From: David Hildenbrand <david@redhat.com>
To: Shakeel Butt <shakeelb@google.com>
Cc: "Suren Baghdasaryan" <surenb@google.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Jann Horn" <jannh@google.com>,
"Kees Cook" <keescook@chromium.org>,
"Jeffrey Vander Stoep" <jeffv@google.com>,
"Minchan Kim" <minchan@kernel.org>,
"Michal Hocko" <mhocko@suse.com>,
"David Rientjes" <rientjes@google.com>,
"Edgar Arriaga García" <edgararriaga@google.com>,
"Tim Murray" <timmurray@google.com>,
"Florian Weimer" <fweimer@redhat.com>,
"Oleg Nesterov" <oleg@redhat.com>,
"James Morris" <jmorris@namei.org>,
"Linux MM" <linux-mm@kvack.org>,
"SElinux list" <selinux@vger.kernel.org>,
"Linux API" <linux-api@vger.kernel.org>,
linux-security-module <linux-security-module@vger.kernel.org>,
stable <stable@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
kernel-team <kernel-team@android.com>
Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
Date: Fri, 5 Mar 2021 18:52:27 +0100 [thread overview]
Message-ID: <c234a564-a052-b586-2a32-8580aaf8ca5d@redhat.com> (raw)
In-Reply-To: <CALvZod6b8H-=N6WVrgMVLE3=pm-ELWerjAO5v5KHSH-ih337+g@mail.gmail.com>
On 05.03.21 18:45, Shakeel Butt wrote:
> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 04.03.21 01:03, Shakeel Butt wrote:
>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>>>
>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote:
>>>>>
>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> 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 <surenb@google.com>
>>>>>> Reviewed-by: Kees Cook <keescook@chromium.org>
>>>>>> Acked-by: Minchan Kim <minchan@kernel.org>
>>>>>> Acked-by: David Rientjes <rientjes@google.com>
>>>>>> ---
>>>>>> changes in v3
>>>>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org>
>>>>>> - 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
next prev parent reply other threads:[~2021-03-05 17:54 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-03 18:58 [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise Suren Baghdasaryan
2021-03-03 23:17 ` Shakeel Butt
2021-03-03 23:34 ` Suren Baghdasaryan
2021-03-04 0:03 ` Shakeel Butt
2021-03-04 1:17 ` Suren Baghdasaryan
2021-03-05 17:37 ` David Hildenbrand
2021-03-05 17:45 ` Shakeel Butt
2021-03-05 17:52 ` David Hildenbrand [this message]
2021-03-05 18:08 ` Suren Baghdasaryan
2021-03-05 18:22 ` David Hildenbrand
2021-03-05 18:36 ` Suren Baghdasaryan
2021-03-05 19:41 ` David Hildenbrand
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=c234a564-a052-b586-2a32-8580aaf8ca5d@redhat.com \
--to=david@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=edgararriaga@google.com \
--cc=fweimer@redhat.com \
--cc=jannh@google.com \
--cc=jeffv@google.com \
--cc=jmorris@namei.org \
--cc=keescook@chromium.org \
--cc=kernel-team@android.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-security-module@vger.kernel.org \
--cc=mhocko@suse.com \
--cc=minchan@kernel.org \
--cc=oleg@redhat.com \
--cc=rientjes@google.com \
--cc=selinux@vger.kernel.org \
--cc=shakeelb@google.com \
--cc=stable@vger.kernel.org \
--cc=surenb@google.com \
--cc=timmurray@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).