Re: [PATCH bpf] xsk: harden userspace-supplied &xdp_desc validation

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Oct 08, 2025 at 06:56:59PM +0200, Alexander Lobakin wrote:
> Turned out certain clearly invalid values passed in &xdp_desc from
> userspace can pass xp_{,un}aligned_validate_desc() and then lead
> to UBs or just invalid frames to be queued for xmit.
> 
> desc->len close to ``U32_MAX`` with a non-zero pool->tx_metadata_len
> can cause positive integer overflow and wraparound, the same way low
> enough desc->addr with a non-zero pool->tx_metadata_len can cause
> negative integer overflow. Both scenarios can then pass the
> validation successfully.

Hmm, when underflow happens the addr would be enormous, passing
existing validation would really be rare. However let us fix it while at
it.

> This doesn't happen with valid XSk applications, but can be used
> to perform attacks.
> 
> Always promote desc->len to ``u64`` first to exclude positive
> overflows of it. Use explicit check_{add,sub}_overflow() when
> validating desc->addr (which is ``u64`` already).
> 
> bloat-o-meter reports a little growth of the code size:
> 
> add/remove: 0/0 grow/shrink: 2/1 up/down: 60/-16 (44)
> Function                                     old     new   delta
> xskq_cons_peek_desc                          299     330     +31
> xsk_tx_peek_release_desc_batch               973    1002     +29
> xsk_generic_xmit                            3148    3132     -16
> 
> but hopefully this doesn't hurt the performance much.

Let us be fully transparent and link the previous discussion here?

I was commenting that breaking up single statement to multiple branches
might affect subtly performance as this code is executed per each
descriptor. Jason tested copy+aligned mode, let us see if zc+unaligned
mode is affected.

<rant>
I am also thinking about test side, but xsk tx metadata came with a
separate test (xdp_hw_metadata), which was rather about testing positive
cases. That is probably a separate discussion, but metadata negative
tests should appear somewhere, I suppose xskxceiver would be a good fit,
but then, should we merge the existing logic from xdp_hw_metadata?
</rant>

> 
> Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
> Cc: stable@vger.kernel.org # 6.8+
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> ---
>  net/xdp/xsk_queue.h | 45 +++++++++++++++++++++++++++++++++++----------
>  1 file changed, 35 insertions(+), 10 deletions(-)
> 
> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
> index f16f390370dc..1eb8d9f8b104 100644
> --- a/net/xdp/xsk_queue.h
> +++ b/net/xdp/xsk_queue.h
> @@ -143,14 +143,24 @@ static inline bool xp_unused_options_set(u32 options)
>  static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
>  					    struct xdp_desc *desc)
>  {
> -	u64 addr = desc->addr - pool->tx_metadata_len;
> -	u64 len = desc->len + pool->tx_metadata_len;
> -	u64 offset = addr & (pool->chunk_size - 1);
> +	u64 len = desc->len;
> +	u64 addr, offset;
>  
> -	if (!desc->len)
> +	if (!len)

This is yet another thing being fixed here as for non-zero tx_metadata_len
we were allowing 0 length descriptors... :< overall feels like we relied
too much on contract with userspace WRT descriptor layout.

If zc perf is fine, then:
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

>  		return false;
>  
> -	if (offset + len > pool->chunk_size)
> +	/* Can overflow if desc->addr < pool->tx_metadata_len */
> +	if (check_sub_overflow(desc->addr, pool->tx_metadata_len, &addr))
> +		return false;
> +
> +	offset = addr & (pool->chunk_size - 1);
> +
> +	/*
> +	 * Can't overflow: @offset is guaranteed to be < ``U32_MAX``
> +	 * (pool->chunk_size is ``u32``), @len is guaranteed
> +	 * to be <= ``U32_MAX``.
> +	 */
> +	if (offset + len + pool->tx_metadata_len > pool->chunk_size)
>  		return false;
>  
>  	if (addr >= pool->addrs_cnt)
> @@ -158,27 +168,42 @@ static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
>  
>  	if (xp_unused_options_set(desc->options))
>  		return false;
> +
>  	return true;
>  }
>  
>  static inline bool xp_unaligned_validate_desc(struct xsk_buff_pool *pool,
>  					      struct xdp_desc *desc)
>  {
> -	u64 addr = xp_unaligned_add_offset_to_addr(desc->addr) - pool->tx_metadata_len;
> -	u64 len = desc->len + pool->tx_metadata_len;
> +	u64 len = desc->len;
> +	u64 addr, end;
>  
> -	if (!desc->len)
> +	if (!len)
>  		return false;
>  
> +	/* Can't overflow: @len is guaranteed to be <= ``U32_MAX`` */
> +	len += pool->tx_metadata_len;
>  	if (len > pool->chunk_size)
>  		return false;
>  
> -	if (addr >= pool->addrs_cnt || addr + len > pool->addrs_cnt ||
> -	    xp_desc_crosses_non_contig_pg(pool, addr, len))
> +	/* Can overflow if desc->addr is close to 0 */
> +	if (check_sub_overflow(xp_unaligned_add_offset_to_addr(desc->addr),
> +			       pool->tx_metadata_len, &addr))
> +		return false;
> +
> +	if (addr >= pool->addrs_cnt)
> +		return false;
> +
> +	/* Can overflow if pool->addrs_cnt is high enough */
> +	if (check_add_overflow(addr, len, &end) || end > pool->addrs_cnt)
> +		return false;
> +
> +	if (xp_desc_crosses_non_contig_pg(pool, addr, len))
>  		return false;
>  
>  	if (xp_unused_options_set(desc->options))
>  		return false;
> +
>  	return true;
>  }
>  
> -- 
> 2.51.0
>