Re: [PATCH] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()

[Tycho Andersen <tycho@docker.com>]

On Wed, Sep 20, 2017 at 03:04:43PM +0200, Oleg Nesterov wrote:
> On 09/20, Oleg Nesterov wrote:
> >
> > @@ -908,13 +912,13 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
> >  	if (!data)
> >  		goto out;
> >
> > -	get_seccomp_filter(task);
> > +	refcount_inc(&filter->usage);
> >  	spin_unlock_irq(&task->sighand->siglock);
> >
> >  	if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
> >  		ret = -EFAULT;
> >
> > -	put_seccomp_filter(task);
> > +	__put_seccomp_filter(filter);
> 
> This is the simple fix for -stable, but again, can't we simplify this
> code? Afaics we can do get_seccomp_filter() at the start and drop siglock
> right after that.
> 
> Something like the untested patch (on top of this one) below?

Yes, this looks good to me, thanks.

> And I can't understand the SECCOMP_MODE_DISABLED check... shouldn't we
> simply remove it?

I think the idea was to prevent some interaction between
seccomp+ptrace+fork that we didn't understand. Since the user of this
code doesn't have seccomp filters attached, it was fine.

Thanks for cleaning this up, I'll be happy to test whatever final
patch we come up with.

Tycho

> Oleg.
> 
> 
> --- x/kernel/seccomp.c
> +++ x/kernel/seccomp.c
> @@ -858,45 +858,36 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
>  long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
>  			void __user *data)
>  {
> -	struct seccomp_filter *filter;
> +	struct seccomp_filter *orig, *filter;
>  	struct sock_fprog_kern *fprog;
> +	unsigned long count;
>  	long ret;
> -	unsigned long count = 0;
>  
>  	if (!capable(CAP_SYS_ADMIN) ||
>  	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
>  		return -EACCES;
>  	}
>  
> +	if (task->seccomp.mode != SECCOMP_MODE_FILTER)
> +		return -EINVAL;
> +
>  	spin_lock_irq(&task->sighand->siglock);
> -	if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
> -		ret = -EINVAL;
> -		goto out;
> -	}
> +	get_seccomp_filter(task);
> +	orig = task->seccomp.filter;
> +	spin_unlock_irq(&task->sighand->siglock);
>  
> -	filter = task->seccomp.filter;
> -	while (filter) {
> -		filter = filter->prev;
> +	count = 0;
> +	for (filter = orig; filter; filter = filter->prev)
>  		count++;
> -	}
>  
>  	if (filter_off >= count) {
>  		ret = -ENOENT;
>  		goto out;
>  	}
> -	count -= filter_off;
>  
> -	filter = task->seccomp.filter;
> -	while (filter && count > 1) {
> -		filter = filter->prev;
> +	count -= filter_off;
> +	for (filter = orig; count > 1; filter = filter->prev)
>  		count--;
> -	}
> -
> -	if (WARN_ON(count != 1 || !filter)) {
> -		/* The filter tree shouldn't shrink while we're using it. */
> -		ret = -ENOENT;
> -		goto out;
> -	}
>  
>  	fprog = filter->prog->orig_prog;
>  	if (!fprog) {
> @@ -912,17 +903,11 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
>  	if (!data)
>  		goto out;
>  
> -	refcount_inc(&filter->usage);
> -	spin_unlock_irq(&task->sighand->siglock);
> -
>  	if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
>  		ret = -EFAULT;
>  
> -	__put_seccomp_filter(filter);
> -	return ret;
> -
>  out:
> -	spin_unlock_irq(&task->sighand->siglock);
> +	__put_seccomp_filter(orig);
>  	return ret;
>  }
>  #endif
>