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

[Suren Baghdasaryan]

On Wed, Mar 3, 2021 at 4:04 PM Shakeel Butt <shakeelb at google.com> 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?

I think so. Destructive hints affect the data, so we will probably
need stricter checks for those hints.