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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 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Stat-Signature: guigkf3pg14u1guquu578ecus48tpeom X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: A22AE90009F9 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf19; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614968589-891141 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 05.03.21 19:08, Suren Baghdasaryan wrote: > On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand wro= te: >> >> On 05.03.21 18:45, Shakeel Butt wrote: >>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand w= rote: >>>> >>>> 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 anoth= er >>>>>>>> process. It effectively removes the security boundary between th= e >>>>>>>> two processes (in one direction). Granting ptrace attach capabil= ity >>>>>>>> even to a system process is considered dangerous since it create= s an >>>>>>>> attack surface. This severely limits the usage of this API. >>>>>>>> The operations process_madvise can perform do not affect the cor= rectness >>>>>>>> of the operation of the target process; they only affect where t= he data >>>>>>>> is physically located (and therefore, how fast it can be accesse= d). >>>>>>>> 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_REA= D >>>>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metad= ata >>>>>>>> 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: htt= ps://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=3Da14= 4f458bad476a3358e3a45023789cb7bb9f993 >>>>>>>> - 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, pi= dfd, const struct iovec __user *, vec, >>>>>>>> goto release_task; >>>>>>>> } >>>>>>>> >>>>>>>> - mm =3D mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); >>>>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metada= ta. */ >>>>>>>> + mm =3D mm_access(task, PTRACE_MODE_READ_FSCREDS); >>>>>>>> if (IS_ERR_OR_NULL(mm)) { >>>>>>>> ret =3D IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; >>>>>>>> goto release_task; >>>>>>>> } >>>>>>>> >>>>>>>> + /* >>>>>>>> + * Require CAP_SYS_NICE for influencing process performa= nce. 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_FRE= E >>>>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNE= ED >>>>>> would be considered destructive hints. >>>>>> Note that process_madvise_behavior_valid() allows only MADV_COLD a= nd >>>>>> MADV_PAGEOUT at the moment, which are both non-destructive. >>>>>> >>>>> >>>>> There is a plan to support MADV_DONTNEED for this syscall. Do we ne= ed >>>>> 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 ca= n >>>> 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 o= n >> the approach while skimming over the discussion :) will have a more >> detailed look) >=20 > 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=20 archive anymore due to automatic cleanups ...) , but introducing=20 MADV_DONTEED on a remote processes, having to tweak range logic because=20 we always want to apply it to the whole MM, just to speed up memory=20 reaping sounds like completely abusing madvise()/process_madvise() to me. You want different semantics than MADV_DONTNEED. You want different=20 semantics than madvise. Simple example: mlock()ed pages in the target process. MADV_DONTNEED=20 would choke on that. For the use case of reaping, you certainly don't car= e. I am not sure if process_madvise() is the right interface to enforce=20 discarding of all target memory. Not to mention that MADV_FREE doesn't make any sense IMHO for memory=20 reaping ... no to mention exposing this via process_madvise(). --=20 Thanks, David / dhildenb