linux-kernel - Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

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Date:   Sat, 16 Jan 2021 21:41:25 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        stable <stable@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>, surenb@...gle.com
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

On Tue, Jan 12, 2021 at 09:43:38PM +0000, Will Deacon wrote:
> On Tue, Jan 12, 2021 at 12:38:34PM -0800, Nadav Amit wrote:
> > > On Jan 12, 2021, at 11:56 AM, Yu Zhao <yuzhao@...gle.com> wrote:
> > > On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote:
> > >> I will send an RFC soon for per-table deferred TLB flushes tracking.
> > >> The basic idea is to save a generation in the page-struct that tracks
> > >> when deferred PTE change took place, and track whenever a TLB flush
> > >> completed. In addition, other users - such as mprotect - would use
> > >> the tlb_gather interface.
> > >> 
> > >> Unfortunately, due to limited space in page-struct this would only
> > >> be possible for 64-bit (and my implementation is only for x86-64).
> > > 
> > > I don't want to discourage you but I don't think this would end up
> > > well. PPC doesn't necessarily follow one-page-struct-per-table rule,
> > > and I've run into problems with this before while trying to do
> > > something similar.
> > 
> > Discourage, discourage. Better now than later.
> > 
> > It will be relatively easy to extend the scheme to be per-VMA instead of
> > per-table for architectures that prefer it this way. It does require
> > TLB-generation tracking though, which Andy only implemented for x86, so I
> > will focus on x86-64 right now.
> 
> Can you remind me of what we're missing on arm64 in this area, please? I'm
> happy to help get this up and running once you have something I can build
> on.

I noticed arm/arm64 don't support ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.
Would it be something worth pursuing? Arm has been using mm_cpumask,
so it might not be too difficult I guess?