linux-kernel - Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Sun, 29 Jul 2018 12:55:05 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm
 refcounting

On Sun, 2018-07-29 at 08:29 -0700, Andy Lutomirski wrote:
> > On Jul 29, 2018, at 5:11 AM, Rik van Riel <riel@...riel.com> wrote:
> > 
> > > On Sat, 2018-07-28 at 21:21 -0700, Andy Lutomirski wrote:
> > > On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@...riel.com>
> > > wrote:
> > > > Conditionally skip lazy TLB mm refcounting. When an
> > > > architecture
> > > > has
> > > > CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is
> > > > used in
> > > > lazy TLB mode anywhere will get shot down from exit_mmap, and
> > > > there
> > > > in no need to incur the cache line bouncing overhead of
> > > > refcounting
> > > > a lazy TLB mm.
> > > 
> > > Unless I've misunderstood something, this patch results in idle
> > > tasks
> > > whose active_mm has been freed still having active_mm pointing at
> > > freed memory. 
> > 
> > Patch 9/10 is supposed to ensure that the lazy TLB CPUs get
> > switched to init_mm before an mm is freed. No CPU should ever
> > have its active_mm pointing at a freed mm.
> > 
> > Your message made me re-read the code, and now I realize that
> > leave_mm does not actually do that.
> > 
> > Looking at the other callers of leave_mm, I might not be the
> > only one surprised by that; xen_drop_mm_ref comes to mind.
> > 
> > I guess I should some code to leave_mm to have it actually
> > clear active_mm and call the conditional refcount drop helper
> > function.
> > 
> > Does that clear up the confusion?
> 
> Kind of. But what’s the point of keeping active_mm?  On architectures
> that opt in to the new mode, there won’t be any code that cares about
> it’s value.  What’s the benefit of keeping it around?  If you ifdef
> it out, then it can’t possibly point to freed memory, and there’s
> nothing to worry about.

I would like to get to that point, but in a way that does
not leave the code too difficult to follow.

Getting rid of ->active_mm in context_switch() is straightforward,
but I am not sure at all what to do about idle_task_exit() for
example.

All the subtleties I ran into just with this phase of the
code suggests (to me at least) that we should probably do
this one step at a time.

I agree on the same end goal, though :)

-- 
All Rights Reversed.
Download attachment "signature.asc" of type "application/pgp-signature" (489 bytes)