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SubjectRe: [BUG][PATCH -mm] avoid BUG() in __stop_machine_run()
Date
On Thursday 19 June 2008 16:59:50 Hidehiro Kawai wrote:
> When a process loads a kernel module, __stop_machine_run() is called, and
> it calls sched_setscheduler() to give newly created kernel threads highest
> priority. However, the process can have no CAP_SYS_NICE which required
> for sched_setscheduler() to increase the priority. For example, SystemTap
> loads its module with only CAP_SYS_MODULE. In this case,
> sched_setscheduler() returns -EPERM, then BUG() is called.

Hi Hidehiro,

Nice catch. This can happen in the current code, it just doesn't
BUG().

> Failure of sched_setscheduler() wouldn't be a real problem, so this
> patch just ignores it.

Well, it can mean that the stop_machine blocks indefinitely. Better
than a BUG(), but we should aim higher.

> Or, should we give the CAP_SYS_NICE capability temporarily?

I don't think so. It can be seen from another thread, and in theory
that should not see something random. Worse, they can change it from
another thread.

How's this?

sched_setscheduler: add a flag to control access checks

Hidehiro Kawai noticed that sched_setscheduler() can fail in
stop_machine: it calls sched_setscheduler() from insmod, which can
have CAP_SYS_MODULE without CAP_SYS_NICE.

This simply introduces a flag to allow us to disable the capability
checks for internal callers (this is simpler than splitting the
sched_setscheduler() function, since it loops checking permissions).

The flag is only "false" (ie. no check) for the following cases, where
it shouldn't matter:
drivers/input/touchscreen/ucb1400_ts.c:ucb1400_ts_thread()
- it's a kthread
drivers/mmc/core/sdio_irq.c:sdio_irq_thread()
- also a kthread
kernel/kthread.c:create_kthread()
- making a kthread (from kthreadd)
kernel/softlockup.c:watchdog()
- also a kthread

And these cases could have failed before:
kernel/softirq.c:cpu_callback()
- CPU hotplug callback
kernel/stop_machine.c:__stop_machine_run()
- Called from various places, including modprobe()

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r 509f0724da6b drivers/input/touchscreen/ucb1400_ts.c
--- a/drivers/input/touchscreen/ucb1400_ts.c Thu Jun 19 17:06:30 2008 +1000
+++ b/drivers/input/touchscreen/ucb1400_ts.c Thu Jun 19 19:36:40 2008 +1000
@@ -287,7 +287,7 @@ static int ucb1400_ts_thread(void *_ucb)
int valid = 0;
struct sched_param param = { .sched_priority = 1 };

- sched_setscheduler(tsk, SCHED_FIFO, &param);
+ sched_setscheduler(tsk, SCHED_FIFO, &param, false);

set_freezable();
while (!kthread_should_stop()) {
diff -r 509f0724da6b drivers/mmc/core/sdio_irq.c
--- a/drivers/mmc/core/sdio_irq.c Thu Jun 19 17:06:30 2008 +1000
+++ b/drivers/mmc/core/sdio_irq.c Thu Jun 19 19:36:40 2008 +1000
@@ -70,7 +70,7 @@ static int sdio_irq_thread(void *_host)
unsigned long period, idle_period;
int ret;

- sched_setscheduler(current, SCHED_FIFO, &param);
+ sched_setscheduler(current, SCHED_FIFO, &param, false);

/*
* We want to allow for SDIO cards to work even on non SDIO
diff -r 509f0724da6b include/linux/sched.h
--- a/include/linux/sched.h Thu Jun 19 17:06:30 2008 +1000
+++ b/include/linux/sched.h Thu Jun 19 19:36:40 2008 +1000
@@ -1654,7 +1654,8 @@ extern int can_nice(const struct task_st
extern int can_nice(const struct task_struct *p, const int nice);
extern int task_curr(const struct task_struct *p);
extern int idle_cpu(int cpu);
-extern int sched_setscheduler(struct task_struct *, int, struct sched_param *);
+extern int sched_setscheduler(struct task_struct *, int, struct sched_param *,
+ bool);
extern struct task_struct *idle_task(int cpu);
extern struct task_struct *curr_task(int cpu);
extern void set_curr_task(int cpu, struct task_struct *p);
diff -r 509f0724da6b kernel/kthread.c
--- a/kernel/kthread.c Thu Jun 19 17:06:30 2008 +1000
+++ b/kernel/kthread.c Thu Jun 19 19:36:40 2008 +1000
@@ -104,7 +104,7 @@ static void create_kthread(struct kthrea
* root may have changed our (kthreadd's) priority or CPU mask.
* The kernel thread should not inherit these properties.
*/
- sched_setscheduler(create->result, SCHED_NORMAL, &param);
+ sched_setscheduler(create->result, SCHED_NORMAL, &param, false);
set_user_nice(create->result, KTHREAD_NICE_LEVEL);
set_cpus_allowed(create->result, CPU_MASK_ALL);
}
diff -r 509f0724da6b kernel/rtmutex-tester.c
--- a/kernel/rtmutex-tester.c Thu Jun 19 17:06:30 2008 +1000
+++ b/kernel/rtmutex-tester.c Thu Jun 19 19:36:40 2008 +1000
@@ -327,7 +327,8 @@ static ssize_t sysfs_test_command(struct
switch (op) {
case RTTEST_SCHEDOT:
schedpar.sched_priority = 0;
- ret = sched_setscheduler(threads[tid], SCHED_NORMAL, &schedpar);
+ ret = sched_setscheduler(threads[tid], SCHED_NORMAL, &schedpar,
+ true);
if (ret)
return ret;
set_user_nice(current, 0);
@@ -335,7 +336,8 @@ static ssize_t sysfs_test_command(struct

