Re: [PATCH net v4] xsk: fix NULL pointer dereference in __xsk_rcv()

On Fri, Aug 14, 2026 at 5:53 AM Cen Zhang (Microsoft) <[email protected]> wrote:
>
> In the __xsk_rcv() multi-buffer path, xsk_buff_alloc() is called in a
> loop without checking its return value. xsk_buff_can_alloc() only
> counts fill queue entries without validating their addresses, so it
> can succeed while xsk_buff_alloc() rejects all remaining entries and
> returns NULL.
>
>   Oops: general protection fault, probably for non-canonical address
>    0xdffffc0000000000
>   KASAN: null-ptr-deref in range
>    [0x0000000000000000-0x0000000000000007]
>   RIP: 0010:__xsk_rcv+0x426/0xc20 (net/xdp/xsk.c:350)
>   Call Trace:
>    xsk_generic_rcv+0x26d/0x5f0
>    xdp_do_generic_redirect+0x3c5/0xcf0
>    do_xdp_generic+0x92f/0xe70
>    __netif_receive_skb_core.constprop.0+0xf7e/0x2b30
>
> Fix this with a two-stage transaction. First allocate and stage all
> buffers required for the packet, recycling all staged buffers with
> xsk_buff_free() if any allocation fails. Only after this stage
> succeeds, copy the data, reserve the RX descriptors, and release the
> buffers in an error-free loop.
>
> Fixes: 804627751b42 ("xsk: add support for AF_XDP multi-buffer on Rx path")
> Reported-by: [email protected]
> Signed-off-by: Cen Zhang (Microsoft) <[email protected]>

Thanks for working on different approaches!

Reviewed-by: Jason Xing <[email protected]>

> ---

I think it'd be better to put the previous discussion links like [1]
here and then explain a bit about what you've changed through
iterations next time. It would be beneficial for network folks to
effortlessly trace your patches.

[1]: https://lore.kernel.org/all/[email protected]/

Thanks,
Jason

>  net/xdp/xsk.c | 30 +++++++++++++++++++++++++++---
>  1 file changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 7855ee09c4b6..33475b180ea6 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -298,9 +298,11 @@ static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
>         u32 frame_size = __xsk_pool_get_rx_frame_size(xs->pool);
>         void *copy_from = xsk_copy_xdp_start(xdp), *copy_to;
>         u32 from_len, meta_len, rem, num_desc;
> -       struct xdp_buff_xsk *xskb;
> +       struct xdp_buff_xsk *xskb, *tmp;
>         struct xdp_buff *xsk_xdp;
> +       LIST_HEAD(xsk_buffs);
>         skb_frag_t *frag;
> +       u32 i;
>
>         from_len = xdp->data_end - copy_from;
>         meta_len = xdp->data - copy_from;
> @@ -343,23 +345,45 @@ static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
>                 frag =  &sinfo->frags[0];
>         }
>
> +       for (i = 0; i < num_desc; i++) {
> +               xsk_xdp = xsk_buff_alloc(xs->pool);
> +               if (!xsk_xdp)
> +                       goto err_alloc;
> +
> +               xskb = container_of(xsk_xdp, struct xdp_buff_xsk, xdp);
> +               if (unlikely(!list_empty(&xskb->list_node)))
> +                       goto err_alloc;
> +
> +               list_add_tail(&xskb->list_node, &xsk_buffs);
> +       }
> +
>         do {
>                 u32 to_len = frame_size + meta_len;
>                 u32 copied;
>
> -               xsk_xdp = xsk_buff_alloc(xs->pool);
> +               xskb = list_first_entry(&xsk_buffs, struct xdp_buff_xsk,
> +                                       list_node);
> +               list_del_init(&xskb->list_node);
> +               xsk_xdp = &xskb->xdp;
>                 copy_to = xsk_xdp->data - meta_len;
>
>                 copied = xsk_copy_xdp(copy_to, &copy_from, to_len, &from_len, &frag, rem);
>                 rem -= copied;
>
> -               xskb = container_of(xsk_xdp, struct xdp_buff_xsk, xdp);
>                 __xsk_rcv_zc_safe(xs, xskb, copied - meta_len,
>                                   rem ? XDP_PKT_CONTD : 0);
>                 meta_len = 0;
>         } while (rem);
>
>         return 0;
> +
> +err_alloc:
> +       list_for_each_entry_safe(xskb, tmp, &xsk_buffs, list_node) {
> +               list_del_init(&xskb->list_node);
> +               xsk_buff_free(&xskb->xdp);
> +       }
> +       xs->rx_dropped++;
> +       return -ENOMEM;
>  }
>
>  static bool xsk_tx_writeable(struct xdp_sock *xs)
> --
> 2.52.0
>