Re: [PATCH mm-unstable] mm/madvise: remove CAP_SYS_ADMIN requirement for process_madvise(MADV_COLLAPSE)

[Zach O'Keefe]

On Tue, Aug 2, 2022 at 2:09 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> Hasn't this been discussed during the MADV_COLLAPSE submission? What has
> changed? Does this need more time to settle with the consensus?
>
> On Mon 01-08-22 14:09:46, Zach O'Keefe wrote:
> > process_madvise(MADV_COLLAPSE) currently requires CAP_SYS_ADMIN when not
> > acting on the caller's own mm.  This is maximally restrictive, and
> > perpetuates existing issues with CAP_SYS_ADMIN.  Remove this requirement.
> >
> > When acting on an external process' memory, the biggest concerns for
> > process_madvise(MADV_COLLAPSE) are (1) being able to influence process
> > performance by moving memory, possibly between nodes, that is mapped
> > into the address space of external process(es), (2) defeat of
> > address-space-layout randomization, and (3), being able to increase
> > process RSS and memcg usage, possibly causing memcg OOM.
> >
> > process_madvise(2) already enforces CAP_SYS_NICE and PTRACE_MODE_READ (in
> > PTRACE_MODE_FSCREDS mode).  A process with these credentials can already
> > accomplish (1) and (2) via move_pages(MPOL_MF_MOVE_ALL), and (3) via
> > process_madvise(MADV_WILLNEED).
> >
> > process_madvise(MADV_COLLAPSE) may also circumvent sysfs THP settings.
> > When acting on one's own memory (which is equivalent to
> > madvise(MADV_COLLAPSE)), this is deemed acceptable, since aside from the
> > possibility of hoarding available hugepages (which is currently already
> > possible) no harm to the system can be done.  When acting on an external
> > process' memory, circumventing sysfs THP settings should provide no
> > additional threat compared to the ones listed.  As such, imposing
> > additional capabilities (such as CAP_SETUID, as a way to ensure the
> > caller could have just altered the sysfs THP settings themselves)
> > provides no extra protection.
> >
> > Fixes: 7ec952341312 ("mm/madvise: add MADV_COLLAPSE to process_madvise()")
> > Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
> > ---
> >  mm/madvise.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index f9e11b6c9916..af97100a0727 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1170,16 +1170,14 @@ madvise_behavior_valid(int behavior)
> >       }
> >  }
> >
> > -static bool
> > -process_madvise_behavior_valid(int behavior, struct task_struct *task)
> > +static bool process_madvise_behavior_valid(int behavior)
> >  {
> >       switch (behavior) {
> >       case MADV_COLD:
> >       case MADV_PAGEOUT:
> >       case MADV_WILLNEED:
> > -             return true;
> >       case MADV_COLLAPSE:
> > -             return task == current || capable(CAP_SYS_ADMIN);
> > +             return true;
> >       default:
> >               return false;
> >       }
> > @@ -1457,7 +1455,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> >               goto free_iov;
> >       }
> >
> > -     if (!process_madvise_behavior_valid(behavior, task)) {
> > +     if (!process_madvise_behavior_valid(behavior)) {
> >               ret = -EINVAL;
> >               goto release_task;
> >       }
> > --
> > 2.37.1.455.g008518b4e5-goog
>
> --
> Michal Hocko
> SUSE Labs

Hey Michal,

Thanks for taking the time to take a look at this.

"mm/madvise: add MADV_COLLAPSE to process_madvise()" in the v7 series
ended with me mentioning a couple options, but ultimately I didn't
present a solution, and no consensus was reached[1]. After taking a
closer look, this is my proposal for what I believe to be the best
path forward. It should be squashed into the original patch. What do you think?

Thanks again,
Zach

[1] https://lore.kernel.org/linux-mm/Ys4aTRqWIbjNs1mI@xxxxxxxxxx/