Messages in this thread | | | Subject | Re: [PATCH v3] virtio: force spec specified alignment on types | From | Jason Wang <> | Date | Tue, 21 Apr 2020 10:39:19 +0800 |
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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 <mst@redhat.com> > --- > > 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
> + > 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
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