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Subject[patch V2 3/6] posix-cpu-timers: Sanitize thread clock permissions
From: Thomas Gleixner <tglx@linutronix.de>

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>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/time/posix-cpu-timers.c | 66 +++++++++++++++++++----------------------
1 file changed, 32 insertions(+), 34 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;

/*
@@ -64,50 +65,47 @@ 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(PROCESS) the task does not need to be
- * the actual group leader. tsk->sighand gives
- * access to the group's clock.
- *
- * Timers need the group leader because they take a
- * reference on it and store the task pointer until the
- * timer is destroyed.
+ * 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 it's own process, so spare
- * the ptrace check below.
+ * 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 {
+ /*
+ * Process wide timers need the group leader because they
+ * take a reference on it and store the task pointer until
+ * the timer is destroyed.
+ */
+ 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,

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