Messages in this thread | | | Subject | Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok() | From | Jason Wang <> | Date | Thu, 4 Jun 2020 14:10:27 +0800 |
| |
On 2020/6/4 上午12:52, Al Viro wrote: > On Wed, Jun 03, 2020 at 01:29:00AM -0400, Michael S. Tsirkin wrote: >> On Wed, Jun 03, 2020 at 02:48:15AM +0100, Al Viro wrote: >>> On Tue, Jun 02, 2020 at 04:45:05AM -0400, Michael S. Tsirkin wrote: >>>> So vhost needs to poke at userspace *a lot* in a quick succession. It >>>> is thus benefitial to enable userspace access, do our thing, then >>>> disable. Except access_ok has already been pre-validated with all the >>>> relevant nospec checks, so we don't need that. Add an API to allow >>>> userspace access after access_ok and barrier_nospec are done. >>> BTW, what are you going to do about vq->iotlb != NULL case? Because >>> you sure as hell do *NOT* want e.g. translate_desc() under STAC. >>> Disable it around the calls of translate_desc()? >>> >>> How widely do you hope to stretch the user_access areas, anyway? >> So ATM I'm looking at adding support for the packed ring format. >> That does something like: >> >> get_user(flags, desc->flags) >> smp_rmb() >> if (flags & VALID) >> copy_from_user(&adesc, desc, sizeof adesc); >> >> this would be a good candidate I think. > Perhaps, once we get stac/clac out of raw_copy_from_user() (coming cycle, > probably). BTW, how large is the structure and how is it aligned?
Each descriptor is 16 bytes, and 16 bytes aligned.
> >>> BTW, speaking of possible annotations: looks like there's a large >>> subset of call graph that can be reached only from vhost_worker() >>> or from several ioctls, with all uaccess limited to that subgraph >>> (thankfully). Having that explicitly marked might be a good idea... >> Sure. What's a good way to do that though? Any examples to follow? >> Or do you mean code comments? > Not sure... FWIW, the part of call graph from "known to be only > used by vhost_worker" (->handle_kick/vhost_work_init callback/ > vhost_poll_init callback) and "part of ->ioctl()" to actual uaccess > primitives is fairly large - the longest chain is > handle_tx_net -> > handle_tx -> > handle_tx_zerocopy -> > get_tx_bufs -> > vhost_net_tx_get_vq_desc -> > vhost_tx_batch -> > vhost_net_signal_used -> > vhost_add_used_and_signal_n -> > vhost_signal -> > vhost_notify -> > vhost_get_avail_flags -> > vhost_get_avail -> > vhost_get_user -> > __get_user() > i.e. 14 levels deep and the graph doesn't factorize well... > > Something along the lines of "all callers of thus annotated function > must be annotated the same way themselves, any implicit conversion > of pointers to such functions to anything other than boolean yields > a warning, explicit cast is allowed only with __force", perhaps? > Then slap such annotations on vhost_{get,put,copy_to,copy_from}_user(), > on ->handle_kick(), a force-cast in the only caller of ->handle_kick() > and force-casts in the 3 callers in ->ioctl(). > > And propagate the annotations until the warnings stop, basically... > > Shouldn't be terribly hard to teach sparse that kind of stuff and it > might be useful elsewhere. It would act as a qualifier on function > pointers, with syntax ultimately expanding to __attribute__((something)). > I'll need to refresh my memories of the parser, but IIRC that shouldn't > require serious modifications. Most of the work would be in > evaluate_call(), just before calling evaluate_symbol_call()... > I'll look into that; not right now, though. > > BTW, __vhost_get_user() might be better off expanded in both callers - > that would get their structure similar to vhost_copy_{to,from}_user(), > especially if you expand __vhost_get_user_slow() as well. > > Not sure I understand what's going with ->meta_iotlb[] - what are the > lifetime rules for struct vhost_iotlb_map
It used to cache the translation for virtqueue address which. Vhost will try to get those addresses from IOTLB and store them in meta_iotlb, and it will be invalidated when userspace update or invalidate a new mapping.
> and what prevents the pointers > from going stale? > >
The vq->mutex is used to synchronize between the invalidation and vhost workers.
Thanks
| |