netdev - Re: [PATCH RFC] uaccess: user_access_begin_after_access

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Date:   Wed, 3 Jun 2020 02:48:15 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

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?

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...

Unrelated question, while we are at it: is there any point having
vhost_get_user() a polymorphic macro?  In all callers the third
argument is __virtio16 __user * and the second one is an explicit
*<something> where <something> is __virtio16 *.  Similar for
vhost_put_user(): in all callers the third arugment is
__virtio16 __user * and the second - cpu_to_vhost16(vq, something).

Incidentally, who had come up with the name __vhost_get_user?
Makes for lovey WTF moment for readers - esp. in vhost_put_user()...