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: