On Mon, Jul 18, 2016 at 1:11 PM, John Stultz <john.stu...@linaro.org> wrote:
> When an interface to allow a task to change another tasks
> timerslack was first proposed, it was suggested that something
> greater then CAP_SYS_NICE would be needed, as a task could be
> delayed further then what normally could be done with nice
> adjustments.
>
> So CAP_SYS_PTRACE was adopted instead for what became the
> /proc/<tid>/timerslack_ns interface. However, for Android (where
> this feature originates), giving the system_server
> CAP_SYS_PTRACE would allow it to observe and modify all tasks
> memory. This is considered too high a privilege level for only
> needing to change the timerslack.
>
> After some discussion, it was realized that a CAP_SYS_NICE
> process can set a task as SCHED_FIFO, so they could fork some
> spinning processes and set them all SCHED_FIFO 99, in effect
> delaying all other tasks for an infinite amount of time.
>
> So as a CAP_SYS_NICE task can already cause trouble for other
> tasks, using it as a required capability for accessing and
> modifying /proc/<tid>/timerslack_ns seems sufficient.
>
> Thus, this patch loosens the capability requirements to
> CAP_SYS_NICE and removes CAP_SYS_PTRACE, simplifying some
> of the code flow as well.
>
> This is technically an ABI change, but as the feature just
> landed in 4.6, I suspect no one is yet using it.
>
> Cc: Kees Cook <keesc...@chromium.org>
> Cc: "Serge E. Hallyn" <se...@hallyn.com>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> CC: Arjan van de Ven <ar...@linux.intel.com>
> Cc: Oren Laadan <or...@cellrox.com>
> Cc: Ruchi Kandoi <kandoiru...@google.com>
> Cc: Rom Lemarchand <rom...@android.com>
> Cc: Todd Kjos <tk...@google.com>
> Cc: Colin Cross <ccr...@android.com>
> Cc: Nick Kralevich <n...@google.com>
> Cc: Dmitry Shmidt <dimitr...@google.com>
> Cc: Elliott Hughes <e...@google.com>
> Cc: Android Kernel Team <kernel-t...@android.com>
> Signed-off-by: John Stultz <john.stu...@linaro.org>
Acked-by: Kees Cook <keesc...@chromium.org>
-Kees
> ---
> v2: Removed CAP_SYS_PTRACE check and simplified code flow
> v3: Tweaked where CAP_SYS_NICE check is made, suggeded by NickK
>
> fs/proc/base.c | 34 ++++++++++++++++++++--------------
> 1 file changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index a11eb71..c94abae 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2281,16 +2281,19 @@ static ssize_t timerslack_ns_write(struct file *file,
> const char __user *buf,
> if (!p)
> return -ESRCH;
>
> - if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> - task_lock(p);
> - if (slack_ns == 0)
> - p->timer_slack_ns = p->default_timer_slack_ns;
> - else
> - p->timer_slack_ns = slack_ns;
> - task_unlock(p);
> - } else
> + if (!capable(CAP_SYS_NICE)) {
> count = -EPERM;
> + goto out;
> + }
> +
> + task_lock(p);
> + if (slack_ns == 0)
> + p->timer_slack_ns = p->default_timer_slack_ns;
> + else
> + p->timer_slack_ns = slack_ns;
> + task_unlock(p);
>
> +out:
> put_task_struct(p);
>
> return count;
> @@ -2300,19 +2303,22 @@ static int timerslack_ns_show(struct seq_file *m,
> void *v)
> {
> struct inode *inode = m->private;
> struct task_struct *p;
> - int err = 0;
> + int err = 0;
>
> p = get_proc_task(inode);
> if (!p)
> return -ESRCH;
>
> - if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> - task_lock(p);
> - seq_printf(m, "%llu\n", p->timer_slack_ns);
> - task_unlock(p);
> - } else
> + if (!capable(CAP_SYS_NICE)) {
> err = -EPERM;
> + goto out;
> + }
>
> + task_lock(p);
> + seq_printf(m, "%llu\n", p->timer_slack_ns);
> + task_unlock(p);
> +
> +out:
> put_task_struct(p);
>
> return err;
> --
> 1.9.1
>
--
Kees Cook
Chrome OS & Brillo Security
