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Fri, 05 Mar 2021 10:08:28 -0800 (PST) MIME-Version: 1.0 References: <20210303185807.2160264-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 5 Mar 2021 10:08:17 -0800 Message-ID: Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise To: David Hildenbrand 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 Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: d3ke8bjd1bftrmhxpycofpqmzznquf3p X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 55CAB80192E8 Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf27; identity=mailfrom; envelope-from=""; helo=mail-yb1-f172.google.com; client-ip=209.85.219.172 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614967707-482298 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 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. > > -- > Thanks, > > David / dhildenb >