netdev - Re: [PATCH net] net: check the minimum value of gso size in virtio_net_hdr_to

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Message-ID: <688235273230f_39271d29430@willemb.c.googlers.com.notmuch>
Date: Thu, 24 Jul 2025 09:29:11 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Wang Liang <wangliang74@...wei.com>, 
 mst@...hat.com, 
 jasowang@...hat.com, 
 xuanzhuo@...ux.alibaba.com, 
 eperezma@...hat.com, 
 pabeni@...hat.com, 
 davem@...emloft.net, 
 willemb@...gle.com, 
 atenart@...nel.org
Cc: yuehaibing@...wei.com, 
 zhangchangzhong@...wei.com, 
 wangliang74@...wei.com, 
 netdev@...r.kernel.org, 
 virtualization@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, 
 steffen.klassert@...unet.com
Subject: Re: [PATCH net] net: check the minimum value of gso size in
 virtio_net_hdr_to_skb()

Wang Liang wrote:
> When sending a packet with virtio_net_hdr to tun device, if the gso_type
> in virtio_net_hdr is SKB_GSO_UDP and the gso_size is less than udphdr
> size, below crash may happen.
> 
>   ------------[ cut here ]------------
>   kernel BUG at net/core/skbuff.c:4572!

The BUG_ON hit:

	void *skb_pull_rcsum(struct sk_buff *skb, unsigned int len)
	{
		unsigned char *data = skb->data;

		BUG_ON(len > skb->len);

>   Oops: invalid opcode: 0000 [#1] SMP NOPTI
>   CPU: 0 UID: 0 PID: 62 Comm: mytest Not tainted 6.16.0-rc7 #203 PREEMPT(voluntary)
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
>   RIP: 0010:skb_pull_rcsum+0x8e/0xa0

>From udp_csum_pull_header

>   Code: 00 00 5b c3 cc cc cc cc 8b 93 88 00 00 00 f7 da e8 37 44 38 00 f7 d8 89 83 88 00 00 00 48 8b 83 c8 00 00 00 5b c3 cc cc cc cc <0f> 0b 0f 0b 66 66 2e 0f 1f 84 00 000
>   RSP: 0018:ffffc900001fba38 EFLAGS: 00000297
>   RAX: 0000000000000004 RBX: ffff8880040c1000 RCX: ffffc900001fb948
>   RDX: ffff888003e6d700 RSI: 0000000000000008 RDI: ffff88800411a062
>   RBP: ffff8880040c1000 R08: 0000000000000000 R09: 0000000000000001
>   R10: ffff888003606c00 R11: 0000000000000001 R12: 0000000000000000
>   R13: ffff888004060900 R14: ffff888004050000 R15: ffff888004060900
>   FS:  000000002406d3c0(0000) GS:ffff888084a19000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000020000040 CR3: 0000000004007000 CR4: 00000000000006f0
>   Call Trace:
>    <TASK>
>    udp_queue_rcv_one_skb+0x176/0x4b0 net/ipv4/udp.c:2445
>    udp_queue_rcv_skb+0x155/0x1f0 net/ipv4/udp.c:2475
>    udp_unicast_rcv_skb+0x71/0x90 net/ipv4/udp.c:2626
>    __udp4_lib_rcv+0x433/0xb00 net/ipv4/udp.c:2690
>    ip_protocol_deliver_rcu+0xa6/0x160 net/ipv4/ip_input.c:205
>    ip_local_deliver_finish+0x72/0x90 net/ipv4/ip_input.c:233
>    ip_sublist_rcv_finish+0x5f/0x70 net/ipv4/ip_input.c:579
>    ip_sublist_rcv+0x122/0x1b0 net/ipv4/ip_input.c:636
>    ip_list_rcv+0xf7/0x130 net/ipv4/ip_input.c:670
>    __netif_receive_skb_list_core+0x21d/0x240 net/core/dev.c:6067
>    netif_receive_skb_list_internal+0x186/0x2b0 net/core/dev.c:6210
>    napi_complete_done+0x78/0x180 net/core/dev.c:6580
>    tun_get_user+0xa63/0x1120 drivers/net/tun.c:1909
>    tun_chr_write_iter+0x65/0xb0 drivers/net/tun.c:1984
>    vfs_write+0x300/0x420 fs/read_write.c:593
>    ksys_write+0x60/0xd0 fs/read_write.c:686
>    do_syscall_64+0x50/0x1c0 arch/x86/entry/syscall_64.c:63
>    </TASK>
> 
> To trigger gso segment in udp_queue_rcv_skb(), we should also set option
> UDP_ENCAP_ESPINUDP to enable udp_sk(sk)->encap_rcv. When the encap_rcv
> hook return 1 in udp_queue_rcv_one_skb(), udp_csum_pull_header() will try
> to pull udphdr, but the skb size has been segmented to gso size, which
> leads to this crash.
> 
> Only udp has this probloem. Add check for the minimum value of gso size in
> virtio_net_hdr_to_skb().
> 
> Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
> Fixes: 3d010c8031e3 ("udp: do not accept non-tunnel GSO skbs landing in a tunnel")
> Signed-off-by: Wang Liang <wangliang74@...wei.com>
> ---
>  include/linux/virtio_net.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 02a9f4dc594d..0533101642bd 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -157,11 +157,13 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  		u16 gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);
>  		unsigned int nh_off = p_off;
>  		struct skb_shared_info *shinfo = skb_shinfo(skb);
> +		u16 min_gso_size = 0;
>  
>  		switch (gso_type & ~SKB_GSO_TCP_ECN) {
>  		case SKB_GSO_UDP:
>  			/* UFO may not include transport header in gso_size. */
>  			nh_off -= thlen;
> +			min_gso_size = sizeof(struct udphdr) + 1;
>  			break;
>  		case SKB_GSO_UDP_L4:
>  			if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
> @@ -172,6 +174,7 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  				return -EINVAL;
>  			if (gso_type != SKB_GSO_UDP_L4)
>  				return -EINVAL;
> +			min_gso_size = sizeof(struct udphdr) + 1;
>  			break;
>  		case SKB_GSO_TCPV4:
>  		case SKB_GSO_TCPV6:
> @@ -182,7 +185,7 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  		}
>  
>  		/* Kernel has a special handling for GSO_BY_FRAGS. */
> -		if (gso_size == GSO_BY_FRAGS)
> +		if ((gso_size == GSO_BY_FRAGS) || (gso_size < min_gso_size))

gso_size is the size of the segment payload, excluding the transport
header.

This is probably not the right approach.

Not sure how a GSO skb can be built that is shorter than even the
transport header. Maybe an skb_dump of the GSO skb can be elucidating.
>  			return -EINVAL;
>  
>  		/* Too small packets are not really GSO ones. */
> -- 
> 2.34.1
>