Re: [PATCH net] net: check the minimum value of gso size in virtio_net_hdr_to_skb()
From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Date: 2025-07-25 08:58:45
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lkml, virtualization
On Thu, 24 Jul 2025 16:30:05 +0800, Wang Liang [off-list ref] 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! 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 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.
Is it correct to access the checksum of a segmented skb?
Only udp has this probloem. Add check for the minimum value of gso size in virtio_net_hdr_to_skb().
Why tcp has not this problem?
quoted hunk ↗ jump to hunk
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 <redacted> --- 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;
why +1? Thanks.
quoted hunk ↗ jump to hunk
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)) return -EINVAL; /* Too small packets are not really GSO ones. */ --2.34.1