linux-kernel - Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to

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Date:   Thu, 22 Oct 2020 12:49:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, x86@...nel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to
 access guest memory

On Tue, Oct 20, 2020 at 01:25:59AM -0700, John Hubbard wrote:
> Should copy_to_guest() use pin_user_pages_unlocked() instead of gup_unlocked?
> We wrote a  "Case 5" in Documentation/core-api/pin_user_pages.rst, just for this
> situation, I think:
> 
> 
> CASE 5: Pinning in order to write to the data within the page
> -------------------------------------------------------------
> Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
> write to a page's data, unpin" can cause a problem. Case 5 may be considered a
> superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
> other words, if the code is neither Case 1 nor Case 2, it may still require
> FOLL_PIN, for patterns like this:
> 
> Correct (uses FOLL_PIN calls):
>     pin_user_pages()
>     write to the data within the pages
>     unpin_user_pages()

Case 5 is crap though.  That bug should have been fixed by getting
the locking right.  ie:

	get_user_pages_fast();
	lock_page();
	kmap();
	set_bit();
	kunmap();
	set_page_dirty()
	unlock_page();

I should have vetoed that patch at the time, but I was busy with other things.