From: Magnus Karlsson magnus.karlsson@gmail.com Date: Tue, 7 Oct 2025 15:52:52 +0200
On Tue, 7 Oct 2025 at 15:34, Ilia Gavrilov Ilia.Gavrilov@infotecs.ru wrote:
On 10/7/25 15:44, Magnus Karlsson wrote:
On Tue, 7 Oct 2025 at 14:11, Alexander Lobakin aleksander.lobakin@intel.com wrote:
From: Ilia Gavrilov Ilia.Gavrilov@infotecs.ru Date: Tue, 7 Oct 2025 11:19:19 +0000
On 10/6/25 18:19, Alexander Lobakin wrote:
From: Ilia Gavrilov Ilia.Gavrilov@infotecs.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.
I'll add the repro from this e-mail in the next patch version, the full source is too long.
(also pls remove those double `@@` from the subject next time)
ok
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.
The limit is already checked in the xp_aligned_validate_desc() function:
What I meant was, let us not introduce a new limit. I like your proposed solution.
[to Ilia]
The netdev and XSk maintainers prefer to fix this themselves after a quick discussion. Let us take over this topic. Thanks for finding this.
Olek