Re: [PATCH net 1/4] sockmap, vsock: For connectible sockets allow only connected

[Michal Luczaj <mhal@rbox.co>]

> ...
> Another design detail is that listening vsocks are not supposed to have any
> transport assigned at all. Which implies they are not supported by the
> sockmap. But this is complicated by the fact that a socket, before
> switching to TCP_LISTEN, may have had some transport assigned during a
> failed connect() attempt. Hence, we may end up with a listening vsock in a
> sockmap, which blows up quickly:
> 
> KASAN: null-ptr-deref in range [0x0000000000000120-0x0000000000000127]
> CPU: 7 UID: 0 PID: 56 Comm: kworker/7:0 Not tainted 6.14.0-rc1+
> Workqueue: vsock-loopback vsock_loopback_work
> RIP: 0010:vsock_read_skb+0x4b/0x90
> Call Trace:
>  sk_psock_verdict_data_ready+0xa4/0x2e0
>  virtio_transport_recv_pkt+0x1ca8/0x2acc
>  vsock_loopback_work+0x27d/0x3f0
>  process_one_work+0x846/0x1420
>  worker_thread+0x5b3/0xf80
>  kthread+0x35a/0x700
>  ret_from_fork+0x2d/0x70
>  ret_from_fork_asm+0x1a/0x30

Perhaps I should have expanded more on the null-ptr-deref itself.

The idea is: force a vsock into assigning a transport and add it to the
sockmap (with a verdict program), but keep it unconnected. Then, drop
the transport and set the vsock to TCP_LISTEN. The moment a new
connection is established:

virtio_transport_recv_pkt()
  virtio_transport_recv_listen()
    sk->sk_data_ready(sk)            i.e. sk_psock_verdict_data_ready()
      ops->read_skb()                i.e. vsock_read_skb()
        vsk->transport->read_skb()   vsk->transport is NULL, boom

Here's a stand-alone repro:

/*
 * # modprobe -a vsock_loopback vhost_vsock
 * # gcc test.c && ./a.out
 */
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <linux/bpf.h>
#include <linux/vm_sockets.h>

static void die(const char *msg)
{
	perror(msg);
	exit(-1);
}

static int sockmap_create(void)
{
	union bpf_attr attr = {
		.map_type = BPF_MAP_TYPE_SOCKMAP,
		.key_size = sizeof(int),
		.value_size = sizeof(int),
		.max_entries = 1
	};
	int map;

	map = syscall(SYS_bpf, BPF_MAP_CREATE, &attr, sizeof(attr));
	if (map < 0)
		die("map_create");

	return map;
}

static void map_update_elem(int fd, int key, int value)
{
	union bpf_attr attr = {
		.map_fd = fd,
		.key = (uint64_t)&key,
		.value = (uint64_t)&value,
		.flags = BPF_ANY
	};

	if (syscall(SYS_bpf, BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr)))
		die("map_update_elem");
}

static int prog_load(void)
{
	/* mov %r0, 1; exit */
	struct bpf_insn insns[] = {
		{ .code = BPF_ALU64 | BPF_MOV | BPF_K, .dst_reg = 0, .imm = 1 },
		{ .code = BPF_JMP | BPF_EXIT }
	};
	union bpf_attr attr = {
		.prog_type = BPF_PROG_TYPE_SK_SKB,
		.insn_cnt = sizeof(insns)/sizeof(insns[0]),
		.insns = (long)insns,
		.license = (long)"",
	};
	
	int prog = syscall(SYS_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
	if (prog < 0)
		die("prog_load");

	return prog;
}

static void link_create(int prog_fd, int target_fd)
{
	union bpf_attr attr = {
		.link_create = {
			.prog_fd = prog_fd,
			.target_fd = target_fd,
			.attach_type = BPF_SK_SKB_VERDICT
		}
	};

	if (syscall(SYS_bpf, BPF_LINK_CREATE, &attr, sizeof(attr)) < 0)
		die("link_create");
}

int main(void)
{
	struct sockaddr_vm addr = {
		.svm_family = AF_VSOCK,
		.svm_cid = VMADDR_CID_LOCAL,
		.svm_port = VMADDR_PORT_ANY
	};
	socklen_t alen = sizeof(addr);
	int s, map, prog, c;

	s = socket(AF_VSOCK, SOCK_SEQPACKET, 0);
	if (s < 0)
		die("socket");

	if (bind(s, (struct sockaddr *)&addr, alen))
		die("bind");

	if (!connect(s, (struct sockaddr *)&addr, alen) || errno != ECONNRESET)
		die("connect #1");

	map = sockmap_create();
	prog = prog_load();
	link_create(prog, map);
	map_update_elem(map, 0, s);

	addr.svm_cid = 0x42424242; /* non-existing */
	if (!connect(s, (struct sockaddr *)&addr, alen) || errno != ESOCKTNOSUPPORT)
		die("connect #2");

	if (listen(s, 1))
		die("listen");

	if (getsockname(s, (struct sockaddr *)&addr, &alen))
		die("getsockname");

	c = socket(AF_VSOCK, SOCK_SEQPACKET, 0);
	if (c < 0)
		die("socket c");

	if (connect(c, (struct sockaddr *)&addr, alen))
		die("connect #3");

	return 0;
}