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

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

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

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)

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.
 

quoted

Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
Cc: stable@vger.kernel.org
Signed-off-by: Ilia Gavrilov <redacted>
---
 net/xdp/xsk_queue.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Thanks,
Olek

Thanks, 
Ilia