linux-kernel - Re: [PATCH RFC] uaccess: user_access_begin_after_access

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Message-ID: <20200602163048.GL23230@ZenIV.linux.org.uk>
Date:   Tue, 2 Jun 2020 17:30:48 +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.

This is the wrong way to do it, and this API is certain to be abused
elsewhere.  NAK - we need to sort out vhost-related problems, but
this is not an acceptable solution.  Sorry.