linux-kernel - Re: [PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages

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Message-ID: <CA+EHjTz6g_0P+t3wzV99hBtf9rd2Lvn-vwYb2oKZaXxSLs5BzQ@mail.gmail.com>
Date: Fri, 16 Aug 2024 12:19:09 +0100
From: Fuad Tabba <tabba@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Elliot Berman <quic_eberman@...cinc.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Patrick Roy <roypat@...zon.co.uk>, qperret@...gle.com, 
	Ackerley Tng <ackerleytng@...gle.com>, linux-coco@...ts.linux.dev, 
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, kvm@...r.kernel.org
Subject: Re: [PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages

On Fri, 16 Aug 2024 at 10:48, David Hildenbrand <david@...hat.com> wrote:
>
> On 15.08.24 09:24, Fuad Tabba wrote:
> > Hi David,
>
> Hi!
>
> >
> > On Tue, 6 Aug 2024 at 14:51, David Hildenbrand <david@...hat.com> wrote:
> >>
> >>>
> >>> -     if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
> >>> +     if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
> >>>                r = guest_memfd_folio_private(folio);
> >>>                if (r)
> >>>                        goto out_err;
> >>> @@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
> >>>    }
> >>>    EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
> >>>
> >>> +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
> >>> +{
> >>> +     unsigned long gmem_flags = (unsigned long)file->private_data;
> >>> +     unsigned long i;
> >>> +     int r;
> >>> +
> >>> +     unmap_mapping_folio(folio);
> >>> +
> >>> +     /**
> >>> +      * We can't use the refcount. It might be elevated due to
> >>> +      * guest/vcpu trying to access same folio as another vcpu
> >>> +      * or because userspace is trying to access folio for same reason
> >>
> >> As discussed, that's insufficient. We really have to drive the refcount
> >> to 1 -- the single reference we expect.
> >>
> >> What is the exact problem you are running into here? Who can just grab a
> >> reference and maybe do nasty things with it?
> >
> > I was wondering, why do we need to check the refcount? Isn't it enough
> > to check for page_mapped() || page_maybe_dma_pinned(), while holding
> > the folio lock?
>
> (folio_mapped() + folio_maybe_dma_pinned())
>
> Not everything goes trough FOLL_PIN. vmsplice() is an example, or just
> some very simple read/write through /proc/pid/mem. Further, some
> O_DIRECT implementations still don't use FOLL_PIN.
>
> So if you see an additional folio reference, as soon as you mapped that
> thing to user space, you have to assume that it could be someone
> reading/writing that memory in possibly sane context. (vmsplice() should
> be using FOLL_PIN|FOLL_LONGTERM, but that's a longer discussion)
>
> (noting that also folio_maybe_dma_pinned() can have false positives in
> some cases due to speculative references or *many* references).

Thanks for the clarification!
/fuad

> --
> Cheers,
>
> David / dhildenb
>