Subject: Re: [PATCH v2] vhost: block speculation of translated descriptors

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 11 Sep 2019 08:25:03 -0400

On Wed, Sep 11, 2019 at 02:16:28PM +0200, Michal Hocko wrote:
> On Wed 11-09-19 08:10:00, 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 <[email-masked]>
> > Acked-by: Jason Wang <[email-masked]>
> > Tested-by: Jason Wang <[email-masked]>
> 
> no need to mark fo stable? Other spectre fixes tend to be backported
> even when the security implications are not really clear. The risk
> should be low and better to be covered in case.

This is not really a fix - more a defence in depth thing,
quite similar to e.g.  commit b3bbfb3fb5d25776b8e3f361d2eedaabb0b496cd
x86: Introduce __uaccess_begin_nospec() and uaccess_try_nospec
in scope.

That one doesn't seem to be tagged for stable. Was it queued
there in practice?

> > ---
> > 
> > changes from v1: fix build on 32 bit
> > 
> >  drivers/vhost/vhost.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 5dc174ac8cac..34ea219936e3 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2071,8 +2071,10 @@ static int translate_desc(struct vhost_virtqueue *vq, u64 addr, u32 len,
> >  		_iov = iov + ret;
> >  		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);
> > +		_iov->iov_base = (void __user *)
> > +			((unsigned long)node->userspace_addr +
> > +			 array_index_nospec((unsigned long)(addr - node->start),
> > +					    node->size));
> >  		s += size;
> >  		addr += size;
> >  		++ret;
> > -- 
> > MST
> 
> -- 
> Michal Hocko
> SUSE Labs