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

[Shakeel Butt <shakeelb@google.com>]

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/