Re: [PATCH v2 2/8] exec: turn self_exec_id into self_privunit

[Oleg Nesterov <oleg@redhat.com>]

forgot to mention...

On 09/30, Oleg Nesterov wrote:
>
> On 09/23, Jann Horn wrote:
> >
> > One reason for doing this is that it prevents an attacker from sending an
> > arbitrary signal to a parent process after performing 2^32-1 execve()
> > calls.

No, sets ->exit_signal = SIGCHLD. So the only problem is that the parent
can do clone(SIGKILL), then do execve() 2^32-1 times, then it can be killed
by SIGKILL from the exiting child.

Honestly, I do not think this is security problem.

> I think we should simply kill self/parent_exec_id's. I am going to send
> the patch below after re-check/testing.

Yes, I think this makes sense anyway.

Oleg.

> --- x/include/linux/sched.h
> +++ x/include/linux/sched.h
> @@ -1677,9 +1677,6 @@ struct task_struct {
>  #endif
>  	struct seccomp seccomp;
>  
> -/* Thread group tracking */
> -   	u32 parent_exec_id;
> -   	u32 self_exec_id;
>  /* Protection of (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed,
>   * mempolicy */
>  	spinlock_t alloc_lock;
> --- x/fs/exec.c
> +++ x/fs/exec.c
> @@ -1163,6 +1163,14 @@ static int de_thread(struct task_struct 
>  no_thread_group:
>  	/* we have changed execution domain */
>  	tsk->exit_signal = SIGCHLD;
> +	if (!list_empty(&father->children)) {
> +		struct task_struct *child;
> +
> +		read_lock(&tasklist_lock);
> +		list_for_each_entry(child, &father->children, sibling)
> +			child->exit_signal = SIGCHLD;
> +		read_unlock(&tasklist_lock);
> +	}
>  
>  	exit_itimers(sig);
>  	flush_itimer_signals();
> @@ -1306,9 +1314,6 @@ void setup_new_exec(struct linux_binprm 
>  			set_dumpable(current->mm, suid_dumpable);
>  	}
>  
> -	/* An exec changes our domain. We are no longer part of the thread
> -	   group */
> -	current->self_exec_id++;
>  	flush_signal_handlers(current, 0);
>  	do_close_on_exec(current->files);
>  }
> --- x/kernel/fork.c
> +++ x/kernel/fork.c
> @@ -1573,13 +1573,10 @@ static struct task_struct *copy_process(
>  	write_lock_irq(&tasklist_lock);
>  
>  	/* CLONE_PARENT re-uses the old parent */
> -	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
> +	if (clone_flags & (CLONE_PARENT|CLONE_THREAD))
>  		p->real_parent = current->real_parent;
> -		p->parent_exec_id = current->parent_exec_id;
> -	} else {
> +	else
>  		p->real_parent = current;
> -		p->parent_exec_id = current->self_exec_id;
> -	}
>  
>  	spin_lock(&current->sighand->siglock);
>  
> --- x/kernel/signal.c
> +++ x/kernel/signal.c
> @@ -1585,15 +1585,6 @@ bool do_notify_parent(struct task_struct
>  	BUG_ON(!tsk->ptrace &&
>  	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
>  
> -	if (sig != SIGCHLD) {
> -		/*
> -		 * This is only possible if parent == real_parent.
> -		 * Check if it has changed security domain.
> -		 */
> -		if (tsk->parent_exec_id != tsk->parent->self_exec_id)
> -			sig = SIGCHLD;
> -	}
> -
>  	info.si_signo = sig;
>  	info.si_errno = 0;
>  	/*