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Fri, 05 Mar 2021 10:36:42 -0800 (PST) MIME-Version: 1.0 References: <20210303185807.2160264-1-surenb@google.com> <3dfb7545-3545-cdbe-d643-8d76fc77a30f@redhat.com> In-Reply-To: <3dfb7545-3545-cdbe-d643-8d76fc77a30f@redhat.com> From: Suren Baghdasaryan Date: Fri, 5 Mar 2021 10:36:31 -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: fj66ttsjkb8mgwjzndxrgxx1t4go8bzm X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 1DD8840001DE Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf02; identity=mailfrom; envelope-from=""; helo=mail-yb1-f170.google.com; client-ip=209.85.219.170 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614969403-567061 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 10:23 AM David Hildenbrand wrote: > > 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(). Yeah, that was the last comment from Christoph Hellwig on https://lore.kernel.org/patchwork/patch/1344418/ I'll be rethinking the whole approach. Previously I proposed couple different approaches that would make reaping a part of the kill by adding a new flag for pidfd_send_signal: https://lore.kernel.org/patchwork/patch/1338196/ https://lore.kernel.org/patchwork/patch/1060407/ but maybe a separate syscall for reaping is indeed the right way to go... > > -- > Thanks, > > David / dhildenb >