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

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Message-ID: <fa7b9dc7-037f-42f7-87e5-19b3d8a3d2c3@intel.com>
Date: Tue, 7 Oct 2025 14:09:52 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Ilia Gavrilov <Ilia.Gavrilov@...otecs.ru>
CC: Song Yoong Siang <yoong.siang.song@...el.com>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>, Jesper Dangaard Brouer <hawk@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, John Fastabend <john.fastabend@...il.com>, "Alexei
 Starovoitov" <ast@...nel.org>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>,
	"Stanislav Fomichev" <sdf@...ichev.me>, Simon Horman <horms@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>, "bpf@...r.kernel.org"
	<bpf@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>, "David S. Miller"
	<davem@...emloft.net>, Magnus Karlsson <magnus.karlsson@...el.com>,
	"lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>
Subject: Re: [lvc-project] [PATCH net] xsk: Fix overflow in descriptor
 validation@@

From: Ilia Gavrilov <Ilia.Gavrilov@...otecs.ru>
Date: Tue, 7 Oct 2025 11:19:19 +0000

> On 10/6/25 18:19, Alexander Lobakin wrote:
>> From: Ilia Gavrilov <Ilia.Gavrilov@...otecs.ru>
>> Date: Mon, 6 Oct 2025 08:53:17 +0000
>>
>>> 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.
> 
> 
>>> 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?

In any case, you raised a good topic.

> 
>> 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