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

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Date:   Tue, 12 Jan 2021 17:57:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     Vinayak Menon <vinmenon@...eaurora.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>, Yu Zhao <yuzhao@...gle.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>,
        Will Deacon <will@...nel.org>, surenb@...gle.com
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

On Tue, Jan 12, 2021 at 04:47:17PM +0100, Laurent Dufour wrote:
> Le 12/01/2021 à 12:43, Vinayak Menon a écrit :

> > Possibility of race against other PTE modifiers
> > 
> > 1) Fork - We have seen a case of SPF racing with fork marking PTEs RO and that
> > is described and fixed here https://lore.kernel.org/patchwork/patch/1062672/

Right, that's exactly the kind of thing I was worried about.

> > 2) mprotect - change_protection in mprotect which does the deferred flush is
> > marked under vm_write_begin/vm_write_end, thus SPF bails out on faults
> > on those VMAs.

Sure, mprotect also changes vm_flags, so it really needs that anyway.

> > 3) userfaultfd - mwriteprotect_range is not protected unlike in (2) above.
> > But SPF does not take UFFD faults.
> > 4) hugetlb - hugetlb_change_protection - called from mprotect and covered by
> > (2) above.

> > 5) Concurrent faults - SPF does not handle all faults. Only anon page faults.

What happened to shared/file-backed stuff? ISTR I had that working.

> > Of which do_anonymous_page and do_swap_page are NONE/NON-PRESENT->PRESENT
> > transitions without tlb flush. And I hope do_wp_page with RO->RW is fine as well.

The tricky one is demotion, specifically write to non-write.

> > I could not see a case where speculative path cannot see a PTE update done via
> > a fault on another CPU.

One you didn't mention is the NUMA balancing scanning crud; although I
think that's fine, loosing a PTE update there is harmless. But I've not
thought overly hard on it.

> You explained it fine. Indeed SPF is handling deferred TLB invalidation by
> marking the VMA through vm_write_begin/end(), as for the fork case you
> mentioned. Once the PTL is held, and the VMA's seqcount is checked, the PTE
> values read are valid.

That should indeed work, but are we really sure we covered them all?
Should we invest in better TLBI APIs to make sure we can't get this
wrong?