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Fri, 5 Mar 2021 17:52:32 +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 BBFAC60BF3; Fri, 5 Mar 2021 17:52:28 +0000 (UTC) Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise To: Shakeel Butt 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 References: <20210303185807.2160264-1-surenb@google.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Fri, 5 Mar 2021 18:52:27 +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: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Stat-Signature: qoxfp113ewp7mggqj1uundchcs6ukw6o X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 1A752C000C66 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf22; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=63.128.21.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614966807-937215 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 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) -- Thanks, David / dhildenb