Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932164AbbLRXRB (ORCPT ); Fri, 18 Dec 2015 18:17:01 -0500 Received: from mail-ob0-f176.google.com ([209.85.214.176]:34597 "EHLO mail-ob0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751674AbbLRXQw (ORCPT ); Fri, 18 Dec 2015 18:16:52 -0500 MIME-Version: 1.0 In-Reply-To: References: <56736BD1.5080700@linux.intel.com> <5673750B.606@linux.intel.com> <567453AF.5060808@linux.intel.com> <56746774.8000707@linux.intel.com> <567476CC.8080805@linux.intel.com> From: Andy Lutomirski Date: Fri, 18 Dec 2015 15:16:32 -0800 Message-ID: Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit? To: Linus Torvalds Cc: Dave Hansen , "H. Peter Anvin" , Oleg Nesterov , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Brian Gerst , "linux-kernel@vger.kernel.org" , Christoph Hellwig Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1877 Lines: 41 On Fri, Dec 18, 2015 at 3:08 PM, Linus Torvalds wrote: > On Fri, Dec 18, 2015 at 2:28 PM, Andy Lutomirski wrote: >> >> Apps that don't want to use the baseline_pkru mechanism could use >> syscalls to claim ownership of protection keys but then manage them >> purely with WRPKRU directly. We could optionally disallow >> mprotect_key on keys that weren't allocated in advance. >> >> Does that seem sane? > > So everything seems sane except for the need for that baseline_pkru. > > I'm not seeing why it couldn't just be a fixed value. Is there any > real downside to it? Yes, I think. If I'm using protection keys to protect some critical data structure (important stuff in shared memory, important memory mapped files, pmem, etc), then I'll allocate a protection key and set PKRU to deny writes. The problem is that I really, really want writes denied except when explicitly enabled in narrow regions of code that use wrpkru to enable them, and I don't want an asynchronous signal delivered in those narrow regions of code or newly cloned threads to pick up the write-allow value. So I want baseline_pkru to have the deny writes entry. I think I would do exactly this in my production code here if my server supported it. Some day... Hrm. We might also want an option to change pkru and/or baseline_pkru in all threads in the current mm. That's optional but it could be handy. Maybe it would be as simple as having the allocate-a-pkey call have an option to set an initial baseline value and an option to propagate that initial value to pre-existing threads. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/