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

[Shakeel Butt <shakeelb@google.com>]

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?

> +        */
> +       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
>