Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752954AbdLMM5p (ORCPT ); Wed, 13 Dec 2017 07:57:45 -0500 Received: from mail-wm0-f68.google.com ([74.125.82.68]:38018 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752175AbdLMM5m (ORCPT ); Wed, 13 Dec 2017 07:57:42 -0500 X-Google-Smtp-Source: ACJfBot3+PYq2ZNIwbveFlZOKzQx31e7TnqLf0H3g9Un6+dNoRpBz9c9J4PYKl0uap2FI46qUvTslA== Date: Wed, 13 Dec 2017 15:57:40 +0300 From: "Kirill A. Shutemov" To: Peter Zijlstra , Dave Hansen Cc: Andy Lutomirski , Thomas Gleixner , LKML , X86 ML , Linus Torvalds , Borislav Petkov , Greg KH , Kees Cook , Hugh Dickins , Brian Gerst , Josh Poimboeuf , Denys Vlasenko , Boris Ostrovsky , Juergen Gross , David Laight , Eduardo Valentin , aliguori@amazon.com, Will Deacon , "linux-mm@kvack.org" , kirill.shutemov@linux.intel.com, aneesh.kumar@linux.vnet.ibm.com Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared Message-ID: <20171213125739.fllckbl3o4nonmpx@node.shutemov.name> References: <20171212173221.496222173@linutronix.de> <20171212173333.669577588@linutronix.de> <20171213122211.bxcb7xjdwla2bqol@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171213122211.bxcb7xjdwla2bqol@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20171208 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2153 Lines: 62 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 wrote: > > > From: Peter Zijstra > > > > > > 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: email@kvack.org -- Kirill A. Shutemov