Re: [lvc-project] [PATCH net] xsk: Fix overflow in descriptor validation@@

On Tue, 7 Oct 2025 at 14:11, Alexander Lobakin
[off-list ref] wrote:

From: Ilia Gavrilov <redacted>
Date: Tue, 7 Oct 2025 11:19:19 +0000

quoted

On 10/6/25 18:19, Alexander Lobakin wrote:

quoted

From: Ilia Gavrilov <redacted>
Date: Mon, 6 Oct 2025 08:53:17 +0000

quoted

The desc->len value can be set up to U32_MAX. If umem tx_metadata_len

In theory. Never in practice.

Hi Alexander,
Thank you for the review.

It seems to me that this problem should be considered not from the point of view of practical use,
but from the point of view of security. An attacker can set any length of the packet in the descriptor
from the user space and descriptor validation will pass.

quoted

quoted

option is also set, then the value of the expression
'desc->len + pool->tx_metadata_len' can overflow and validation
of the incorrect descriptor will be successfully passed.
This can lead to a subsequent chain of arithmetic overflows
in the xsk_build_skb() function and incorrect sk_buff allocation.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with SVACE.

I think the general rule for sending fixes is that a fix must fix a real
bug which can be reproduced in real life scenarios.

I agree with that, so I make a test program (PoC). Something like that:

      struct xdp_umem_reg umem_reg;
      umem_reg.addr = (__u64)(void *)umem;
      ...
      umem_reg.chunk_size = 4096;
      umem_reg.tx_metadata_len = 16;
      umem_reg.flags = XDP_UMEM_TX_METADATA_LEN;
      setsockopt(sfd, SOL_XDP, XDP_UMEM_REG, &umem_reg, sizeof(umem_reg));
      ...

      xsk_ring_prod__reserve(tq, batch_size, &idx);

      for (i = 0; i < nr_packets; ++i) {
              struct xdp_desc *tx_desc = xsk_ring_prod__tx_desc(tq, idx + i);
              tx_desc->addr = packets[i].addr;
              tx_desc->addr += umem->tx_metadata_len;
              tx_desc->options = XDP_TX_METADATA;
              tx_desc->len = UINT32_MAX;
      }

      xsk_ring_prod__submit(tq, nr_packets);
      ...
      sendto(sfd, NULL, 0, MSG_DONTWAIT, NULL, 0);

Since the check of an invalid descriptor has passed, kernel try to allocate
a skb with size of 'hr + len + tr' in the sock_alloc_send_pskb() function
and this is where the next overflow occurs.
skb allocates with a size of 63. Next the skb_put() is called, which adds U32_MAX to skb->tail and skb->end.
Next the skb_store_bits() tries to copy -1 bytes, but fails.

 __xsk_generic_xmit
      xsk_build_skb
              len = desc->len; // from descriptor
              sock_alloc_send_skb(..., hr + len + tr, ...) // the next overflow
                      sock_alloc_send_pskb
                              alloc_skb_with_frags
              skb_put(skb, len)  // len casts to int
              skb_store_bits(skb, 0, buffer, len)

Oh, so you actually have a repro for this. This is good. I suggest you
resubmitting the patch and include this repro in the commit message, so
that it will be clear that it's actually possible to trigger the problem
in the kernel using a malicious/broken userspace application.

(also pls remove those double `@@` from the subject next time)

I'd also like to hear from Maciej and/or others what they think about
this problem (that the userspace can set packet len to U32_MAX). Should
we just go with this proposed u64 propagation or maybe we need to limit
the maximum length which could be sent from the userspace?

I prefer that we do not set a limit on it and go with the proposed
solution since I do not know what a future proof size limit would be.
Somebody could come up with a new virtual device that can send really
large packets, who knows.

In any case, you raised a good topic.

quoted

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Static Analysis Tools have no idea that nobody sends 4 Gb sized network
packets.

That's right. Static analyzer is only a tool, but in this case, the overflow
highlighted by the static analyzer can be used for malicious purposes.

+1

Also I really do hope Infotecs stayed independent from the govs and
doesn't take part in any dual-purpose/gov-related projects.

Thanks,
Olek