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Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for
 process_madvise
To:     Shakeel Butt <shakeelb@google.com>,
        Suren Baghdasaryan <surenb@google.com>
Cc:     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>,
        =?UTF-8?Q?Edgar_Arriaga_Garc=c3=ada?= <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>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <b45d9599-b917-10c3-6b86-6ecd8db16d43@redhat.com>
Date:   Fri, 5 Mar 2021 18:37:39 +0100
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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?

-- 
Thanks,

David / dhildenb