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

[Suren Baghdasaryan <surenb@google.com>]

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.

>
> > +        */
> > +       if (!capable(CAP_SYS_NICE)) {
> > +               ret = -EPERM;
> > +               goto release_mm;
> > +       }
> > +
> >         total_len = iov_iter_count(&iter);
> >
> >         while (iov_iter_count(&iter)) {
> > @@ -1218,6 +1228,7 @@ 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);
> > --
> > 2.30.1.766.gb4fecdf3b7-goog
> >