From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31E9BC67790 for ; Wed, 25 Jul 2018 16:37:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DD635205C9 for ; Wed, 25 Jul 2018 16:37:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DD635205C9 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729202AbeGYRtx (ORCPT ); Wed, 25 Jul 2018 13:49:53 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:41946 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728633AbeGYRtx (ORCPT ); Wed, 25 Jul 2018 13:49:53 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id EDD6080D; Wed, 25 Jul 2018 09:37:26 -0700 (PDT) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id BEBBA3F575; Wed, 25 Jul 2018 09:37:26 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id BBF9A1AE1110; Wed, 25 Jul 2018 17:37:26 +0100 (BST) Date: Wed, 25 Jul 2018 17:37:26 +0100 From: Will Deacon To: Dave Martin Cc: linux-kernel@vger.kernel.org, arnd@arndb.de, oleg@redhat.com, linux@dominikbrodowski.net, viro@zeniv.linux.org.uk, akpm@linux-foundation.org, steve.mcintyre@arm.com, linux-arm-kernel@lists.infradead.org, ebiederm@xmission.com Subject: Re: [PATCH 1/2] signal: Introduce COMPAT_SIGMINSTKSZ for use in compat_sys_sigaltstack Message-ID: <20180725163726.GE6866@arm.com> References: <1532526312-26993-1-git-send-email-will.deacon@arm.com> <1532526312-26993-2-git-send-email-will.deacon@arm.com> <20180725155427.GR4240@e103592.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180725155427.GR4240@e103592.cambridge.arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 25, 2018 at 04:54:27PM +0100, Dave Martin wrote: > On Wed, Jul 25, 2018 at 02:45:11PM +0100, Will Deacon wrote: > > @@ -3476,7 +3478,8 @@ int restore_altstack(const stack_t __user *uss) > > stack_t new; > > if (copy_from_user(&new, uss, sizeof(stack_t))) > > return -EFAULT; > > - (void)do_sigaltstack(&new, NULL, current_user_stack_pointer()); > > + (void)do_sigaltstack(&new, NULL, current_user_stack_pointer(), > > + MINSIGSTKSZ); > > Why can't this fail? > > If this fails here we silently go wrong, but... > > > /* squash all but EFAULT for now */ > > return 0; > > } > > @@ -3510,7 +3513,8 @@ static int do_compat_sigaltstack(const compat_stack_t __user *uss_ptr, > > uss.ss_size = uss32.ss_size; > > } > > ret = do_sigaltstack(uss_ptr ? &uss : NULL, &uoss, > > - compat_user_stack_pointer()); > > + compat_user_stack_pointer(), > > + COMPAT_MINSIGSTKSZ); > > If this fails on arm64, we seem to SEGV (see compat_sys_rt_sigreturn()). > > This patch doesn't introduce this inconsistency, this might be a good > opportunity to clean it up. I don't think there's an inconsistency here -- both restore_altstack and compat_restore_altstack suppress all non--EFAULT errors (remember that uoss is NULL in both cases, so the copy_from_user() immediately before the do_sigaltstack() call for the native path is all we care about). I think the behaviour is: on a sigreturn, if you set the altstack to be an unmapped address then you get a SEGV, otherwise if you make it invalid in some other way (e.g. too small) then it's ignored and the old altstack remains intact. Will