Re: [RFC][PATCH] proc: Relax /proc/<tid>/timerslack_ns capability requirements

[Kees Cook]

On Thu, Jul 14, 2016 at 11:50 AM, John Stultz <john.stultz@xxxxxxxxxx> 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.
>
> For ABI preservation, it still allows CAP_SYS_PTRACE tasks to
> access/modify timerslack values, but I'm fine with removing
> this if others agree.

Is anything actually using this ABI yet? (Regardless, I'm fine
allowing both caps.)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Cc: Oren Laadan <orenl@xxxxxxxxxxx>
> Cc: Ruchi Kandoi <kandoiruchi@xxxxxxxxxx>
> Cc: Rom Lemarchand <romlem@xxxxxxxxxxx>
> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
> Cc: Colin Cross <ccross@xxxxxxxxxxx>
> Cc: Nick Kralevich <nnk@xxxxxxxxxx>
> Cc: Dmitry Shmidt <dimitrysh@xxxxxxxxxx>
> Cc: Elliott Hughes <enh@xxxxxxxxxx>
> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
>  fs/proc/base.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index a11eb71..d32033e 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2281,7 +2281,8 @@ 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)) {
> +       if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) ||
> +           capable(CAP_SYS_NICE)) {
>                 task_lock(p);
>                 if (slack_ns == 0)
>                         p->timer_slack_ns = p->default_timer_slack_ns;
> @@ -2306,7 +2307,8 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>         if (!p)
>                 return -ESRCH;
>
> -       if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> +       if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) ||
> +           capable(CAP_SYS_NICE)) {
>                 task_lock(p);
>                 seq_printf(m, "%llu\n", p->timer_slack_ns);
>                 task_unlock(p);
> --
> 1.9.1
>



-- 
Kees Cook
Chrome OS & Brillo Security