uvms/pkgs/taps/main.c

#define _GNU_SOURCE

#include <stdlib.h> /* secure_getenv */
#include <sys/socket.h>
#include <unistd.h>
#include <sys/un.h>
#include <string.h>
#include <stdbool.h>
#include <ctype.h>
#include <pwd.h>
#include <fnmatch.h>
#include <fcntl.h> /* open, O_NONBLOCK, &c */
#include <err.h>
#include <error.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <signal.h>

#define __UAPI_DEF_IF_IFNAMSIZ 1
#include <linux/if_tun.h>
#include <linux/if.h>

#include "sendfd.h"

// From `man unix`
#define SUN_PATH_SZ 108
#define N_CONNS 16

char *TEMP_PATHS[1024] = { 0 }; 
int LAST_TEMP_PATH = -1;

#define IFR_FLAGS_ALLOWED (IFF_NO_PI | IFF_TAP | IFF_TUN | IFF_VNET_HDR | IFF_MULTI_QUEUE | IFF_PERSIST)
#define IFR_FLAGS_DEFAULT (IFF_NO_PI | IFF_TAP | IFF_VNET_HDR | IFF_PERSIST)

#define PTR_OR_DIE(expr) TRUE_OR_DIE((expr) != NULL)
#define DO_OR_DIE(expr) TRUE_OR_DIE((expr) != -1)
#define TRUE_OR_DIE(expr, ...) TRUE_OR_(EXIT_FAILURE, expr, __VA_ARGS__)
#define TRUE_OR_WARN(expr, ...) TRUE_OR_(0, expr, __VA_ARGS__)
#define TRUE_OR_(status, expr, ...) \
    do if (!(expr)) { \
        error(status, errno, "Failed assertion: " #expr "." __VA_ARGS__); \
    } while(false)

struct allow_pattern {
    // enum { USER = 1, GROUP = 2 } type;
    // union { uid_t uid, gid_t gid } xid;
    char *name;
};
struct allow_patterns {
    size_t n;
    struct allow_pattern *patterns;
};

/* Running on the same host, not caring for alignment */
struct tap_request {
    short ifrFlags; /* 0 to use defaults: IFF_TAP | IFF_NO_PI | IFF_VNET_HDR */
    char name[IFNAMSIZ];
};

struct tap_reply {
    enum { OK = 0, AUTH_ERROR = 1 } status;
    char name[IFNAMSIZ];
};

int tuntap_alloc(char *dev, short openFlags, short ifrFlags, int *out_fd);

bool match_mask(const char *test_addr, const char *expected_addr, const char *mask, int n) {
    for (int octet = 0; octet < n; ++octet) {
        if ((test_addr[octet] & mask[octet]) != expected_addr[octet]) {
            return false;
        }
    }
    return true;
}

/* 
 * Adapted from spectrum's `mktuntap.c` (2019 Alyssa Ross <hi@alyssa.is>
 * GPL-2.0-only), which in turn adapts `tun_alloc` from
 * `linux/Documentation/networking/tuntap.rst`.
 *
 * ifrFlags: IFF_TUN   - TUN device (no Ethernet headers)
 *           IFF_TAP   - TAP device
 *
 *           IFF_NO_PI - Do not provide packet information
 */
int tuntap_alloc(char *dev, short openFlags, short ifrFlags, int *out_fd) {
  struct ifreq ifr = { 0 };
  int fd = -1, err = 0;

  DO_OR_DIE(fd = open("/dev/net/tun", openFlags));

  if (dev != NULL) {
    int devLen = strlen(dev);
    if (devLen >= IFNAMSIZ) {
      /* If client requests a name, we do want the entire name to fit */
      errno = EINVAL;
      return EINVAL;
    }
    strncpy(ifr.ifr_name, dev, IFNAMSIZ - 1);
  }
  ifr.ifr_flags = ifrFlags;

  TRUE_OR_WARN((err = ioctl(fd, TUNSETIFF, (void *)&ifr)) == 0);
  if (err != 0) {
    close(fd);
    return err;
  }

  strncpy(dev, ifr.ifr_name, IFNAMSIZ);
  *out_fd = fd;
  return 0;
}

int acceptRequests(const char *requestsPath, const struct allow_patterns *patterns) {
    int listener;
    struct sockaddr_un addr;
    const int t = 1;

    DO_OR_DIE(listener = socket(AF_UNIX, SOCK_SEQPACKET, 0));
    DO_OR_DIE(setsockopt(listener, SOL_SOCKET, SO_PASSCRED, &t, sizeof(t)));

