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SubjectRe: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value
Oleg Nesterov <oleg@redhat.com> writes:

> On 08/30, Eric W. Biederman wrote:
>>
>> --- a/kernel/exit.c
>> +++ b/kernel/exit.c
>> @@ -182,6 +182,24 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
>> put_task_struct(tsk);
>> }
>>
>> +void put_dead_task_struct(struct task_struct *task)
>> +{
>> + bool delay = false;
>> + unsigned long flags;
>> +
>> + /* Is the task both reaped and no longer being scheduled? */
>> + raw_spin_lock_irqsave(&task->pi_lock, flags);
>> + if ((task->state == TASK_DEAD) &&
>> + (cmpxchg(&task->exit_state, EXIT_DEAD, EXIT_RCU) == EXIT_DEAD))
>> + delay = true;
>> + raw_spin_lock_irqrestore(&task->pi_lock, flags);
>> +
>> + /* If both are true use rcu delay the put_task_struct */
>> + if (delay)
>> + call_rcu(&task->rcu, delayed_put_task_struct);
>> + else
>> + put_task_struct(task);
>> +}
>>
>> void release_task(struct task_struct *p)
>> {
>> @@ -222,76 +240,13 @@ void release_task(struct task_struct *p)
>>
>> write_unlock_irq(&tasklist_lock);
>> release_thread(p);
>> - call_rcu(&p->rcu, delayed_put_task_struct);
>> + put_dead_task_struct(p);
>
> I had a similar change in mind, see below. This is subjective, but to me
> it looks more simple and clean.
>
> Oleg.
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8dc1811..1f9b021 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1134,7 +1134,10 @@ struct task_struct {
>
> struct tlbflush_unmap_batch tlb_ubc;
>
> - struct rcu_head rcu;
> + union {
> + bool xxx;
> + struct rcu_head rcu;
> + };
>
> /* Cache last used pipe for splice(): */
> struct pipe_inode_info *splice_pipe;
> diff --git a/kernel/exit.c b/kernel/exit.c
> index a75b6a7..baacfce 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -182,6 +182,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
> put_task_struct(tsk);
> }
>
> +void call_delayed_put_task_struct(struct task_struct *p)
> +{
> + if (xchg(&p->xxx, 1))
> + call_rcu(&p->rcu, delayed_put_task_struct);
> +}

I like using the storage we will later use for the rcu_head.

Is the intention your new variable xxx start as 0, and the only
on the second write it becomes 1 and we take action?

That should work but it is a funny way to encode a decrement. I think
it would be more straight forward to use refcount_dec_and_test.

So something like this:

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9f51932bd543..99a4518b9b17 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1142,7 +1142,10 @@ struct task_struct {

struct tlbflush_unmap_batch tlb_ubc;

- struct rcu_head rcu;
+ union {
+ refcount_t rcu_users;
+ struct rcu_head rcu;
+ };

/* Cache last used pipe for splice(): */
struct pipe_inode_info *splice_pipe;
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0497091e40c1..8bd51af44bf8 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -115,7 +115,7 @@ static inline void put_task_struct(struct task_struct *t)
__put_task_struct(t);
}

-struct task_struct *task_rcu_dereference(struct task_struct **ptask);
+void put_task_struct_rcu_user(struct task_struct *task);

#ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT
extern int arch_task_struct_size __read_mostly;
diff --git a/kernel/exit.c b/kernel/exit.c
index 5b4a5dcce8f8..a42fd8889036 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -182,6 +182,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
put_task_struct(tsk);
}

+void put_task_struct_rcu_user(struct task_struct *task)
+{
+ if (refcount_dec_and_test(&task->rcu_users))
+ call_rcu(&task->rcu, delayed_put_task_struct);
+}

void release_task(struct task_struct *p)
{
@@ -222,10 +227,10 @@ void release_task(struct task_struct *p)

write_unlock_irq(&tasklist_lock);
release_thread(p);
- call_rcu(&p->rcu, delayed_put_task_struct);
+ put_task_struct_rcu_user(p);

p = leader;
if (unlikely(zap_leader))
goto repeat;
}

diff --git a/kernel/fork.c b/kernel/fork.c
index 2852d0e76ea3..dc4799210e05 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -900,11 +900,15 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
if (orig->cpus_ptr == &orig->cpus_mask)
tsk->cpus_ptr = &tsk->cpus_mask;

- /*
- * One for us, one for whoever does the "release_task()" (usually
- * parent)
+ /* One for the user space visible state that goes away when
+ * the processes zombie is reaped.
+ * One for the reference from the scheduler.
+ *
+ * The reference count is ignored and free_task is called
+ * directly until copy_process completes.
*/
- refcount_set(&tsk->usage, 2);
+ refcount_set(&tsk->rcu_users, 2);
+ refcount_set(&tsk->usage, 1); /* One for the rcu users */
#ifdef CONFIG_BLK_DEV_IO_TRACE
tsk->btrace_seq = 0;
#endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2b037f195473..69015b7c28da 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3135,7 +3135,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
/* Task is done with its stack. */
put_task_stack(prev);

- put_task_struct(prev);
+ put_task_struct_rcu_user(prev);
}

tick_nohz_task_switch();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 802b1f3405f2..082f8ba2b1f4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -892,7 +892,7 @@ struct rq {
*/
unsigned long nr_uninterruptible;

- struct task_struct *curr;
+ struct task_struct __rcu *curr;
struct task_struct *idle;
struct task_struct *stop;
unsigned long next_balance;

Eric

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