linux-kernel - Re: [PULL] vhost: cleanups and fixes

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Date:   Thu, 1 Nov 2018 19:55:53 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        bijan.mottahedeh@...cle.com, gedwards@....com, joe@...ches.com,
        lenaic@...ard.fr, liang.z.li@...el.com, mhocko@...nel.org,
        mhocko@...e.com, stefanha@...hat.com, wei.w.wang@...el.com
Subject: Re: [PULL] vhost: cleanups and fixes

On Thu, Nov 01, 2018 at 04:06:19PM -0700, Linus Torvalds wrote:
> On Thu, Nov 1, 2018 at 4:00 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > +       memset(&rsp, 0, sizeof(rsp));
> > +       rsp.response = VIRTIO_SCSI_S_FUNCTION_REJECTED;
> > +       resp = vq->iov[out].iov_base;
> > +       ret = __copy_to_user(resp, &rsp, sizeof(rsp));
> >
> > Is it actually safe to trust that iov_base has passed an earlier
> > access_ok() check here? Why not just use copy_to_user() instead?
> 
> Good point.
> 
> We really should have removed those double-underscore things ages ago.

Well in case of vhost there are a bunch of reasons to keep them.

One is that all access_ok checks take place
way earlier in context of the owner task. Result is saved
and then used for access repeatedly. Skipping reding access_ok twice
did seem to give a small speedup in the past.
In fact I am looking
into switching some of the uses to unsafe_put_user/unsafe_get_user
after doing something like barrier_nospec after the
access_ok checks. Seems to give a measureable speedup.

Another is that the double underscore things actually can be done
in softirq context if you do preempt_disable/preempt_enable.
IIUC Jason's looking into using that to cut down the latency
for when the access is very small.

> Also, apart from the address, what about the size? Wouldn't it be
> better to use copy_to_iter() rather than implement it badly by hand?
> 
>                Linus

Generally size is checked when we retrieve the iov but I will recheck
this case and reply here.