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

[Suren Baghdasaryan <surenb@google.com>]

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.

>
> --
> Thanks,
>
> David / dhildenb
>