Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

[Suren Baghdasaryan]

On Mon, Jan 11, 2021 at 11:46 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 11-01-21 09:06:22, Suren Baghdasaryan 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).
>
> Yes it doesn't influence the correctness but it is still a very
> sensitive operation because it can allow a targeted side channel timing
> attacks so we should be really careful.

Sorry, I missed this comment in my answer. Possibility of affecting
the target's performance including side channel attack is why we
require CAP_SYS_NICE.

>
> > 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.
>
> I have to say that ptrace modes are rather obscure to me. So I cannot
> really judge whether MODE_READ is sufficient. My understanding has
> always been that this is requred to RO access to the address space. But
> this operation clearly has a visible side effect. Do we have any actual
> documentation for the existing modes?
>
> I would be really curious to hear from Jann and Oleg (now Cced).
>
> Is CAP_SYS_NICE requirement really necessary?
>
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > ---
> >  mm/madvise.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 6a660858784b..a9bcd16b5d95 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1197,12 +1197,22 @@ 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.
> > +      */
> > +     if (!capable(CAP_SYS_NICE)) {
> > +             ret = -EPERM;
> > +             goto release_mm;
> > +     }
> > +
> >       total_len = iov_iter_count(&iter);
> >
> >       while (iov_iter_count(&iter)) {
> > @@ -1217,6 +1227,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> >       if (ret == 0)
> >               ret = total_len - iov_iter_count(&iter);
> >
> > +release_mm:
> >       mmput(mm);
> >  release_task:
> >       put_task_struct(task);
> > --
> > 2.30.0.284.gd98b1dd5eaa7-goog
> >
>
> --
> Michal Hocko
> SUSE Labs