Re: [PATCH net v2] xsk: reject undersized tx_metadata at runtime validation

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Stanislav Fomichev <sdf.kernel@gmail.com>
Date: Wed, 22 Jul 2026 10:55:17 -0700

> On 07/22, Cen Zhang (Microsoft) wrote:
>> There is no metadata length check before accessing struct xsk_tx_metadata
>> fields in both xdp_umem_reg() (registration) and
>> xsk_buff_valid_tx_metadata() (runtime).  Thus, an invalid small
>> tx_metadata_len (e.g. 8 bytes) passes registration and later causes an
>> out-of-bounds read when xsk_skb_metadata() accesses fields beyond the
>> declared metadata region.

[...]

> @@ -773,7 +773,7 @@ struct xdp_desc_ctx xp_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr)
>  	addr = __xp_raw_get_addr(pool, addr);
>  
>  	ret.dma = __xp_raw_get_dma(pool, addr);
> -	ret.meta = __xsk_buff_get_metadata(pool, __xp_raw_get_data(pool, addr));
> +	xsk_buff_read_metadata(pool, addr, &ret.meta);

That effectively disables some micro-optimizations that were the reason
I created this helper -- __xp_raw_get_data() is inline, while
xsk_buff_read_metadata() uses the external version, probably some other
stuff (note the underscored __xsk_buff_get_metadata()) =\ This function
expanded to have no external calls on clang -O2, with this patch I'm not
sure it would.

>  
>  	return ret;
>  }
Thanks,
Olek