From: David Hildenbrand <david@redhat.com>
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>,
"Edgar Arriaga García" <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>
Subject: Re: [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
Date: Fri, 5 Mar 2021 18:37:39 +0100 [thread overview]
Message-ID: <b45d9599-b917-10c3-6b86-6ecd8db16d43@redhat.com> (raw)
In-Reply-To: <CALvZod4nZ6W05N-4ostUEz5EbCuEvuBpc4LRYfAFgwQU-wb9dQ@mail.gmail.com>
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
next prev parent reply other threads:[~2021-03-05 17:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-03 18:58 [PATCH v3 1/1] mm/madvise: replace ptrace attach requirement for process_madvise Suren Baghdasaryan
2021-03-03 23:17 ` Shakeel Butt
2021-03-03 23:34 ` Suren Baghdasaryan
2021-03-04 0:03 ` Shakeel Butt
2021-03-04 1:17 ` Suren Baghdasaryan
2021-03-05 17:37 ` David Hildenbrand [this message]
2021-03-05 17:45 ` Shakeel Butt
2021-03-05 17:52 ` David Hildenbrand
2021-03-05 18:08 ` Suren Baghdasaryan
2021-03-05 18:22 ` David Hildenbrand
2021-03-05 18:36 ` Suren Baghdasaryan
2021-03-05 19:41 ` David Hildenbrand
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