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=-22.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 41F05C433DB for ; Fri, 5 Mar 2021 18:24:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 132EB650A4 for ; Fri, 5 Mar 2021 18:24:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229497AbhCESXn (ORCPT ); Fri, 5 Mar 2021 13:23:43 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:53193 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229875AbhCESXL (ORCPT ); Fri, 5 Mar 2021 13:23:11 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614968590; 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=UBlr5iwi3txToIZalk7mH8mEF/smQnuT6Yz6L+zX5F0=; b=P0GG2hK9nPvuQsXt6EP9Fhz/ASvCDVDKTexXCfnNEqFP4tW9t+b89ro0zGLIOsXDuBgGGS oCWgfRTPFeTosZ/KTMBxWdwWjajFnPV/XcIHJ0CDA6/OzZm3G2M8v5eblV6JvROxgKNy/H LxHLQo4YNcLYiMVXhUQouPaXE2vV9E0= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-331-uLxmRfCJN8O63EK29dBDng-1; Fri, 05 Mar 2021 13:23:07 -0500 X-MC-Unique: uLxmRfCJN8O63EK29dBDng-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 03696108BD07; Fri, 5 Mar 2021 18:23:04 +0000 (UTC) Received: from [10.36.112.194] (ovpn-112-194.ams2.redhat.com [10.36.112.194]) by smtp.corp.redhat.com (Postfix) with ESMTP id 12A3D1001B2C; Fri, 5 Mar 2021 18:22:56 +0000 (UTC) To: Suren Baghdasaryan Cc: Shakeel Butt , Andrew Morton , Jann Horn , Kees Cook , Jeffrey Vander Stoep , Minchan Kim , Michal Hocko , David Rientjes , =?UTF-8?Q?Edgar_Arriaga_Garc=c3=ada?= , Tim Murray , Florian Weimer , Oleg Nesterov , James Morris , Linux MM , SElinux list , Linux API , linux-security-module , stable , LKML , kernel-team References: <20210303185807.2160264-1-surenb@google.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise Message-ID: <3dfb7545-3545-cdbe-d643-8d76fc77a30f@redhat.com> Date: Fri, 5 Mar 2021 19:22:56 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On 05.03.21 19:08, Suren Baghdasaryan wrote: > On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand wrote: >> >> On 05.03.21 18:45, Shakeel Butt wrote: >>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand wrote: >>>> >>>> On 04.03.21 01:03, Shakeel Butt wrote: >>>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan wrote: >>>>>> >>>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt wrote: >>>>>>> >>>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan wrote: >>>>>>>> >>>>>>>> process_madvise currently requires ptrace attach capability. >>>>>>>> PTRACE_MODE_ATTACH gives one process complete control over another >>>>>>>> process. It effectively removes the security boundary between the >>>>>>>> two processes (in one direction). Granting ptrace attach capability >>>>>>>> even to a system process is considered dangerous since it creates an >>>>>>>> attack surface. This severely limits the usage of this API. >>>>>>>> The operations process_madvise can perform do not affect the correctness >>>>>>>> of the operation of the target process; they only affect where the data >>>>>>>> is physically located (and therefore, how fast it can be accessed). >>>>>>>> What we want is the ability for one process to influence another process >>>>>>>> in order to optimize performance across the entire system while leaving >>>>>>>> the security boundary intact. >>>>>>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ >>>>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata >>>>>>>> and CAP_SYS_NICE for influencing process performance. >>>>>>>> >>>>>>>> Cc: stable@vger.kernel.org # 5.10+ >>>>>>>> Signed-off-by: Suren Baghdasaryan >>>>>>>> Reviewed-by: Kees Cook >>>>>>>> Acked-by: Minchan Kim >>>>>>>> Acked-by: David Rientjes >>>>>>>> --- >>>>>>>> changes in v3 >>>>>>>> - Added Reviewed-by: Kees Cook >>>>>>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 >>>>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request >>>>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request >>>>>>>> >>>>>>>> mm/madvise.c | 13 ++++++++++++- >>>>>>>> 1 file changed, 12 insertions(+), 1 deletion(-) >>>>>>>> >>>>>>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>>>>>> index df692d2e35d4..01fef79ac761 100644 >>>>>>>> --- a/mm/madvise.c >>>>>>>> +++ b/mm/madvise.c >>>>>>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, >>>>>>>> goto release_task; >>>>>>>> } >>>>>>>> >>>>>>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); >>>>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ >>>>>>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); >>>>>>>> if (IS_ERR_OR_NULL(mm)) { >>>>>>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; >>>>>>>> goto release_task; >>>>>>>> } >>>>>>>> >>>>>>>> + /* >>>>>>>> + * Require CAP_SYS_NICE for influencing process performance. Note that >>>>>>>> + * only non-destructive hints are currently supported. >>>>>>> >>>>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? >>>>>> >>>>>> Non-destructive in this context means the data is not lost and can be >>>>>> recovered. I follow the logic described in >>>>>> https://lwn.net/Articles/794704/ where Minchan was introducing >>>>>> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE >>>>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED >>>>>> would be considered destructive hints. >>>>>> Note that process_madvise_behavior_valid() allows only MADV_COLD and >>>>>> MADV_PAGEOUT at the moment, which are both non-destructive. >>>>>> >>>>> >>>>> There is a plan to support MADV_DONTNEED for this syscall. Do we need >>>>> to change these access checks again with that support? >>>> >>>> Eh, I absolutely don't think letting another process discard memory in >>>> another process' address space is a good idea. The target process can >>>> observe that easily and might even run into real issues. >>>> >>>> What's the use case? >>>> >>> >>> Userspace oom reaper. Please look at >>> https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/ >>> >> >> Thanks, somehow I missed that (not that it really changed my opinion on >> the approach while skimming over the discussion :) will have a more >> detailed look) > > The latest version of that patchset is: > https://lore.kernel.org/patchwork/patch/1344419/ > Yeah, memory reaping is a special case when we are operating on a > dying process to speed up the release of its memory. I don't know if > for that particular case we need to make the checks stricter. It's a > dying process anyway and the data is being destroyed. Allowing to > speed up that process probably can still use CAP_SYS_NICE. I know, unrelated discussion (sorry, I don't have above thread in my archive anymore due to automatic cleanups ...) , but introducing MADV_DONTEED on a remote processes, having to tweak range logic because we always want to apply it to the whole MM, just to speed up memory reaping sounds like completely abusing madvise()/process_madvise() to me. You want different semantics than MADV_DONTNEED. You want different semantics than madvise. Simple example: mlock()ed pages in the target process. MADV_DONTNEED would choke on that. For the use case of reaping, you certainly don't care. I am not sure if process_madvise() is the right interface to enforce discarding of all target memory. Not to mention that MADV_FREE doesn't make any sense IMHO for memory reaping ... no to mention exposing this via process_madvise(). -- Thanks, David / dhildenb