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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ACF54C77B76 for ; Wed, 19 Apr 2023 03:24:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230390AbjDSDYv (ORCPT ); Tue, 18 Apr 2023 23:24:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50572 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229633AbjDSDYq (ORCPT ); Tue, 18 Apr 2023 23:24:46 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D9914D2; Tue, 18 Apr 2023 20:24:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=Y0jC6pG5iyPnBukJ2foYlHjpDWbyen476EaJAQv4Kk0=; b=Fh61qubkIA4NQ/t+TuAGmvG6/Z 8ae3FW5/kyHKxUStAxLa7mpvkzLtwMNYgJQbnpb+2CAPczsqmR4FOhqqXBgjrj3RsvK6cBbn2Z23m y1eMdPe58ajYGb7XwBpgtdn2A0zmspDoqqDjOG9EbGqZzF0c2i0KcZMghhQK1Rnk3/CMPeBof4m/8 PNA4p/h1AHaAGp0lUFAhoFL0Uyxc8DKnpiw1DPdsayJ4imPlaJRwJ2XRbzFpJFCymq5GNcX8nyqlY 882ux3LVtdcusDvul2SLTD2YkUiZ6PGrENVpWkanrH7HMAJKUBMiU2CalsAYoac+84Z/fgVGzFWyd 9PPgBDkQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1poyQm-00CvFm-U9; Wed, 19 Apr 2023 03:24:28 +0000 Date: Wed, 19 Apr 2023 04:24:28 +0100 From: Matthew Wilcox To: Waiman Long Cc: Hugh Dickins , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joe Mario , Barry Marson , Rafael Aquini , Paul Moore , Stephen Smalley , Eric Paris , selinux@vger.kernel.org, James Morris , "Serge E. Hallyn" , linux-security-module@vger.kernel.org Subject: Re: [PATCH] mm/mmap: Map MAP_STACK to VM_STACK Message-ID: References: <20230418210230.3495922-1-longman@redhat.com> <20230418141852.75e551e57e97f4b522957c5c@linux-foundation.org> <6c3c68b1-c4d4-dd82-58e8-f7013fb6c8e5@redhat.com> <22aee5ea-dd6b-ac2b-0b28-a25ee6602b48@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <22aee5ea-dd6b-ac2b-0b28-a25ee6602b48@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 18, 2023 at 09:45:34PM -0400, Waiman Long wrote: > > On 4/18/23 21:36, Hugh Dickins wrote: > > On Tue, 18 Apr 2023, Waiman Long wrote: > > > On 4/18/23 17:18, Andrew Morton wrote: > > > > On Tue, 18 Apr 2023 17:02:30 -0400 Waiman Long wrote: > > > > > > > > > One of the flags of mmap(2) is MAP_STACK to request a memory segment > > > > > suitable for a process or thread stack. The kernel currently ignores > > > > > this flags. Glibc uses MAP_STACK when mmapping a thread stack. However, > > > > > selinux has an execstack check in selinux_file_mprotect() which disallows > > > > > a stack VMA to be made executable. > > > > > > > > > > Since MAP_STACK is a noop, it is possible for a stack VMA to be merged > > > > > with an adjacent anonymous VMA. With that merging, using mprotect(2) > > > > > to change a part of the merged anonymous VMA to make it executable may > > > > > fail. This can lead to sporadic failure of applications that need to > > > > > make those changes. > > > > "Sporadic failure of applications" sounds quite serious. Can you > > > > provide more details? > > > The problem boils down to the fact that it is possible for user code to mmap a > > > region of memory and then for the kernel to merge the VMA for that memory with > > > the VMA for one of the application's thread stacks. This is causing random > > > SEGVs with one of our large customer application. > > > > > > At a high level, this is what's happening: > > > > > >  1) App runs creating lots of threads. > > >  2) It mmap's 256K pages of anonymous memory. > > >  3) It writes executable code to that memory. > > >  4) It calls mprotect() with PROT_EXEC on that memory so > > >     it can subsequently execute the code. > > > > > > The above mprotect() will fail if the mmap'd region's VMA gets merged with the > > > VMA for one of the thread stacks.  That's because the default RHEL SELinux > > > policy is to not allow executable stacks. > > Then wouldn't the bug be at the SELinux end? VMAs may have been merged > > already, but the mprotect() with PROT_EXEC of the good non-stack range > > will then split that area off from the stack again - maybe the SELinux > > check does not understand that must happen? > > The SELinux check is done per VMA, not a region within a VMA. After VMA > merging, SELinux is probably not able to determine which part of a VMA is a > stack unless we keep that information somewhere and provide an API for > SELinux to query. That can be quite a lot of work. So the easiest way to > prevent this problem is to avoid merging a stack VMA with a regular > anonymous VMA. To paraphrase you, "Yes, SELinux is buggy, but we don't want to fix it". Cc'ing the SELinux people so it can be fixed properly.