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=-28.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL 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 E9439C43332 for ; Fri, 5 Mar 2021 17:46:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BCD4E650C6 for ; Fri, 5 Mar 2021 17:46:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229899AbhCERpg (ORCPT ); Fri, 5 Mar 2021 12:45:36 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48286 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229919AbhCERpV (ORCPT ); Fri, 5 Mar 2021 12:45:21 -0500 Received: from mail-lj1-x22e.google.com (mail-lj1-x22e.google.com [IPv6:2a00:1450:4864:20::22e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 39F55C061761 for ; Fri, 5 Mar 2021 09:45:21 -0800 (PST) Received: by mail-lj1-x22e.google.com with SMTP id u18so3842357ljd.3 for ; Fri, 05 Mar 2021 09:45:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=fjmoW72/ku1lnLXMuAm6ZbrP9WWp23FQh9M1c5WKSl8=; b=A9upIdY3NrsolQX2lFBy06Za+fmkomHdOwQcUyiHv6xHc7YqCQkQn8C6s0QHzCTaev SPR5qphZ57qK3qBIyHtSjH7FuFLszlLhqzw1qfcJhyzvX40a4/NJhPfKIvpxqznGT893 DRtYpo3aDFSUVvdgLQxmy+u29XYt2x56ELWBlMwArbOh3mRWENcJlAdQXEzeRInYX4Dj 8eSHEoYHoj08rC2K0VG4xfUTeh8kxNYHZZVMA08IIc5dA/y4QPSpteJOU1GAgKZG4bCt ryJoO/evucYsYRC1Tic8ime0iJWw/SzNmpqAA9Zadn9IUvBgXQcNW2l5TBAXTeQPZyL7 FDRQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=fjmoW72/ku1lnLXMuAm6ZbrP9WWp23FQh9M1c5WKSl8=; b=SfjYzSbwxyd3YdhL5teG50XXd/4ZjZYd9RsfQjB/jzkHYA4Tj3YYtkOK1R0u9YJ90W JfB8ifB4XlCSlA5G0mmwGG355OL6eCDtaZEybG41+1M2ZKtaA0/XXLWPmZWrDjgeLOMu BUN4gwXCRGsefbssWYHicScwB6g3LLBNsBlicyN5npmYU/C0FrtAa4nMUkQa286dJRq4 q0kw7xOx0pYW5av5rH5NOeRr1fXmdXq/RvEPXPrAt77fHbkrwnARaNLfYFoKg5zGqMWu m1A7P9BJCZRm82mIpBNA/T8nGidbiYLZBucFe6Nx4jxzkVNjwCo+NrNlUfPWKurrqQ+U Otuw== X-Gm-Message-State: AOAM530CH6rqG6tqNIDka4TDgROM7FDR4Sq5nU+9PhMDRi9XIdBn8fmN JchrZnOdub/R4Mj6FVqxDwZh1q862ytvxqGIDYcQ9w== X-Google-Smtp-Source: ABdhPJyK/dVc1k+/qBKUx8d5GEaTi/dVKzYSI4dp4RHRGb8oEpa+bUgfpktgJn4unjvrnTrj45o6DabtPYORirmP6Rw= X-Received: by 2002:a2e:9195:: with SMTP id f21mr5616413ljg.160.1614966319338; Fri, 05 Mar 2021 09:45:19 -0800 (PST) MIME-Version: 1.0 References: <20210303185807.2160264-1-surenb@google.com> In-Reply-To: From: Shakeel Butt Date: Fri, 5 Mar 2021 09:45:07 -0800 Message-ID: Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise To: David Hildenbrand Cc: Suren Baghdasaryan , 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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/