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

[Peter Zijlstra <peterz@infradead.org>]

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@linutronix.de> wrote:
> > From: Peter Zijstra <peterz@infradead.org>
> >
> > 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.

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 */