Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752834AbbLCS2T (ORCPT ); Thu, 3 Dec 2015 13:28:19 -0500 Received: from relais.videotron.ca ([24.201.245.36]:13220 "EHLO relais.videotron.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752267AbbLCS2S (ORCPT ); Thu, 3 Dec 2015 13:28:18 -0500 MIME-version: 1.0 Content-transfer-encoding: 7BIT Content-type: text/plain; CHARSET=US-ASCII Date: Thu, 03 Dec 2015 13:28:12 -0500 (EST) From: Nicolas Pitre To: Russell King - ARM Linux Cc: Peter Rosin , "'linux-kernel@vger.kernel.org'" , "'linux-arm-kernel@lists.infradead.org'" , Will Deacon Subject: Re: Domain faults when CONFIG_CPU_SW_DOMAIN_PAN is enabled In-reply-to: <20151203172708.GT8644@n2100.arm.linux.org.uk> Message-id: References: <1267b061b66c4d1fa8af1955825bc97a@EMAIL.axentia.se> <20151203110041.GK8644@n2100.arm.linux.org.uk> <20151203115143.GM8644@n2100.arm.linux.org.uk> <533b0f1f264e42e78b94bbff9570fb50@EMAIL.axentia.se> <20151203133727.GN8644@n2100.arm.linux.org.uk> <94580382ca344ae2b64f66fb778c6ff7@EMAIL.axentia.se> <20151203164118.GR8644@n2100.arm.linux.org.uk> <20151203172708.GT8644@n2100.arm.linux.org.uk> User-Agent: Alpine 2.20 (LFD 67 2015-01-07) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2445 Lines: 53 On Thu, 3 Dec 2015, Russell King - ARM Linux wrote: > On Thu, Dec 03, 2015 at 04:41:18PM +0000, Russell King - ARM Linux wrote: > > On Thu, Dec 03, 2015 at 04:12:06PM +0000, Peter Rosin wrote: > > > * uaccess_with_memcpy.c:__copy_to_user() has a mode in which it copies > > > "non-atomically" (if faulthandler_disabled() returns 0). If a fault > > > happens during __copy_to_user, what prevents some other thread from > > > clobbering DACR? > > > > See the second point above. Moreover, if we sleep in down_read(), > > then __switch_to() reads the current DACR value and saves it in the > > thread information, and will restore that value when resuming the > > thread - even if the thread has been migrated to a different CPU. > > I thought this was correct, but it isn't - that's what my original solution > did, but I think when Will reviewed it, we decided it wasn't necessary - > and it isn't necessary for every single case with the exception of this > one. This is exactly what's going wrong: the down_read() in these paths > calls into the scheduler, which switches away. When we come back, the > DACR value is reset by the other thread to 0x51. > > There's a few ways to solve this: > > 1. Make the thread switching code save and restore the DACR register as > it would do for domains. This imposes an overhead on every single > context switch whether or not we happen to be in this _single_ > troublesome code. (Patch attached - as there's several, I'm attaching > them.) > > 2. Add additional code to the uaccess-with-memcpy stuff to reset the > DACR value prior to using memcpy() or memset(). (Patch attached.) > > 3. Make uaccess-with-memcpy depend on !CPU_SW_DOMAINS_PAN (suggested by > Will) > > 4. Delete the uaccess-with-memcpy code (also suggested by Will.) > > I think the best thing I can do is say... "Discuss amongst yourselves" :) Personally, I'd advocate for #2 or #4. Prior commit 0f64b247e6 I was already leaning towards #4. So if some people are still relying on uaccess-with-memcpy and #2 fixes it then it's all good. I'd suggest surrounding the DACR accesses with #ifdef CONFIG_CPU_SW_DOMAIN_PAN in the final patch. Nicolas -- 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/