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

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Message-ID: <CA+EHjTxNNinn7EzV_o1X1d0kwhEwrbj_O7H8WgDtEy2CwURZFQ@mail.gmail.com>
Date: Thu, 15 Aug 2024 08:24:32 +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

Hi David,

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?

Thanks!
/fuad

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