Re: [PATCH] Fix race in process_vm_rw_core

[Oleg Nesterov <oleg@redhat.com>]

On 01/16, Christopher Yeoh wrote:
>
> +struct mm_struct *get_check_task_mm(struct task_struct *task, unsigned int mode)
> +{
> +	struct mm_struct *mm;
> +	int err;
> +
> +	err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
> +	if (err)
> +		return ERR_PTR(err);
> +
> +	task_lock(task);
> +	mm = task->mm;
> +	if (mm != current->mm && __ptrace_may_access(task, mode)) {
> +		mm = ERR_PTR(-EACCES);
> +		goto out;
> +	}
> +
> +	if (mm) {
> +		if (task->flags & PF_KTHREAD)
> +			mm = NULL;
> +		else
> +			atomic_inc(&mm->mm_users);
> +	}

This still looks a bit strange, we call __ptrace_may_access()
before we check ->mm != NULL even if this is safe... Really, we
would simply fix the bug then try to microoptimize this code.

But OK, I promised I won't argue ;)

I believe the patch is correct and fixes the problem.

Oleg.