Re: [PULL] vhost: cleanups and fixes

From: Linus Torvalds <torvalds@linux-foundation.org>
To: mst@redhat.com, Al Viro <viro@zeniv.linux.org.uk>
Cc: mark.rutland@arm.com, Kees Cook <keescook@chromium.org>,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	bijan.mottahedeh@oracle.com, gedwards@ddn.com, joe@perches.com,
	lenaic@lhuard.fr, liang.z.li@intel.com, mhocko@kernel.org,
	mhocko@suse.com, stefanha@redhat.com, wei.w.wang@intel.com,
	jasowang@redhat.com
Subject: Re: [PULL] vhost: cleanups and fixes
Date: Fri, 2 Nov 2018 10:15:56 -0700	[thread overview]
Message-ID: <CAHk-=wj5ZnqmpDuXBCcND8nMNitNyRf_4KQSzSUfQvX2-wOYsg@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wibzqo7C+mS+BgZxRbgdWe2w5F39EhuFUhZUxvotoGLuA@mail.gmail.com>

On Fri, Nov 2, 2018 at 10:10 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Don't you take over the VM with "use_mm()" when you do the copies? So
> yes, it's a kernel thread, but it has a user VM, and though that
> should have the user limits.

Oooh. *Just* as I sent this, I realized that "use_mm()" doesn't update
the thread addr_limit.

That actually looks like a bug to me - although one that you've
apparently been aware of and worked around.

Wouldn't it be nicer to just make "use_mm()" do

        set_fs(USER_DS);

instead? And undo it on unuse_mm()?

And, in fact, maybe we should default kernel threads to have a zero
address limit, so that they can't do any user accesses at all without
doing this?

Adding Al to the cc, because I think he's been looking at set_fs() in general.

                  Linus