Linux Virtualization - Sep 2019 - [RFC PATCH untested] vhost: block speculation of translated descriptors

iovec addresses coming from vhost are assumed to be
pre-validated, but in fact can be speculated to a value
out of range.

Userspace address are later validated with array_index_nospec so we can
be sure kernel info does not leak through these addresses, but vhost
must also not leak userspace info outside the allowed memory table to
guests.

Following the defence in depth principle, make sure
the address is not validated out of node range.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 drivers/vhost/vhost.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 5dc174ac8cac..0ee375fb7145 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2072,7 +2072,9 @@ static int translate_desc(struct vhost_virtqueue *vq, u64
addr, u32 len,
 		size = node->size - addr + node->start;
 		_iov->iov_len = min((u64)len - s, size);
 		_iov->iov_base = (void __user *)(unsigned long)
-			(node->userspace_addr + addr - node->start);
+			(node->userspace_addr +
+			 array_index_nospec(addr - node->start,
+					    node->size));
 		s += size;
 		addr += size;
 		++ret;
-- 
MST

On 2019/9/8 ??7:05, Michael S. Tsirkin wrote:
> iovec addresses coming from vhost are assumed to be
> pre-validated, but in fact can be speculated to a value
> out of range.
>
> Userspace address are later validated with array_index_nospec so we can
> be sure kernel info does not leak through these addresses, but vhost
> must also not leak userspace info outside the allowed memory table to
> guests.
>
> Following the defence in depth principle, make sure
> the address is not validated out of node range.
>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>   drivers/vhost/vhost.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 5dc174ac8cac..0ee375fb7145 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2072,7 +2072,9 @@ static int translate_desc(struct vhost_virtqueue *vq,
u64 addr, u32 len,
>   		size = node->size - addr + node->start;
>   		_iov->iov_len = min((u64)len - s, size);
>   		_iov->iov_base = (void __user *)(unsigned long)
> -			(node->userspace_addr + addr - node->start);
> +			(node->userspace_addr +
> +			 array_index_nospec(addr - node->start,
> +					    node->size));
>   		s += size;
>   		addr += size;
>   		++ret;

I've tried this on Kaby Lake smap off metadata acceleration off using 
testpmd (virtio-user) + vhost_net. I don't see obvious performance 
difference with TX PPS.

Thanks

On Mon, Sep 09, 2019 at 03:19:55PM +0800, Jason Wang
wrote:
> 
> On 2019/9/8 ??7:05, Michael S. Tsirkin wrote:
> > iovec addresses coming from vhost are assumed to be
> > pre-validated, but in fact can be speculated to a value
> > out of range.
> > 
> > Userspace address are later validated with array_index_nospec so we
can
> > be sure kernel info does not leak through these addresses, but vhost
> > must also not leak userspace info outside the allowed memory table to
> > guests.
> > 
> > Following the defence in depth principle, make sure
> > the address is not validated out of node range.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > ---
> >   drivers/vhost/vhost.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 5dc174ac8cac..0ee375fb7145 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2072,7 +2072,9 @@ static int translate_desc(struct vhost_virtqueue
*vq, u64 addr, u32 len,
> >   		size = node->size - addr + node->start;
> >   		_iov->iov_len = min((u64)len - s, size);
> >   		_iov->iov_base = (void __user *)(unsigned long)
> > -			(node->userspace_addr + addr - node->start);
> > +			(node->userspace_addr +
> > +			 array_index_nospec(addr - node->start,
> > +					    node->size));
> >   		s += size;
> >   		addr += size;
> >   		++ret;
> 
> 
> I've tried this on Kaby Lake smap off metadata acceleration off using
> testpmd (virtio-user) + vhost_net. I don't see obvious performance
> difference with TX PPS.
> 
> Thanks
Should I push this to Linus right now then? It's a security thing so
maybe we better do it ASAP ... what's your opinion?

-- 
MST