net: do not allow gso_size to be set to GSO_BY_FRAGS

One missing check in virtio_net_hdr_to_skb() allowed
syzbot to crash kernels again [1]

Do not allow gso_size to be set to GSO_BY_FRAGS (0xffff),
because this magic value is used by the kernel.

[1]
general protection fault, probably for non-canonical address 0xdffffc000000000e: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000070-0x0000000000000077]
CPU: 0 PID: 5039 Comm: syz-executor401 Not tainted 6.5.0-rc5-next-20230809-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/26/2023
RIP: 0010:skb_segment+0x1a52/0x3ef0 net/core/skbuff.c:4500
Code: 00 00 00 e9 ab eb ff ff e8 6b 96 5d f9 48 8b 84 24 00 01 00 00 48 8d 78 70 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e ea 21 00 00 48 8b 84 24 00 01
RSP: 0018:ffffc90003d3f1c8 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 000000000001fffe RCX: 0000000000000000
RDX: 000000000000000e RSI: ffffffff882a3115 RDI: 0000000000000070
RBP: ffffc90003d3f378 R08: 0000000000000005 R09: 000000000000ffff
R10: 000000000000ffff R11: 5ee4a93e456187d6 R12: 000000000001ffc6
R13: dffffc0000000000 R14: 0000000000000008 R15: 000000000000ffff
FS: 00005555563f2380(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020020000 CR3: 000000001626d000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
udp6_ufo_fragment+0x9d2/0xd50 net/ipv6/udp_offload.c:109
ipv6_gso_segment+0x5c4/0x17b0 net/ipv6/ip6_offload.c:120
skb_mac_gso_segment+0x292/0x610 net/core/gso.c:53
__skb_gso_segment+0x339/0x710 net/core/gso.c:124
skb_gso_segment include/net/gso.h:83 [inline]
validate_xmit_skb+0x3a5/0xf10 net/core/dev.c:3625
__dev_queue_xmit+0x8f0/0x3d60 net/core/dev.c:4329
dev_queue_xmit include/linux/netdevice.h:3082 [inline]
packet_xmit+0x257/0x380 net/packet/af_packet.c:276
packet_snd net/packet/af_packet.c:3087 [inline]
packet_sendmsg+0x24c7/0x5570 net/packet/af_packet.c:3119
sock_sendmsg_nosec net/socket.c:727 [inline]
sock_sendmsg+0xd9/0x180 net/socket.c:750
____sys_sendmsg+0x6ac/0x940 net/socket.c:2496
___sys_sendmsg+0x135/0x1d0 net/socket.c:2550
__sys_sendmsg+0x117/0x1e0 net/socket.c:2579
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7ff27cdb34d9

Fixes: 3953c46c ("sk_buff: allow segmenting based on frag sizes")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230816142158.1779798-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/virtio_net.h

@@ -155,6 +155,10 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 		if (gso_type & SKB_GSO_UDP)
 			nh_off -= thlen;
 
+		/* Kernel has a special handling for GSO_BY_FRAGS. */
+		if (gso_size == GSO_BY_FRAGS)
+			return -EINVAL;
+
 		/* Too small packets are not really GSO ones. */
 		if (skb->len - nh_off > gso_size) {
 			shinfo->gso_size = gso_size;