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

Thomas Gleixner
2019-09-05T12:12:18Z

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))
+ 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;
}

static struct task_struct *__get_task_for_clock(const clockid_t clock,