thr3ads.net - Linux Virtualization - [PATCH] vhost: force spec specified alignment on types [Apr 2020]

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Michael S. Tsirkin

2020-Apr-06 13:55 UTC

[PATCH] vhost: force spec specified alignment on types

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)
> >   {

Jason Wang

2020-Apr-06 14:09 UTC

head link

[PATCH] vhost: force spec specified alignment on types

On 2020/4/6 ??9:55, Michael S. Tsirkin wrote:> 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.

Yes, but I meant when userspace is complied with apcs-gnu, then it still 
assumes 8 byte alignment?

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)
>>>    {

Michael S. Tsirkin

2020-Apr-06 14:34 UTC

head link

[PATCH] vhost: force spec specified alignment on types

On Mon, Apr 06, 2020 at 10:09:27PM +0800, Jason Wang
wrote:> 
> On 2020/4/6 ??9:55, Michael S. Tsirkin wrote:
> > 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.
> 
> 
> Yes, but I meant when userspace is complied with apcs-gnu, then it still
> assumes 8 byte alignment?
> 
> Thanks

That's not a problem since with vhost userspace is doing the allocation.
So it can increase alignment with no bad effect.

I agree it's probably safest not to touch struct vring at all though.

> 
> > 
> > > >    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)
> > > >    {

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