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From:   David Hildenbrand <david@redhat.com>
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
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On 05.03.21 19:08, Suren Baghdasaryan wrote:
> On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> 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)
> 
> 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().

-- 
Thanks,

David / dhildenb