Re: [PATCH v2 1/5] KVM: do not allow mapping valid but non-refcounted pages

[Paolo Bonzini]

On 25/06/21 09:58, Christian Borntraeger wrote:
> On 25.06.21 09:36, David Stevens wrote:
> > From: Nicholas Piggin <npiggin@xxxxxxxxx>
> >
> > It's possible to create a region which maps valid but non-refcounted
> > pages (e.g., tail pages of non-compound higher order allocations). These
> > host pages can then be returned by gfn_to_page, gfn_to_pfn, etc., family
> > of APIs, which take a reference to the page, which takes it from 0 to 1.
> > When the reference is dropped, this will free the page incorrectly.
> >
> > Fix this by only taking a reference on the page if it was non-zero,
> > which indicates it is participating in normal refcounting (and can be
> > released with put_page).
> >
> > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
>
> I guess this would be the small fix for stable? Do we want to add that cc?
>
> Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>

Yes, this one is going to Linus today.  The rest is for 5.15.

Paolo

> > ---
> >   virt/kvm/kvm_main.c | 19 +++++++++++++++++--
> >   1 file changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 3dcc2abbfc60..f7445c3bcd90 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2175,6 +2175,13 @@ static bool vma_is_valid(struct vm_area_struct 
> > *vma, bool write_fault)
> >       return true;
> >   }
> >
> > +static int kvm_try_get_pfn(kvm_pfn_t pfn)
> > +{
> > +    if (kvm_is_reserved_pfn(pfn))
> > +        return 1;
> > +    return get_page_unless_zero(pfn_to_page(pfn));
> > +}
> > +
> >   static int hva_to_pfn_remapped(struct vm_area_struct *vma,
> >                      unsigned long addr, bool *async,
> >                      bool write_fault, bool *writable,
> > @@ -2224,13 +2231,21 @@ static int hva_to_pfn_remapped(struct 
> > vm_area_struct *vma,
> >        * Whoever called remap_pfn_range is also going to call e.g.
> >        * unmap_mapping_range before the underlying pages are freed,
> >        * causing a call to our MMU notifier.
> > +     *
> > +     * Certain IO or PFNMAP mappings can be backed with valid
> > +     * struct pages, but be allocated without refcounting e.g.,
> > +     * tail pages of non-compound higher order allocations, which
> > +     * would then underflow the refcount when the caller does the
> > +     * required put_page. Don't allow those pages here.
> >        */
> > -    kvm_get_pfn(pfn);
> > +    if (!kvm_try_get_pfn(pfn))
> > +        r = -EFAULT;
> >
> >   out:
> >       pte_unmap_unlock(ptep, ptl);
> >       *p_pfn = pfn;
> > -    return 0;
> > +
> > +    return r;
> >   }
> >
> >   /*