Re: [PATCH RFC 09/10] vfio/type1: Use follow_pfn for VM_FPNMAP VMAs

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: linux-nvdimm@lists.01.org,
	Dan Williams <dan.j.williams@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	kvm@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Liran Alon <liran.alon@oracle.com>,
	Nikita Leshenko <nikita.leshchenko@oracle.com>,
	Barret Rhoden <brho@google.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Matthew Wilcox <willy@infradead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH RFC 09/10] vfio/type1: Use follow_pfn for VM_FPNMAP VMAs
Date: Fri, 7 Feb 2020 17:08:31 -0400	[thread overview]
Message-ID: <20200207210831.GA31015@ziepe.ca> (raw)
In-Reply-To: <20200110190313.17144-10-joao.m.martins@oracle.com>

On Fri, Jan 10, 2020 at 07:03:12PM +0000, Joao Martins wrote:
> From: Nikita Leshenko <nikita.leshchenko@oracle.com>
> 
> Unconditionally interpreting vm_pgoff as a PFN is incorrect.
> 
> VMAs created by /dev/mem do this, but in general VM_PFNMAP just means
> that the VMA doesn't have an associated struct page and is being managed
> directly by something other than the core mmu.
> 
> Use follow_pfn like KVM does to find the PFN.
> 
> Signed-off-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
>  drivers/vfio/vfio_iommu_type1.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 2ada8e6cdb88..1e43581f95ea 100644
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -362,9 +362,9 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  	vma = find_vma_intersection(mm, vaddr, vaddr + 1);
>  
>  	if (vma && vma->vm_flags & VM_PFNMAP) {
> -		*pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> -		if (is_invalid_reserved_pfn(*pfn))
> -			ret = 0;
> +		ret = follow_pfn(vma, vaddr, pfn);
> +		if (!ret && !is_invalid_reserved_pfn(*pfn))
> +			ret = -EOPNOTSUPP;
>  	}

FWIW this existing code is a huge hack and a security problem.

I'm not sure how you could be successfully using this path on actual
memory without hitting bad bugs?

Fudamentally VFIO can't retain a reference to a page from within a VMA
without some kind of recount/locking/etc to allow the thing that put
the page there to know it is still being used (ie programmed in a
IOMMU) by VFIO.

Otherwise it creates use-after-free style security problems on the
page.

This code needs to be deleted, not extended :(

Jason