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 01500C77B75 for ; Wed, 19 Apr 2023 01:46:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231400AbjDSBq3 (ORCPT ); Tue, 18 Apr 2023 21:46:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37894 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230153AbjDSBq0 (ORCPT ); Tue, 18 Apr 2023 21:46:26 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4653A7AB9 for ; Tue, 18 Apr 2023 18:45:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1681868738; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=uxbho8QxMR+zxIIpkOmO13HkVCORBZijDHQqdGD2kSE=; b=fIfOQmKjIsgvch3FSliyF/AFqQ6D6T0w8ynhEks4NBaqeNrhBsHAITXvZRnfwNHyjAsS4F BLLfACAPliM9Xraoj5nSTZrvTNA6jidSY2Czbf0kahRrF5rIDKazt5T8qiraEuF5nFOOvL WoZqpaQjANkiOS5SR06vGWJYfrXjdV4= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-499-nt_oy41jNOyHJiCmN5TsSg-1; Tue, 18 Apr 2023 21:45:35 -0400 X-MC-Unique: nt_oy41jNOyHJiCmN5TsSg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5E2E68996E0; Wed, 19 Apr 2023 01:45:35 +0000 (UTC) Received: from [10.22.34.98] (unknown [10.22.34.98]) by smtp.corp.redhat.com (Postfix) with ESMTP id ED1485AB7A; Wed, 19 Apr 2023 01:45:34 +0000 (UTC) Message-ID: <22aee5ea-dd6b-ac2b-0b28-a25ee6602b48@redhat.com> Date: Tue, 18 Apr 2023 21:45:34 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH] mm/mmap: Map MAP_STACK to VM_STACK Content-Language: en-US To: Hugh Dickins Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joe Mario , Barry Marson , Rafael Aquini References: <20230418210230.3495922-1-longman@redhat.com> <20230418141852.75e551e57e97f4b522957c5c@linux-foundation.org> <6c3c68b1-c4d4-dd82-58e8-f7013fb6c8e5@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Cheers, Longman