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From:   David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
Date:   Fri, 5 Mar 2021 20:41:04 +0100
Message-Id: <245612A8-56DA-47D8-BB18-613FF9C8AF96@redhat.com>
References: <CAJuCfpHDtu0R6zZ0uo0YZgCE=dyhy6bsToUU2+reo3pBV0PcBg@mail.gmail.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Shakeel Butt <shakeelb@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>,
        =?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>
In-Reply-To: <CAJuCfpHDtu0R6zZ0uo0YZgCE=dyhy6bsToUU2+reo3pBV0PcBg@mail.gmail.com>
To:     Suren Baghdasaryan <surenb@google.com>
Precedence: bulk


> Am 05.03.2021 um 19:36 schrieb Suren Baghdasaryan <surenb@google.com>:
>=20
> =EF=BB=BFOn Fri, Mar 5, 2021 at 10:23 AM David Hildenbrand <david@redhat.c=
om> wrote:
>>=20
>>> On 05.03.21 19:08, Suren Baghdasaryan wrote:
>>> On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@redhat.com> wrot=
e:
>>>>=20
>>>> On 05.03.21 18:45, Shakeel Butt wrote:
>>>>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wr=
ote:
>>>>>>=20
>>>>>> On 04.03.21 01:03, Shakeel Butt wrote:
>>>>>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com=
> wrote:
>>>>>>>>=20
>>>>>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> w=
rote:
>>>>>>>>>=20
>>>>>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.=
com> wrote:
>>>>>>>>>>=20
>>>>>>>>>> process_madvise currently requires ptrace attach capability.
>>>>>>>>>> PTRACE_MODE_ATTACH gives one process complete control over anothe=
r
>>>>>>>>>> process. It effectively removes the security boundary between the=

>>>>>>>>>> two processes (in one direction). Granting ptrace attach capabili=
ty
>>>>>>>>>> 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 corr=
ectness
>>>>>>>>>> of the operation of the target process; they only affect where th=
e data
>>>>>>>>>> is physically located (and therefore, how fast it can be accessed=
).
>>>>>>>>>> What we want is the ability for one process to influence another p=
rocess
>>>>>>>>>> in order to optimize performance across the entire system while l=
eaving
>>>>>>>>>> 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 metada=
ta
>>>>>>>>>> and CAP_SYS_NICE for influencing process performance.
>>>>>>>>>>=20
>>>>>>>>>> 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: http=
s://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=3Da144f45=
8bad476a3358e3a45023789cb7bb9f993
>>>>>>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request
>>>>>>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's r=
equest
>>>>>>>>>>=20
>>>>>>>>>>    mm/madvise.c | 13 ++++++++++++-
>>>>>>>>>>    1 file changed, 12 insertions(+), 1 deletion(-)
>>>>>>>>>>=20
>>>>>>>>>> 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, pid=
fd, const struct iovec __user *, vec,
>>>>>>>>>>                   goto release_task;
>>>>>>>>>>           }
>>>>>>>>>>=20
>>>>>>>>>> -       mm =3D mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
>>>>>>>>>> +       /* Require PTRACE_MODE_READ to avoid leaking ASLR metadat=
a. */
>>>>>>>>>> +       mm =3D mm_access(task, PTRACE_MODE_READ_FSCREDS);
>>>>>>>>>>           if (IS_ERR_OR_NULL(mm)) {
>>>>>>>>>>                   ret =3D IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
>>>>>>>>>>                   goto release_task;
>>>>>>>>>>           }
>>>>>>>>>>=20
>>>>>>>>>> +       /*
>>>>>>>>>> +        * Require CAP_SYS_NICE for influencing process performan=
ce. Note that
>>>>>>>>>> +        * only non-destructive hints are currently supported.
>>>>>>>>>=20
>>>>>>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive?
>>>>>>>>=20
>>>>>>>> Non-destructive in this context means the data is not lost and can b=
e
>>>>>>>> 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_DONTNEE=
D
>>>>>>>> would be considered destructive hints.
>>>>>>>> Note that process_madvise_behavior_valid() allows only MADV_COLD an=
d
>>>>>>>> MADV_PAGEOUT at the moment, which are both non-destructive.
>>>>>>>>=20
>>>>>>>=20
>>>>>>> There is a plan to support MADV_DONTNEED for this syscall. Do we nee=
d
>>>>>>> to change these access checks again with that support?
>>>>>>=20
>>>>>> Eh, I absolutely don't think letting another process discard memory i=
n
>>>>>> another process' address space is a good idea. The target process can=

>>>>>> observe that easily and might even run into real issues.
>>>>>>=20
>>>>>> What's the use case?
>>>>>>=20
>>>>>=20
>>>>> Userspace oom reaper. Please look at
>>>>> https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/=
T/
>>>>>=20
>>>>=20
>>>> 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)
>>>=20
>>> 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.
>>=20
>> 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.=

>>=20
>> You want different semantics than MADV_DONTNEED. You want different
>> semantics than madvise.
>>=20
>> 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 car=
e.
>>=20
>> I am not sure if process_madvise() is the right interface to enforce
>> discarding of all target memory.
>>=20
>>=20
>> Not to mention that MADV_FREE doesn't make any sense IMHO for memory
>> reaping ... no to mention exposing this via process_madvise().
>=20
> 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!

>=20