From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f44.google.com (mail-la0-f44.google.com [209.85.215.44]) by kanga.kvack.org (Postfix) with ESMTP id B213782F7F for ; Thu, 24 Sep 2015 15:17:58 -0400 (EDT) Received: by lacdq2 with SMTP id dq2so19771486lac.1 for ; Thu, 24 Sep 2015 12:17:57 -0700 (PDT) Received: from mail-la0-f43.google.com (mail-la0-f43.google.com. [209.85.215.43]) by mx.google.com with ESMTPS id y191si6333587lfd.174.2015.09.24.12.17.57 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 24 Sep 2015 12:17:57 -0700 (PDT) Received: by lahh2 with SMTP id h2so73868117lah.0 for ; Thu, 24 Sep 2015 12:17:56 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <56044A88.7030203@sr71.net> References: <20150916174903.E112E464@viggo.jf.intel.com> <20150916174913.AF5FEA6D@viggo.jf.intel.com> <20150920085554.GA21906@gmail.com> <55FF88BA.6080006@sr71.net> <20150924094956.GA30349@gmail.com> <56044A88.7030203@sr71.net> From: Andy Lutomirski Date: Thu, 24 Sep 2015 12:17:37 -0700 Message-ID: Subject: Re: [PATCH 26/26] x86, pkeys: Documentation Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: Ingo Molnar , X86 ML , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Linus Torvalds , Andrew Morton , Peter Zijlstra , Andy Lutomirski , Borislav Petkov , Kees Cook On Thu, Sep 24, 2015 at 12:10 PM, Dave Hansen wrote: > On 09/24/2015 02:49 AM, Ingo Molnar wrote: >> * Dave Hansen wrote: >>>> Another question, related to enumeration as well: I'm wondering whether >>>> there's any way for the kernel to allocate a bit or two for its own purposes - >>>> such as protecting crypto keys? Or is the facility fundamentally intended for >>>> user-space use only? >>> >>> No, that's not possible with the current setup. >> >> Ok, then another question, have you considered the following usecase: >> >> AFAICS pkeys only affect data loads and stores. Instruction fetches are notably >> absent from the documentation. Can you clarify that instructions can be fetched >> and executed from PTE_READ but pkeys-all-access-disabled pags? > > That is my understanding. I don't have a test for it, but I'll go make one. > >> If yes then this could be a significant security feature / usecase for pkeys: >> executable sections of shared libraries and binaries could be mapped with pkey >> access disabled. If I read the Intel documentation correctly then that should be >> possible. > > Agreed. I've even heard from some researchers who are interested in this: > > https://www.infsec.cs.uni-saarland.de/wp-content/uploads/sites/2/2014/10/nuernberger2014ccs_disclosure.pdf > >> I.e. AFAICS pkeys could be used to create true '--x' permissions for executable >> (user-space) pages. > > Just remember that all of the protections are dependent on the contents > of PKRU. If an attacker controls the Access-Disable bit in PKRU for the > executable-only region, you're sunk. > > But, that either requires being able to construct and execute arbitrary > code *or* call existing code that sets PKRU to the desired values. > Which, I guess, gets harder to do if all of the the wrpkru's are *in* > the execute-only area. > This may mean that we want to have a way for binaries to indicate that they want their --x segments to be loaded with a particular protection key. The right way to do that might be using an ELF note, and I also want to use ELF notes to allow turning off vsyscalls, so maybe it's time to write an ELF note parser in the kernel. --Andy -- 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