Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

[David Hildenbrand <david@redhat.com>]

> Am 05.03.2021 um 19:36 schrieb Suren Baghdasaryan <surenb@google.com>:
> 
> On Fri, Mar 5, 2021 at 10:23 AM David Hildenbrand <david@redhat.com> wrote:
>> 
>>> 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().
> 
> 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...

Yeah, most likely!

>