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

[Jason Xing <kerneljasonxing@gmail.com>]

On Thu, Oct 9, 2025 at 12:59 AM Alexander Lobakin
<aleksander.lobakin@intel.com> 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.
> 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.

I don't see an evident point that might affect the performance. Since
you said that, I tested by running './xdpsock -i eth1 -t -S -s 64' and
didn't spot any degradation.

>
> Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
> Cc: stable@vger.kernel.org # 6.8+
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>

Thanks for the fix!

Reviewed-by: Jason Xing <kerneljasonxing@gmail.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)
>                 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;
> +

nit?

>         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
>
>