linux-kernel - Re: [PATCH v2] vfio/type1: check pfn valid before converting to struct page

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Message-ID: <20230523141527.6186a044.alex.williamson@redhat.com>
Date:   Tue, 23 May 2023 14:15:27 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kevin.tian@...el.com>, <jgg@...dia.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2] vfio/type1: check pfn valid before converting to
 struct page

On Tue, 23 May 2023 13:48:22 +0800
Yan Zhao <yan.y.zhao@...el.com> wrote:

> On Mon, May 22, 2023 at 01:00:30PM -0600, Alex Williamson wrote:
> > On Fri, 19 May 2023 14:58:43 +0800
> > Yan Zhao <yan.y.zhao@...el.com> wrote:
> >   
> > > Check physical PFN is valid before converting the PFN to a struct page
> > > pointer to be returned to caller of vfio_pin_pages().
> > > 
> > > vfio_pin_pages() pins user pages with contiguous IOVA.
> > > If the IOVA of a user page to be pinned belongs to vma of vm_flags
> > > VM_PFNMAP, pin_user_pages_remote() will return -EFAULT without returning
> > > struct page address for this PFN. This is because usually this kind of PFN
> > > (e.g. MMIO PFN) has no valid struct page address associated.
> > > Upon this error, vaddr_get_pfns() will obtain the physical PFN directly.
> > > 
> > > While previously vfio_pin_pages() returns to caller PFN arrays directly,
> > > after commit
> > > 34a255e67615 ("vfio: Replace phys_pfn with pages for vfio_pin_pages()"),
> > > PFNs will be converted to "struct page *" unconditionally and therefore
> > > the returned "struct page *" array may contain invalid struct page
> > > addresses.
> > > 
> > > Given current in-tree users of vfio_pin_pages() only expect "struct page *
> > > returned, check PFN validity and return -EINVAL to let the caller be
> > > aware of IOVAs to be pinned containing PFN not able to be returned in
> > > "struct page *" array. So that, the caller will not consume the returned
> > > pointer (e.g. test PageReserved()) and avoid error like "supervisor read
> > > access in kernel mode".
> > > 
> > > Fixes: 34a255e67615 ("vfio: Replace phys_pfn with pages for vfio_pin_pages()")
> > > Cc: Sean Christopherson <seanjc@...gle.com>
> > > Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> > > Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> > > 
> > > ---
> > > v2: update commit message to explain background/problem clearly. (Sean)
> > > ---
> > >  drivers/vfio/vfio_iommu_type1.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > index 493c31de0edb..0620dbe5cca0 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > @@ -860,6 +860,11 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
> > >  		if (ret)
> > >  			goto pin_unwind;
> > >  
> > > +		if (!pfn_valid(phys_pfn)) {  
> > 
> > Why wouldn't we use our is_invalid_reserved_pfn() test here?  Doing
> > so would also make it more consistent why we don't need to call
> > put_pfn() or rewind accounting for this page.  Thanks,
> >   
> I actually struggled in choosing is_invalid_reserved_pfn() or
> pfn_valid() when writing this patch.
> 
> Choosing pfn_valid() is because invalid PFN obviously cannot have
> struct page address and it's a bug fix.
> 
> While declining reserved pages will have the IOVA range supported by
> vfio_pin_pages() even more reduced. So I don't know if there's enough
> justification to do so, given that (1) device zone memory usually has
> PG_reserved set. (2) vm_normal_page() also contains reserved page.

Based on the exclusion we have in vaddr_get_pfn() where we unpin
zero-page pfns because they hit on the is_invalid_reserved_pfn() test
and break our accounting otherwise, this does seem like the correct
choice.  I can imagine a scenario where the device wants to do a DMA
read from VM memory backed by the zero page.  Ok.  Thanks,

Alex