Re: [PATCH v5 03/13] X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: KarimAllah Ahmed <karahmed@amazon.de>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	pbonzini@redhat.com, rkrcmar@redhat.com,
	Filippo Sironi <sironi@amazon.de>
Subject: Re: [PATCH v5 03/13] X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs
Date: Wed, 23 Jan 2019 12:31:29 -0500	[thread overview]
Message-ID: <20190123173051.GH19289@Konrads-MacBook-Pro.local> (raw)
In-Reply-To: <1547026933-31226-4-git-send-email-karahmed@amazon.de>

On Wed, Jan 09, 2019 at 10:42:03AM +0100, KarimAllah Ahmed wrote:
> From: Filippo Sironi <sironi@amazon.de>
> 
> cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
> pages and the respective struct page to map 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 back 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>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>

I personally would have used some crafty goto statements to jump to
'err' label which would have
	up_read(&current->mm->mmap_sem);
	return -EFAULT;

which would be after
 154         return (ret != orig_pte);                                               

But that is bike-shedding so feel free to ignore it.

Either way:

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

> ---
>  arch/x86/kvm/paging_tmpl.h | 38 +++++++++++++++++++++++++++++---------
>  1 file changed, 29 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 6bdca39..c40af67 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -141,15 +141,35 @@ 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
> 