Subject: Re: [PATCH net-next] net: datagram: Bypass usercopy checks for kernel iterators

From: Jakub Kicinski <[email-masked]>
Date: Fri, 27 Feb 2026 19:19:56 -0800

On Wed, 25 Feb 2026 11:25:32 -0500 Chuck Lever wrote:
> From: Chuck Lever <[email-masked]>
> 
> Profiling NFSD under an iozone workload showed that hardened
> usercopy checks consume roughly 1.3% of CPU in the TCP receive path.
> These checks validate memory regions during copies, but provide no
> security benefit when both source (skb data) and destination (kernel
> pages in BVEC/KVEC iterators) reside in kernel address space.
> 
> Modify simple_copy_to_iter() and crc32c_and_copy_to_iter() to call
> _copy_to_iter() directly when the destination is a kernel-only
> iterator, bypassing the usercopy hardening validation. User-backed
> iterators (ITER_UBUF, ITER_IOVEC) continue to use copy_to_iter()
> with full validation.
> 
> This benefits kernel consumers of TCP receive such as the NFS client
> and server and NVMe-TCP, which use ITER_BVEC for their receive
> buffers.

If it makes such a difference why not make copy_to_iter()
check the iter type? Why force callers to check it?

> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index c285c6465923..e83cf0125008 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -490,7 +490,10 @@ static size_t crc32c_and_copy_to_iter(const void *addr, size_t bytes,
>  	u32 *crcp = _crcp;
>  	size_t copied;
>  
> -	copied = copy_to_iter(addr, bytes, i);
> +	if (user_backed_iter(i))
> +		copied = copy_to_iter(addr, bytes, i);
> +	else
> +		copied = _copy_to_iter(addr, bytes, i);
>  	*crcp = crc32c(*crcp, addr, copied);
>  	return copied;
>  }
> @@ -515,10 +518,17 @@ int skb_copy_and_crc32c_datagram_iter(const struct sk_buff *skb, int offset,
>  EXPORT_SYMBOL(skb_copy_and_crc32c_datagram_iter);
>  #endif /* CONFIG_NET_CRC32C */
>  
> +/*
> + * Bypass usercopy hardening for kernel-only iterators: no data
> + * crosses the user/kernel boundary, so the slab whitelist check
> + * on the source buffer is unnecessary overhead.
> + */
>  static size_t simple_copy_to_iter(const void *addr, size_t bytes,
>  		void *data __always_unused, struct iov_iter *i)
>  {
> -	return copy_to_iter(addr, bytes, i);
> +	if (user_backed_iter(i))
> +		return copy_to_iter(addr, bytes, i);
> +	return _copy_to_iter(addr, bytes, i);
>  }
>  
>  /**