Re: [PATCH 3/4] seccomp: Introduce SECCOMP_ATTACH_FILTER operation

[Kees Cook]

On Mon, Oct 09, 2023 at 12:40:45PM +0000, Hengqi Chen wrote:
> The SECCOMP_ATTACH_FILTER operation is used to attach
> a loaded filter to the current process. The loaded filter
> is represented by a fd which is either returned by the
> SECCOMP_LOAD_FILTER operation or obtained from bpffs using
> bpf syscall.
> 
> Signed-off-by: Hengqi Chen <hengqi.chen@xxxxxxxxx>
> ---
>  include/uapi/linux/seccomp.h |  1 +
>  kernel/seccomp.c             | 68 +++++++++++++++++++++++++++++++++---
>  2 files changed, 64 insertions(+), 5 deletions(-)
> 
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index ee2c83697810..fbe30262fdfc 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -17,6 +17,7 @@
>  #define SECCOMP_GET_ACTION_AVAIL	2
>  #define SECCOMP_GET_NOTIF_SIZES		3
>  #define SECCOMP_LOAD_FILTER		4
> +#define SECCOMP_ATTACH_FILTER		5
>  
>  /* Valid flags for SECCOMP_SET_MODE_FILTER */
>  #define SECCOMP_FILTER_FLAG_TSYNC		(1UL << 0)
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 3ae43db3b642..9f9d8a7a1d6e 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -523,7 +523,10 @@ static inline pid_t seccomp_can_sync_threads(void)
>  static inline void seccomp_filter_free(struct seccomp_filter *filter)
>  {
>  	if (filter) {
> -		bpf_prog_destroy(filter->prog);
> +		if (filter->prog->type == BPF_PROG_TYPE_SECCOMP)
> +			bpf_prog_put(filter->prog);
> +		else
> +			bpf_prog_destroy(filter->prog);
>  		kfree(filter);
>  	}
>  }
> @@ -894,7 +897,7 @@ static void seccomp_cache_prepare(struct seccomp_filter *sfilter)
>  #endif /* SECCOMP_ARCH_NATIVE */
>  
>  /**
> - * seccomp_attach_filter: validate and attach filter
> + * seccomp_do_attach_filter: validate and attach filter
>   * @flags:  flags to change filter behavior
>   * @filter: seccomp filter to add to the current process
>   *
> @@ -905,8 +908,8 @@ static void seccomp_cache_prepare(struct seccomp_filter *sfilter)
>   *     seccomp mode or did not have an ancestral seccomp filter
>   *   - in NEW_LISTENER mode: the fd of the new listener
>   */
> -static long seccomp_attach_filter(unsigned int flags,
> -				  struct seccomp_filter *filter)
> +static long seccomp_do_attach_filter(unsigned int flags,
> +				     struct seccomp_filter *filter)
>  {
>  	unsigned long total_insns;
>  	struct seccomp_filter *walker;
> @@ -2001,7 +2004,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  		goto out;
>  	}
>  
> -	ret = seccomp_attach_filter(flags, prepared);
> +	ret = seccomp_do_attach_filter(flags, prepared);
>  	if (ret)
>  		goto out;
>  	/* Do not free the successfully attached filter. */
> @@ -2058,6 +2061,51 @@ static long seccomp_load_filter(const char __user *filter)
>  		bpf_prog_put(prog);
>  	return ret;
>  }
> +
> +static long seccomp_attach_filter(const char __user *ufd)
> +{
> +	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
> +	struct seccomp_filter *sfilter;
> +	struct bpf_prog *prog;
> +	int flags = 0;
> +	int fd, ret;
> +
> +	if (copy_from_user(&fd, ufd, sizeof(fd)))
> +		return -EFAULT;
> +
> +	prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_SECCOMP);
> +	if (IS_ERR(prog))
> +		return PTR_ERR(prog);
> +
> +	sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL | __GFP_NOWARN);
> +	if (!sfilter) {
> +		bpf_prog_put(prog);
> +		return -ENOMEM;
> +	}
> +
> +	sfilter->prog = prog;
> +	refcount_set(&sfilter->refs, 1);
> +	refcount_set(&sfilter->users, 1);
> +	mutex_init(&sfilter->notify_lock);
> +	init_waitqueue_head(&sfilter->wqh);
> +
> +	spin_lock_irq(&current->sighand->siglock);
> +
> +	ret = -EINVAL;
> +	if (!seccomp_may_assign_mode(seccomp_mode))
> +		goto out;
> +
> +	ret = seccomp_do_attach_filter(flags, sfilter);
> +	if (ret)
> +		goto out;
> +
> +	sfilter = NULL;
> +	seccomp_assign_mode(current, seccomp_mode, flags);
> +out:
> +	spin_unlock_irq(&current->sighand->siglock);
> +	seccomp_filter_free(sfilter);
> +	return ret;
> +}

This is duplicating part of seccomp_set_mode_filter() but without
handling flags at all. This isn't really workable, since we need things
like TSYNC, etc. I think it would be better to adjust
SECCOMP_SET_MODE_FILTER to take a new flag that indicates that the user
arg is an fd, not a filter. Then the middle of seccomp_set_mode_filter()
can choosen between seccomp_prepare_user_filter() and a wrapped call to
bpf_prog_get_type() on the fd, etc.

-- 
Kees Cook