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

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Jan 8, 2021 at 2:15 PM Minchan Kim <minchan@kernel.org> wrote:
>
> On Fri, Jan 08, 2021 at 12:58:57PM -0800, 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).
> > 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.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> It sounds logical to me.
> If security folks don't see any concern and fix below,
>
> Acked-by: Minchan Kim <minchan@kernel.org>
>
> > @@ -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_task;
>
> mmput?

Ouch! Thanks for pointing it out! Will include in the next respin.

>
> > +     }
> > +
> >       total_len = iov_iter_count(&iter);
> >
> >       while (iov_iter_count(&iter)) {
> > --
> > 2.30.0.284.gd98b1dd5eaa7-goog
> >
>
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