In the Linux kernel, the following vulnerability has been resolved: calipso: Fix null-ptr-deref in calipso_req_{set,del}attr(). syzkaller reported a null-ptr-deref in sock_omalloc() while allocating a CALIPSO option. [0] The NULL is of struct sock, which was fetched by sk_to_full_sk() in calipso_req_setattr(). Since commit a1a5344ddbe8 ("tcp: avoid two atomic ops for syncookies"), reqsk->rsk_listener could be NULL when SYN Cookie is returned to its client, as hinted by the leading SYN Cookie log. Here are 3 options to fix the bug: 1) Return 0 in calipso_req_setattr() 2) Return an error in calipso_req_setattr() 3) Alaways set rsk_listener 1) is no go as it bypasses LSM, but 2) effectively disables SYN Cookie for CALIPSO. 3) is also no go as there have been many efforts to reduce atomic ops and make TCP robust against DDoS. See also commit 3b24d854cb35 ("tcp/dccp: do not touch listener sk_refcnt under synflood"). As of the blamed commit, SYN Cookie already did not need refcounting, and no one has stumbled on the bug for 9 years, so no CALIPSO user will care about SYN Cookie. Let's return an error in calipso_req_setattr() and calipso_req_delattr() in the SYN Cookie case. This can be reproduced by [1] on Fedora and now connect() of nc times out. [0]: TCP: request_sock_TCPv6: Possible SYN flooding on port [::]:20002. Sending cookies. Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] PREEMPT SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] CPU: 3 UID: 0 PID: 12262 Comm: syz.1.2611 Not tainted 6.14.0 #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 RIP: 0010:read_pnet include/net/net_namespace.h:406 [inline] RIP: 0010:sock_net include/net/sock.h:655 [inline] RIP: 0010:sock_kmalloc+0x35/0x170 net/core/sock.c:2806 Code: 89 d5 41 54 55 89 f5 53 48 89 fb e8 25 e3 c6 fd e8 f0 91 e3 00 48 8d 7b 30 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 26 01 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b RSP: 0018:ffff88811af89038 EFLAGS: 00010216 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffff888105266400 RDX: 0000000000000006 RSI: ffff88800c890000 RDI: 0000000000000030 RBP: 0000000000000050 R08: 0000000000000000 R09: ffff88810526640e R10: ffffed1020a4cc81 R11: ffff88810526640f R12: 0000000000000000 R13: 0000000000000820 R14: ffff888105266400 R15: 0000000000000050 FS: 00007f0653a07640(0000) GS:ffff88811af80000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f863ba096f4 CR3: 00000000163c0005 CR4: 0000000000770ef0 PKRU: 80000000 Call Trace: <IRQ> ipv6_renew_options+0x279/0x950 net/ipv6/exthdrs.c:1288 calipso_req_setattr+0x181/0x340 net/ipv6/calipso.c:1204 calipso_req_setattr+0x56/0x80 net/netlabel/netlabel_calipso.c:597 netlbl_req_setattr+0x18a/0x440 net/netlabel/netlabel_kapi.c:1249 selinux_netlbl_inet_conn_request+0x1fb/0x320 security/selinux/netlabel.c:342 selinux_inet_conn_request+0x1eb/0x2c0 security/selinux/hooks.c:5551 security_inet_conn_request+0x50/0xa0 security/security.c:4945 tcp_v6_route_req+0x22c/0x550 net/ipv6/tcp_ipv6.c:825 tcp_conn_request+0xec8/0x2b70 net/ipv4/tcp_input.c:7275 tcp_v6_conn_request+0x1e3/0x440 net/ipv6/tcp_ipv6.c:1328 tcp_rcv_state_process+0xafa/0x52b0 net/ipv4/tcp_input.c:6781 tcp_v6_do_rcv+0x8a6/0x1a40 net/ipv6/tcp_ipv6.c:1667 tcp_v6_rcv+0x505e/0x5b50 net/ipv6/tcp_ipv6.c:1904 ip6_protocol_deliver_rcu+0x17c/0x1da0 net/ipv6/ip6_input.c:436 ip6_input_finish+0x103/0x180 net/ipv6/ip6_input.c:480 NF_HOOK include/linux/netfilter.h:314 [inline] NF_HOOK include/linux/netfilter.h:308 [inline] ip6_input+0x13c/0x6b0 net/ipv6/ip6_input.c:491 dst_input include/net/dst.h:469 [inline] ip6_rcv_finish net/ipv6/ip6_input.c:79 [inline] ip6_rcv_finish+0xb6/0x490 net/ipv6/ip6_input.c:69 NF_HOOK include/linux/netfilter.h:314 [inline] NF_HOOK include/linux/netf ---truncated---