Linux Virtualization - Apr 2020 - [PATCH] vhost: force spec specified alignment on types

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.

I verified that the produced binary is exactly identical on x86.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---

This is my preferred way to handle the ARM incompatibility issues
(in preference to kconfig hacks).
I will push this into next now.
Comments?

 drivers/vhost/vhost.h            |  6 ++---
 include/uapi/linux/virtio_ring.h | 41 ++++++++++++++++++++++++--------
 2 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index cc82918158d2..a67bda9792ec 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -74,9 +74,9 @@ 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 vhost_desc *descs;
diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 559f42e73315..cd6e0b2eaf2f 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -118,16 +118,6 @@ struct vring_used {
 	struct vring_used_elem ring[];
 };
 
-struct vring {
-	unsigned int num;
-
-	struct vring_desc *desc;
-
-	struct vring_avail *avail;
-
-	struct vring_used *used;
-};
-
 /* Alignment requirements for vring elements.
  * When using pre-virtio 1.0 layout, these fall out naturally.
  */
@@ -164,6 +154,37 @@ struct vring {
 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
 #define vring_avail_event(vr) (*(__virtio16
*)&(vr)->used->ring[(vr)->num])
 
+/*
+ * 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 __attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
+	vring_desc_t;
+typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
+	vring_avail_t;
+typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
+	vring_used_t;
+
+struct vring {
+	unsigned int num;
+
+	vring_desc_t *desc;
+
+	vring_avail_t *avail;
+
+	vring_used_t *used;
+};
+
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 			      unsigned long align)
 {
-- 
MST

On 2020/4/6 ??8:50, 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.
>
> I verified that the produced binary is exactly identical on x86.
>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>
> This is my preferred way to handle the ARM incompatibility issues
> (in preference to kconfig hacks).
> I will push this into next now.
> Comments?

I'm not sure if it's too late to fix. It would still be still 
problematic for the userspace that is using old uapi headers?

Thanks

>
>   drivers/vhost/vhost.h            |  6 ++---
>   include/uapi/linux/virtio_ring.h | 41 ++++++++++++++++++++++++--------
>   2 files changed, 34 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index cc82918158d2..a67bda9792ec 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -74,9 +74,9 @@ 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 vhost_desc *descs;
> diff --git a/include/uapi/linux/virtio_ring.h
b/include/uapi/linux/virtio_ring.h
> index 559f42e73315..cd6e0b2eaf2f 100644
> --- a/include/uapi/linux/virtio_ring.h
> +++ b/include/uapi/linux/virtio_ring.h
> @@ -118,16 +118,6 @@ struct vring_used {
>   	struct vring_used_elem ring[];
>   };
>   
> -struct vring {
> -	unsigned int num;
> -
> -	struct vring_desc *desc;
> -
> -	struct vring_avail *avail;
> -
> -	struct vring_used *used;
> -};
> -
>   /* Alignment requirements for vring elements.
>    * When using pre-virtio 1.0 layout, these fall out naturally.
>    */
> @@ -164,6 +154,37 @@ struct vring {
>   #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
>   #define vring_avail_event(vr) (*(__virtio16
*)&(vr)->used->ring[(vr)->num])
>   
> +/*
> + * 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 __attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
> +	vring_desc_t;
> +typedef struct vring_avail
__attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
> +	vring_avail_t;
> +typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
> +	vring_used_t;
> +
> +struct vring {
> +	unsigned int num;
> +
> +	vring_desc_t *desc;
> +
> +	vring_avail_t *avail;
> +
> +	vring_used_t *used;
> +};
> +
>   static inline void vring_init(struct vring *vr, unsigned int num, void
*p,
>   			      unsigned long align)
>   {

On Mon, Apr 06, 2020 at 09:34:00PM +0800, Jason Wang
wrote:
> 
> On 2020/4/6 ??8:50, 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.
> > 
> > I verified that the produced binary is exactly identical on x86.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > ---
> > 
> > This is my preferred way to handle the ARM incompatibility issues
> > (in preference to kconfig hacks).
> > I will push this into next now.
> > Comments?
> 
> 
> I'm not sure if it's too late to fix. It would still be still
problematic
> for the userspace that is using old uapi headers?
> 
> Thanks
It's not a problem in userspace. The problem is when
userspace/guest uses 2 byte alignment and passes it to kernel
assuming 8 byte alignment. The fix is for host not to
make these assumptions.
> 
> > 
> >   drivers/vhost/vhost.h            |  6 ++---
> >   include/uapi/linux/virtio_ring.h | 41
++++++++++++++++++++++++--------
> >   2 files changed, 34 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index cc82918158d2..a67bda9792ec 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -74,9 +74,9 @@ 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 vhost_desc *descs;
> > diff --git a/include/uapi/linux/virtio_ring.h
b/include/uapi/linux/virtio_ring.h
> > index 559f42e73315..cd6e0b2eaf2f 100644
> > --- a/include/uapi/linux/virtio_ring.h
> > +++ b/include/uapi/linux/virtio_ring.h
> > @@ -118,16 +118,6 @@ struct vring_used {
> >   	struct vring_used_elem ring[];
> >   };
> > -struct vring {
> > -	unsigned int num;
> > -
> > -	struct vring_desc *desc;
> > -
> > -	struct vring_avail *avail;
> > -
> > -	struct vring_used *used;
> > -};
> > -
> >   /* Alignment requirements for vring elements.
> >    * When using pre-virtio 1.0 layout, these fall out naturally.
> >    */
> > @@ -164,6 +154,37 @@ struct vring {
> >   #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
> >   #define vring_avail_event(vr) (*(__virtio16
*)&(vr)->used->ring[(vr)->num])
> > +/*
> > + * 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
__attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
> > +	vring_desc_t;
> > +typedef struct vring_avail
__attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
> > +	vring_avail_t;
> > +typedef struct vring_used
__attribute__((aligned(VRING_USED_ALIGN_SIZE)))
> > +	vring_used_t;
> > +
> > +struct vring {
> > +	unsigned int num;
> > +
> > +	vring_desc_t *desc;
> > +
> > +	vring_avail_t *avail;
> > +
> > +	vring_used_t *used;
> > +};
> > +
> >   static inline void vring_init(struct vring *vr, unsigned int num,
void *p,
> >   			      unsigned long align)
> >   {