Re: [PATCH RFC] rdma_rxe: Stop passing AV from user space

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Oct 19, 2020 at 02:06:30PM -0500, Bob Pearson wrote:
> On 10/19/20 1:53 PM, Jason Gunthorpe wrote:
> > On Fri, Oct 16, 2020 at 12:01:48PM -0500, Bob Pearson wrote:
> >>  
> >> +static struct ib_ah *get_ah_from_handle(struct rxe_qp *qp, u32 handle)
> >> +{
> >> +	struct ib_uverbs_file *ufile;
> >> +	struct uverbs_api *uapi;
> >> +	const struct uverbs_api_object *type;
> >> +	struct ib_uobject *uobj;
> >> +
> >> +	ufile = qp->ibqp.uobject->uevent.uobject.ufile;
> >> +	uapi = ufile->device->uapi;
> >> +	type = uapi_get_object(uapi, UVERBS_OBJECT_AH);
> >> +	if (IS_ERR(type))
> >> +		return NULL;
> >> +	uobj = rdma_lookup_get_uobject(type, ufile, (s64)handle,
> >> +				       UVERBS_LOOKUP_READ, NULL);
> >> +	if (IS_ERR(uobj)) {
> >> +		pr_warn("unable to lookup ah handle\n");
> >> +		return NULL;
> >> +	}
> >> +
> >> +	rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_READ);
> > 
> > It can't be put and then return the data pointer, it is a use after free:
> > 
> >> +	return uobj->object;
> > 
> >> @@ -562,11 +563,6 @@ static int init_send_wqe(struct rxe_qp *qp, const struct ib_send_wr *ibwr,
> >>  
> >>  	init_send_wr(qp, &wqe->wr, ibwr);
> >>  
> >> -	if (qp_type(qp) == IB_QPT_UD ||
> >> -	    qp_type(qp) == IB_QPT_SMI ||
> >> -	    qp_type(qp) == IB_QPT_GSI)
> >> -		memcpy(&wqe->av, &to_rah(ud_wr(ibwr)->ah)->av, sizeof(wqe->av));
> > 
> > It needs some kind of negotiated compat, can't just break userspace
> > like this
> > 
> > Jason
> > 
> 
> 1st point. I get it. uobj->object contains the address of one of the ib_xxx verbs objects.
> Normally the driver never looks at this level but presumably has a kref on that object so it makes
> sense to look it up. Perhaps better would be:
> 
> 	void *object;
> 
> 	...
> 
> 	uobj = rdma_lookup_get_uobject(...);
> 
> 	object = uobj->object;
> 
> 	rdma_lookup_put_uobject(...);
> 
> 	return (struct ib_ah *)object;
> 
> Here the caller has created the ib_ah but has not yet destroyed it so it must hold a kref on it.

Drivers are not supposed to keep using object after it has been
destroyed, so some kind of interlock is needed to prevent/defer
destruction here.

The uobject layer does not provide something usable to drivers

> 2nd point. I also get. This suggestion imagines that there will come
> a day when we can change the user API.  May be a rare day but must
> happen occasionally. The current design is just plain wrong and
> needs to get fixed eventually.

You can have some cap negotiation to switch the mode AH mode in the
WQEs - 'Use WQE format 2' for instance. Most of the HW drivers have
multiple WQE formats the userspace selects.

Jason