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

[David Hildenbrand]

On 05.03.21 18:45, Shakeel Butt wrote:
> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david at redhat.com> wrote:
>>
>> On 04.03.21 01:03, Shakeel Butt wrote:
>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb at google.com> wrote:
>>>>
>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb at google.com> wrote:
>>>>>
>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb at 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 at vger.kernel.org # 5.10+
>>>>>> Signed-off-by: Suren Baghdasaryan <surenb at google.com>
>>>>>> Reviewed-by: Kees Cook <keescook at chromium.org>
>>>>>> Acked-by: Minchan Kim <minchan at kernel.org>
>>>>>> Acked-by: David Rientjes <rientjes at google.com>
>>>>>> ---
>>>>>> changes in v3
>>>>>> - Added Reviewed-by: Kees Cook <keescook at 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 at vger.kernel.org # 5.10+ per Andrew's request
>>>>>> - cc'ed linux-security-module at 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