netdev - Re: [PATCH RFC] uaccess: user_access_begin_after_access

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Message-ID: <20200602174430.GN23230@ZenIV.linux.org.uk>
Date:   Tue, 2 Jun 2020 18:44:30 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

On Tue, Jun 02, 2020 at 10:18:09AM -0700, Linus Torvalds wrote:

> You have exactly two cases:
> 
>  (a) the access_ok() would be right above the code and can't be missed
> 
>  (b) not

   (c) what you really want is not quite access_ok().

Again, that "not quite access_ok()" should be right next to STAC, and
come from the same primitive - I'm not saying the current model is
anywhere near sane.  We need a range-checking primitive right next
to memory access; it's just that for KVM and vhost we might want
a different check and, for things like s390 and sparc (mips as well,
in some configs), potentially different part that would do the memory
access itself as well.