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

Fri Mar 5 18:36:31 UTC 2021

On Fri, Mar 5, 2021 at 10:23 AM David Hildenbrand <david at redhat.com> wrote:
>
> On 05.03.21 19:08, Suren Baghdasaryan wrote:
> > On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david at redhat.com> wrote:
> >>
> >> On 05.03.21 18:45, Shakeel Butt wrote:
> >>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david at redhat.com> wrote:
> >>>>
> >>>> On 04.03.21 01:03, Shakeel Butt wrote:
> >>>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb at google.com> wrote:
> >>>>>>
> >>>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb at google.com> wrote:
> >>>>>>>
> >>>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb at 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 at vger.kernel.org # 5.10+
> >>>>>>>> Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> >>>>>>>> Reviewed-by: Kees Cook <keescook at chromium.org>
> >>>>>>>> Acked-by: Minchan Kim <minchan at kernel.org>
> >>>>>>>> Acked-by: David Rientjes <rientjes at google.com>
> >>>>>>>> ---
> >>>>>>>> changes in v3
> >>>>>>>> - Added Reviewed-by: Kees Cook <keescook at 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 at vger.kernel.org # 5.10+ per Andrew's request
> >>>>>>>> - cc'ed linux-security-module at 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.
>
> 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.
>
> You want different semantics than MADV_DONTNEED. You want different
> semantics than madvise.
>
> 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 care.
>
> I am not sure if process_madvise() is the right interface to enforce
> discarding of all target memory.
>
>
> Not to mention that MADV_FREE doesn't make any sense IMHO for memory
> reaping ... no to mention exposing this via process_madvise().

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...

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