Messages in this thread | | | Date | Wed, 13 Dec 2017 15:57:40 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [patch 05/16] mm: Allow special mappings with user access cleared |
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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@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.
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 */ > > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
-- Kirill A. Shutemov
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