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

[Konrad Rzeszutek Wilk]

On Wed, Jan 09, 2019 at 10:42:03AM +0100, KarimAllah Ahmed wrote:
> From: Filippo Sironi <sir...@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 <sir...@amazon.de>
> Signed-off-by: KarimAllah Ahmed <karah...@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.w...@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
>