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[PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs

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  [PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs Filippo Sironi <sironi@amazon.de> - 2017-04-05 15:20 +0200
    Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when  touching GPTEs Radim Krčmář <rkrcmar@redhat.com> - 2017-04-06 16:30 +0200

#1616917 — [PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs

cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
pages and the respective struct pages for mapping in the kernel virtual
address space.
This doesn't work if get_user_pages_fast() is invoked with a userspace
virtual address that's backed by PFNs outside of kernel reach (e.g.,
when limiting the kernel memory with mem= in the command line and using
/dev/mem to map memory).

If get_user_pages_fast() fails, look up the VMA that backs the userspace
virtual address, compute the PFN and the physical address, and map it in
the kernel virtual address space with memremap().

Signed-off-by: Filippo Sironi <sironi@amazon.de>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/kvm/paging_tmpl.h | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index a01105485315..ab4d6617238c 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -147,15 +147,36 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
 	struct page *page;
 
 	npages = get_user_pages_fast((unsigned long)ptep_user, 1, 1, &page);
-	/* Check if the user is doing something meaningless. */
-	if (unlikely(npages != 1))
-		return -EFAULT;
-
-	table = kmap_atomic(page);
-	ret = CMPXCHG(&table[index], orig_pte, new_pte);
-	kunmap_atomic(table);
-
-	kvm_release_page_dirty(page);
+	if (likely(npages == 1)) {
+		table = kmap_atomic(page);
+		ret = CMPXCHG(&table[index], orig_pte, new_pte);
+		kunmap_atomic(table);
+
+		kvm_release_page_dirty(page);
+	} else {
+		struct vm_area_struct *vma;
+		unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK;
+		unsigned long pfn;
+		unsigned long paddr;
+
+		down_read(&current->mm->mmap_sem);
+		vma = find_vma_intersection(current->mm, vaddr,
+					    vaddr + PAGE_SIZE);
+		if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
+			up_read(&current->mm->mmap_sem);
+			return -EFAULT;
+		}
+		pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+		paddr = pfn << PAGE_SHIFT;
+		table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);
+		if (!table) {
+			up_read(&current->mm->mmap_sem);
+			return -EFAULT;
+		}
+		ret = CMPXCHG(&table[index], orig_pte, new_pte);
+		memunmap(table);
+		up_read(&current->mm->mmap_sem);
+	}
 
 	return (ret != orig_pte);
 }
-- 
2.7.4

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#1618068 — Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs

2017-04-05 15:07+0200, Filippo Sironi:
> cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
> pages and the respective struct pages for mapping in the kernel virtual
> address space.
> This doesn't work if get_user_pages_fast() is invoked with a userspace
> virtual address that's backed by PFNs outside of kernel reach (e.g.,
> when limiting the kernel memory with mem= in the command line and using
> /dev/mem to map memory).
> 
> If get_user_pages_fast() fails, look up the VMA that backs the userspace
> virtual address, compute the PFN and the physical address, and map it in
> the kernel virtual address space with memremap().

What is the reason for a configuration that voluntarily restricts access
to memory that it needs?

> Signed-off-by: Filippo Sironi <sironi@amazon.de>
> Cc: Anthony Liguori <aliguori@amazon.com>
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> @@ -147,15 +147,36 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>  	struct page *page;
>  
>  	npages = get_user_pages_fast((unsigned long)ptep_user, 1, 1, &page);
> -	/* Check if the user is doing something meaningless. */
> -	if (unlikely(npages != 1))
> -		return -EFAULT;
> -
> -	table = kmap_atomic(page);
> -	ret = CMPXCHG(&table[index], orig_pte, new_pte);
> -	kunmap_atomic(table);
> -
> -	kvm_release_page_dirty(page);
> +	if (likely(npages == 1)) {
> +		table = kmap_atomic(page);
> +		ret = CMPXCHG(&table[index], orig_pte, new_pte);
> +		kunmap_atomic(table);
> +
> +		kvm_release_page_dirty(page);
> +	} else {
> +		struct vm_area_struct *vma;
> +		unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK;
> +		unsigned long pfn;
> +		unsigned long paddr;
> +
> +		down_read(&current->mm->mmap_sem);
> +		vma = find_vma_intersection(current->mm, vaddr,
> +					    vaddr + PAGE_SIZE);

Hm, with the argument order like this, we check that

  vaddr < vma->vm_end && vaddr + PAGE_SIZE > vma->vm_start

but shouldn't we actually check that the whole page is there, i.e.

  vaddr + PAGE_SIZE < vma->vm_end && vaddr > vma->vm_start

?

Thanks.

> +		if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
> +			up_read(&current->mm->mmap_sem);
> +			return -EFAULT;
> +		}
> +		pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> +		paddr = pfn << PAGE_SHIFT;
> +		table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);

(I don't undestand why there isn't a wrapper for this ...
 Looks like we're doing something unexpected.)

> +		if (!table) {
> +			up_read(&current->mm->mmap_sem);
> +			return -EFAULT;
> +		}
> +		ret = CMPXCHG(&table[index], orig_pte, new_pte);
> +		memunmap(table);
> +		up_read(&current->mm->mmap_sem);
> +	}
>  
>  	return (ret != orig_pte);
>  }
> -- 
> 2.7.4
> 

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