From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 899F26B0033 for ; Thu, 14 Dec 2017 15:55:04 -0500 (EST) Received: by mail-pf0-f199.google.com with SMTP id n6so5652389pfg.19 for ; Thu, 14 Dec 2017 12:55:04 -0800 (PST) Received: from bombadil.infradead.org (bombadil.infradead.org. [65.50.211.133]) by mx.google.com with ESMTPS id n7si3365500pgu.398.2017.12.14.12.55.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 14 Dec 2017 12:55:00 -0800 (PST) Date: Thu, 14 Dec 2017 21:54:50 +0100 From: Peter Zijlstra Subject: Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted() Message-ID: <20171214205450.GI3326@worktop> References: <20171214112726.742649793@infradead.org> <20171214113851.146259969@infradead.org> <20171214124117.wfzcjdczyta2sery@hirez.programming.kicks-ass.net> <20171214143730.s6w7sd6c7b5t6fqp@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, x86@kernel.org, Linus Torvalds , Andy Lutomirsky , Borislav Petkov , Greg KH , keescook@google.com, hughd@google.com, 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, dan.j.williams@intel.com On Thu, Dec 14, 2017 at 12:44:58PM -0800, Dave Hansen wrote: > On 12/14/2017 06:37 AM, Peter Zijlstra wrote: > > I'm also looking at pte_access_permitted() in handle_pte_fault(); that > > looks very dodgy to me. How does that not result in endlessly CoW'ing > > the same page over and over when we have a PKEY disallowing write access > > on that page? > > I'm not seeing the pte_access_permitted() in handle_pte_fault(). I > assume that's something you added in this series. No, Dan did in 5c9d2d5c269c4. > But, one of the ways that we keep pkeys from causing these kinds of > repeating loops when interacting with other things is this hunk in the > page fault code: > > > static inline int > > access_error(unsigned long error_code, struct vm_area_struct *vma) > > { > ... > > /* > > * Read or write was blocked by protection keys. This is > > * always an unconditional error and can never result in > > * a follow-up action to resolve the fault, like a COW. > > */ > > if (error_code & PF_PK) > > return 1; > > That short-circuits the page fault pretty quickly. So, basically, the > rule is: if the hardware says you tripped over pkey permissions, you > die. We don't try to do anything to the underlying page *before* saying > that you die. That only works when you trip the fault from hardware. Not if you do a software fault using gup(). AFAIK __get_user_pages(FOLL_FORCE|FOLL_WRITE|FOLL_GET) will loop indefinitely on the case I described. -- 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