Re: [PATCH 2/2] Yama: add PTRACE exception tracking

[Serge E. Hallyn]

Quoting Kees Cook (kees.cook@xxxxxxxxxxxxx):
> Some application suites have external crash handlers that depend on
> being able to use PTRACE to generate crash reports (KDE, Chromium, etc).
> Since the inferior process generally knows the PID of the debugger,
> it can use PR_SET_PTRACER to allow a specific PID and its descendants
> to perform the PTRACE instead of only a direct ancestor.
> 
> Signed-off-by: Kees Cook <kees.cook@xxxxxxxxxxxxx>
> ---

Hi Kees - very nice, overall.  One little note though:

> @@ -32,27 +204,20 @@ static int yama_ptrace_access_check(struct task_struct *child,
>  {
>  	int rc;
>  
> +	/* If standard caps disallows it, so does Yama.  We should
> +	 * should only tighten restrictions further.
> +	 */
>  	rc = cap_ptrace_access_check(child, mode);

This means that if capable(CAP_SYS_PTRACE) we'll always shortcut
here, so

> -	if (rc != 0)
> +	if (rc)
>  		return rc;
>  
>  	/* require ptrace target be a child of ptracer on attach */
> -	if (mode == PTRACE_MODE_ATTACH && ptrace_scope &&
> -	    !capable(CAP_SYS_PTRACE)) {
> -		struct task_struct *walker = child;
> -
> -		rcu_read_lock();
> -		read_lock(&tasklist_lock);
> -		while (walker->pid > 0) {
> -			if (walker == current)
> -				break;
> -			walker = walker->real_parent;
> -		}
> -		if (walker->pid == 0)
> -			rc = -EPERM;
> -		read_unlock(&tasklist_lock);
> -		rcu_read_unlock();
> -	}
> +	if (mode == PTRACE_MODE_ATTACH &&
> +	    ptrace_scope &&
> +	    !capable(CAP_SYS_PTRACE) &&

You don't need the CAP_SYS_PTRACE check here AFAICS.

> +	    !task_is_descendant(current, child) &&
> +	    !ptracer_exception_found(current, child))
> +		rc = -EPERM;
>  
>  	if (rc) {
>  		char name[sizeof(current->comm)];
> @@ -170,6 +335,8 @@ static struct security_operations yama_ops = {
>  	.ptrace_access_check =	yama_ptrace_access_check,
>  	.inode_follow_link =	yama_inode_follow_link,
>  	.path_link =		yama_path_link,
> +	.task_prctl =		yama_task_prctl,
> +	.task_free =		yama_task_free,
>  };
>  
>  #ifdef CONFIG_SYSCTL
> @@ -221,6 +388,8 @@ static __init int yama_init(void)
>  
>  	printk(KERN_INFO "Yama: becoming mindful.\n");
>  
> +	spin_lock_init(&ptracer_relations_lock);
> +
>  	if (register_security(&yama_ops))
>  		panic("Yama: kernel registration failed.\n");
>  
> -- 
> 1.7.1
> 
> 
> -- 
> Kees Cook
> Ubuntu Security Team
> --
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