204448.377168-1-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 21, 2020 at 10:39:19AM +0800, Jason Wang wrote: > > On 2020/4/21 上午4:46, Michael S. Tsirkin wrote: > > The ring element addresses are passed between components with different > > alignments assumptions. Thus, if guest/userspace selects a pointer and > > host then gets and dereferences it, we might need to decrease the > > compiler-selected alignment to prevent compiler on the host from > > assuming pointer is aligned. > > > > This actually triggers on ARM with -mabi=apcs-gnu - which is a > > deprecated configuration, but it seems safer to handle this > > generally. > > > > Note that userspace that allocates the memory is actually OK and does > > not need to be fixed, but userspace that gets it from guest or another > > process does need to be fixed. The later doesn't generally talk to the > > kernel so while it might be buggy it's not talking to the kernel in the > > buggy way - it's just using the header in the buggy way - so fixing > > header and asking userspace to recompile is the best we can do. > > > > I verified that the produced kernel binary on x86 is exactly identical > > before and after the change. > > > > Signed-off-by: Michael S. Tsirkin > > --- > > > > changes from v2: > > add vring_used_elem_t to ensure alignment for substructures > > changes from v1: > > swicth all __user to the new typedefs > > > > drivers/vhost/vhost.c | 8 +++--- > > drivers/vhost/vhost.h | 6 ++--- > > drivers/vhost/vringh.c | 6 ++--- > > include/linux/vringh.h | 6 ++--- > > include/uapi/linux/virtio_ring.h | 43 ++++++++++++++++++++++++-------- > > 5 files changed, 45 insertions(+), 24 deletions(-) > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index d450e16c5c25..bc77b0f465fd 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -1244,9 +1244,9 @@ static int vhost_iotlb_miss(struct vhost_virtqueue *vq, u64 iova, int access) > > } > > static bool vq_access_ok(struct vhost_virtqueue *vq, unsigned int num, > > - struct vring_desc __user *desc, > > - struct vring_avail __user *avail, > > - struct vring_used __user *used) > > + vring_desc_t __user *desc, > > + vring_avail_t __user *avail, > > + vring_used_t __user *used) > > { > > return access_ok(desc, vhost_get_desc_size(vq, num)) && > > @@ -2301,7 +2301,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq, > > struct vring_used_elem *heads, > > unsigned count) > > { > > - struct vring_used_elem __user *used; > > + vring_used_elem_t __user *used; > > u16 old, new; > > int start; > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h > > index f8403bd46b85..60cab4c78229 100644 > > --- a/drivers/vhost/vhost.h > > +++ b/drivers/vhost/vhost.h > > @@ -67,9 +67,9 @@ struct vhost_virtqueue { > > /* The actual ring of buffers. */ > > struct mutex mutex; > > unsigned int num; > > - struct vring_desc __user *desc; > > - struct vring_avail __user *avail; > > - struct vring_used __user *used; > > + vring_desc_t __user *desc; > > + vring_avail_t __user *avail; > > + vring_used_t __user *used; > > const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS]; > > struct file *kick; > > struct eventfd_ctx *call_ctx; > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c > > index ba8e0d6cfd97..e059a9a47cdf 100644 > > --- a/drivers/vhost/vringh.c > > +++ b/drivers/vhost/vringh.c > > @@ -620,9 +620,9 @@ static inline int xfer_to_user(const struct vringh *vrh, > > */ > > int vringh_init_user(struct vringh *vrh, u64 features, > > unsigned int num, bool weak_barriers, > > - struct vring_desc __user *desc, > > - struct vring_avail __user *avail, > > - struct vring_used __user *used) > > + vring_desc_t __user *desc, > > + vring_avail_t __user *avail, > > + vring_used_t __user *used) > > { > > /* Sane power of 2 please! */ > > if (!num || num > 0xffff || (num & (num - 1))) { > > diff --git a/include/linux/vringh.h b/include/linux/vringh.h > > index 9e2763d7c159..59bd50f99291 100644 > > --- a/include/linux/vringh.h > > +++ b/include/linux/vringh.h > > @@ -105,9 +105,9 @@ struct vringh_kiov { > > /* Helpers for userspace vrings. */ > > int vringh_init_user(struct vringh *vrh, u64 features, > > unsigned int num, bool weak_barriers, > > - struct vring_desc __user *desc, > > - struct vring_avail __user *avail, > > - struct vring_used __user *used); > > + vring_desc_t __user *desc, > > + vring_avail_t __user *avail, > > + vring_used_t __user *used); > > static inline void vringh_iov_init(struct vringh_iov *iov, > > struct iovec *iovec, unsigned num) > > diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h > > index 9223c3a5c46a..b2c20f794472 100644 > > --- a/include/uapi/linux/virtio_ring.h > > +++ b/include/uapi/linux/virtio_ring.h > > @@ -86,6 +86,13 @@ > > * at the end of the used ring. Guest should ignore the used->flags field. */ > > #define VIRTIO_RING_F_EVENT_IDX 29 > > +/* Alignment requirements for vring elements. > > + * When using pre-virtio 1.0 layout, these fall out naturally. > > + */ > > +#define VRING_AVAIL_ALIGN_SIZE 2 > > +#define VRING_USED_ALIGN_SIZE 4 > > +#define VRING_DESC_ALIGN_SIZE 16 > > + > > /* Virtio ring descriptors: 16 bytes. These can chain together via "next". */ > > struct vring_desc { > > /* Address (guest-physical). */ > > @@ -112,29 +119,43 @@ struct vring_used_elem { > > __virtio32 len; > > }; > > +typedef struct vring_used_elem __aligned(VRING_USED_ALIGN_SIZE) > > + vring_used_elem_t; > > + > > struct vring_used { > > __virtio16 flags; > > __virtio16 idx; > > - struct vring_used_elem ring[]; > > + vring_used_elem_t ring[]; > > }; > > +/* > > + * The ring element addresses are passed between components with different > > + * alignments assumptions. Thus, we might need to decrease the compiler-selected > > + * alignment, and so must use a typedef to make sure the __aligned attribute > > + * actually takes hold: > > + * > > + * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#Common-Type-Attributes > > + * > > + * When used on a struct, or struct member, the aligned attribute can only > > + * increase the alignment; in order to decrease it, the packed attribute must > > + * be specified as well. When used as part of a typedef, the aligned attribute > > + * can both increase and decrease alignment, and specifying the packed > > + * attribute generates a warning. > > + */ > > +typedef struct vring_desc __aligned(VRING_DESC_ALIGN_SIZE) vring_desc_t; > > +typedef struct vring_avail __aligned(VRING_AVAIL_ALIGN_SIZE) vring_avail_t; > > +typedef struct vring_used __aligned(VRING_USED_ALIGN_SIZE) vring_used_t; > > > I wonder whether we can simply use __attribute__(packed) instead? > > Thanks Packed is 1 byte alignment. As such generates very bad code for accesses. > > > + > > struct vring { > > unsigned int num; > > - struct vring_desc *desc; > > + vring_desc_t *desc; > > - struct vring_avail *avail; > > + vring_avail_t *avail; > > - struct vring_used *used; > > + vring_used_t *used; > > }; > > -/* Alignment requirements for vring elements. > > - * When using pre-virtio 1.0 layout, these fall out naturally. > > - */ > > -#define VRING_AVAIL_ALIGN_SIZE 2 > > -#define VRING_USED_ALIGN_SIZE 4 > > -#define VRING_DESC_ALIGN_SIZE 16 > > - > > #ifndef VIRTIO_RING_NO_LEGACY > > /* The standard layout for the ring is a continuous chunk of memory which looks