Re: [patch 5/6] posix-cpu-timers: Sanitize thread clock permissions

Frederic Weisbecker
2019-09-23T14:03:54Z

On Thu, Sep 05, 2019 at 02:03:44PM +0200, Thomas Gleixner wrote:
> The thread clock permissions are restricted to tasks of the same thread
> group, but that also prevents a ptracer from reading them. This is
> inconsistent vs. the process restrictions and unnecessary strict.
>
> Relax it to ptrace permissions in the same way as process permissions are
> handled.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> kernel/time/posix-cpu-timers.c | 56 +++++++++++++++++++++--------------------
> 1 file changed, 29 insertions(+), 27 deletions(-)
>
> --- a/kernel/time/posix-cpu-timers.c
> +++ b/kernel/time/posix-cpu-timers.c
> @@ -51,6 +51,7 @@ void update_rlimit_cpu(struct task_struc
> static struct task_struct *lookup_task(const pid_t pid, bool thread,
> bool gettime)
> {
> + unsigned int mode = PTRACE_MODE_ATTACH_REALCREDS;
> struct task_struct *p;
>
> /*
> @@ -64,44 +65,45 @@ static struct task_struct *lookup_task(c
> if (!p)
> return p;
>
> - if (thread)
> - return same_thread_group(p, current) ? p : NULL;
> -
> if (gettime) {
> /*
> * For clock_gettime() the task does not need to be the
> * actual group leader. tsk->sighand gives access to the
> - * group's clock. current can obviously access itself, so
> - * spare the ptrace check below.
> + * group's clock.
> + *
> + * The trivial case is that p is current or in the same
> + * thread group, i.e. sharing p->signal. Spare the ptrace
> + * check in that case.
> */
> - if (p == current)
> + if (same_thread_group(p, current))
> return p;
>
> - if (!thread_group_leader(p))
> - return NULL;
> + mode = PTRACE_MODE_READ_REALCREDS;
>
> - if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
> - return NULL;
> - return p;
> - }
> + } else if (thread) {
> + /*
> + * Timer is going to be attached to a thread. If p is
> + * current or in the same thread group, granted.
> + */
> + if (same_thread_group(p, current))
> + return p;
>
> - /*
> - * For processes require that p is group leader.
> - */
> - if (!has_group_leader_pid(p))
> - return NULL;
> + } else {
> + /*
> + * For processes require that p is group leader.
> + */
> + if (!has_group_leader_pid(p))
> + return NULL;
>
> - /*
> - * Avoid the ptrace overhead when this is current's process
> - */
> - if (same_thread_group(p, current))
> - return p;
> + /*
> + * Avoid the ptrace overhead when this is current's process
> + */
> + if (same_thread_group(p, current))

Should it be "if (p == current)" ?

Other than that:

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

> + return p;
> + }
>
> - /*
> - * Creating timers on processes which cannot be ptraced is not
> - * permitted:
> - */
> - return ptrace_may_access(p, PTRACE_MODE_ATTACH_REALCREDS) ? p : NULL;
> + /* Decide based on the ptrace permissions. */
> + return ptrace_may_access(p, mode) ? p : NULL;
> }