case RTTEST_SCHEDRT:
schedpar.sched_priority = dat;
- ret = sched_setscheduler(threads[tid], SCHED_FIFO, &schedpar);
+ ret = sched_setscheduler(threads[tid], SCHED_FIFO, &schedpar,
+ true);
if (ret)
return ret;
break;
diff -r 509f0724da6b kernel/sched.c
--- a/kernel/sched.c Thu Jun 19 17:06:30 2008 +1000
+++ b/kernel/sched.c Thu Jun 19 19:36:40 2008 +1000
@@ -4749,11 +4749,12 @@ __setscheduler(struct rq *rq, struct tas
* @p: the task in question.
* @policy: new policy.
* @param: structure containing the new RT priority.
+ * @user: do checks to ensure this thread has permission
*
* NOTE that the task may be already dead.
*/
int sched_setscheduler(struct task_struct *p, int policy,
- struct sched_param *param)
+ struct sched_param *param, bool user)
{
int retval, oldprio, oldpolicy = -1, on_rq, running;
unsigned long flags;
@@ -4785,7 +4786,7 @@ recheck:
/*
* Allow unprivileged RT tasks to decrease priority:
*/
- if (!capable(CAP_SYS_NICE)) {
+ if (user && !capable(CAP_SYS_NICE)) {
if (rt_policy(policy)) {
unsigned long rlim_rtprio;

@@ -4821,7 +4822,8 @@ recheck:
* Do not allow realtime tasks into groups that have no runtime
* assigned.
*/
- if (rt_policy(policy) && task_group(p)->rt_bandwidth.rt_runtime == 0)
+ if (user
+ && rt_policy(policy) && task_group(p)->rt_bandwidth.rt_runtime == 0)
return -EPERM;
#endif

@@ -4888,7 +4890,7 @@ do_sched_setscheduler(pid_t pid, int pol
retval = -ESRCH;
p = find_process_by_pid(pid);
if (p != NULL)
- retval = sched_setscheduler(p, policy, &lparam);
+ retval = sched_setscheduler(p, policy, &lparam, true);
rcu_read_unlock();

return retval;
diff -r 509f0724da6b kernel/softirq.c
--- a/kernel/softirq.c Thu Jun 19 17:06:30 2008 +1000
+++ b/kernel/softirq.c Thu Jun 19 19:36:40 2008 +1000
@@ -645,7 +645,7 @@ static int __cpuinit cpu_callback(struct

p = per_cpu(ksoftirqd, hotcpu);
per_cpu(ksoftirqd, hotcpu) = NULL;
- sched_setscheduler(p, SCHED_FIFO, &param);
+ sched_setscheduler(p, SCHED_FIFO, &param, false);
kthread_stop(p);
takeover_tasklets(hotcpu);
break;
diff -r 509f0724da6b kernel/softlockup.c
--- a/kernel/softlockup.c Thu Jun 19 17:06:30 2008 +1000
+++ b/kernel/softlockup.c Thu Jun 19 19:36:40 2008 +1000
@@ -211,7 +211,7 @@ static int watchdog(void *__bind_cpu)
struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
int this_cpu = (long)__bind_cpu;

- sched_setscheduler(current, SCHED_FIFO, &param);
+ sched_setscheduler(current, SCHED_FIFO, &param, false);

/* initialize timestamp */
touch_softlockup_watchdog();
diff -r 509f0724da6b kernel/stop_machine.c
--- a/kernel/stop_machine.c Thu Jun 19 17:06:30 2008 +1000
+++ b/kernel/stop_machine.c Thu Jun 19 19:36:40 2008 +1000
@@ -187,7 +187,7 @@ struct task_struct *__stop_machine_run(i
struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };

/* One high-prio thread per cpu. We'll do this one. */
- sched_setscheduler(p, SCHED_FIFO, &param);
+ sched_setscheduler(p, SCHED_FIFO, &param, false);
kthread_bind(p, cpu);
wake_up_process(p);
wait_for_completion(&smdata.done);

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