Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Kirill A. Shutemov]

On Wed, Dec 13, 2017 at 01:22:11PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 12, 2017 at 10:00:08AM -0800, Andy Lutomirski wrote:
> > On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > > From: Peter Zijstra <peterz@xxxxxxxxxxxxx>
> > >
> > > In order to create VMAs that are not accessible to userspace create a new
> > > VM_NOUSER flag. This can be used in conjunction with
> > > install_special_mapping() to inject 'kernel' data into the userspace map.
> > >
> > > Similar to how arch_vm_get_page_prot() allows adding _PAGE_flags to
> > > pgprot_t, introduce arch_vm_get_page_prot_excl() which masks
> > > _PAGE_flags from pgprot_t and use this to implement VM_NOUSER for x86.
> > 
> > How does this interact with get_user_pages(), etc?
> 
> So I went through that code and I think I found a bug related to this.
> 
> get_user_pages_fast() will ultimately end up doing
> pte_access_permitted() before getting the page, follow_page OTOH does
> not do this, which makes for a curious difference between the two.
> 
> So I'm thinking we want the below irrespective of the VM_NOUSER patch,
> but with VM_NOUSER it would mean write(2) will no longer be able to
> access the page.

Oh..

We do call pte_access_permitted(), but only for write access.
See can_follow_write_pte().

The issue seems bigger: we also need such calls for other page table levels :-/

Dave, what is effect of this on protection keys?

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index dfcde13f289a..b852f37a2b0c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -153,6 +153,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  	}
>  
>  	if (flags & FOLL_GET) {
> +		if (!pte_access_permitted(pte, !!(flags & FOLL_WRITE))) {
> +			page = ERR_PTR(-EFAULT);
> +			goto out;
> +		}
> +
>  		get_page(page);
>  
>  		/* drop the pgmap reference now that we hold the page */
> 
> 
> 
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-- 
 Kirill A. Shutemov