    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, requestsPath, SUN_PATH_SZ - 1);
    DO_OR_DIE (bind(listener, &addr, sizeof(addr)));
    PTR_OR_DIE(TEMP_PATHS[++LAST_TEMP_PATH] = strdup(requestsPath));

    DO_OR_DIE(listen(listener, N_CONNS));

    for (;;) {
        /* Already changed my mind about looking at ucred, but keeping the code around for now */
        int sock = -1;
        struct ucred cred = { 0 };
        struct msghdr msg = { 0 };
        struct cmsghdr *cmsg = NULL;
        struct iovec iov = { 0 };
        struct tap_request req = { 0 };

        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;

        iov.iov_base = &req;
        iov.iov_len = sizeof(struct tap_request);

        DO_OR_DIE((sock = accept(listener, NULL, NULL)));

        TRUE_OR_DIE(recvmsg(sock, &msg, 0) > 0);
        req.name[IFNAMSIZ] = 0;
        for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
            if (cmsg->cmsg_level != SOL_SOCKET) { continue; }
            if (cmsg->cmsg_type != SCM_CREDENTIALS) { continue; }
            if (CMSG_LEN(cmsg) < sizeof(struct ucred)) { continue; }
            memcpy(&cred, CMSG_DATA(cmsg), sizeof(struct ucred));
            break;
        }

        if (req.ifrFlags == 0) {
            req.ifrFlags = IFR_FLAGS_DEFAULT;
        }

        bool allowed = false;
        for (int i = 0; !allowed && i < patterns->n; ++i) {
            bool ifnameOk = fnmatch(patterns->patterns[i].name, req.name, 0) == 0;
            bool flagsOk = (req.ifrFlags & IFR_FLAGS_ALLOWED) == req.ifrFlags;
            allowed = ifnameOk && flagsOk;
        }

        struct tap_reply reply = { 0 };
        if (!allowed) { reply.status = AUTH_ERROR; }
        if (allowed) {
            /* O_CLOEXEC? */
            int fd = -1;
            TRUE_OR_DIE(tuntap_alloc(req.name, O_RDWR | O_NONBLOCK, req.ifrFlags, &fd) == 0);
            struct iovec iov = { 0 };
            iov.iov_base = &reply;
            iov.iov_len = sizeof(struct tap_reply);
            TRUE_OR_DIE(send_fd(sock, fd, &iov) > 0);
            close(fd); 
        }
        close(sock);
    }
    close(listener);
}

struct allow_patterns parsePatterns(const char *raw) {
    const size_t rawLen = strlen(raw);

    size_t nPatterns = 0;
    for (int i = 0; i < rawLen; ++i) {
        const int start = i;
        if (isspace(raw[i])) { continue; }
        for (; i < rawLen && !isspace(raw[i]); ++i) { }
        if (start < i) { ++nPatterns; }
    }

    struct allow_pattern *patterns = NULL;
    PTR_OR_DIE(patterns = calloc(nPatterns, sizeof(struct allow_pattern)));

    int iPattern = 0;
    for (int i = 0; i < rawLen; ++i) {
        if (isspace(raw[i])) { continue; }
        /* used to have per-group/per-user patterns, "u:$username:$pattern", &c - gone */
        {
            const int start = i;
            for (; i < rawLen && !isspace(raw[i]); ++i) { }
            if (start < i) {
                PTR_OR_DIE(patterns[iPattern].name = strndup(&raw[start], i - start));
                iPattern += 1;
            }
        }
    }
    struct allow_patterns out = {
        .n = nPatterns,
        .patterns = patterns
    };
    return out;
}

int get(const char *servePath, const char *ifname, short ifrFlags) {
    /* TODO: sock: move out */
    int sock;
    struct sockaddr_un addr;

    DO_OR_DIE(sock = socket(AF_UNIX, SOCK_SEQPACKET, 0));

    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, servePath, SUN_PATH_SZ - 1);
    DO_OR_DIE (connect(sock, &addr, sizeof(addr)));

    struct msghdr msg = { 0 };
    struct cmsghdr *cmsg = NULL;
    struct iovec iov = { 0 };
    struct tap_request req = { 0 };
    strncpy(req.name, ifname, IFNAMSIZ - 1);
    req.ifrFlags = ifrFlags;

    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    iov.iov_base = &req;
    iov.iov_len = sizeof(struct tap_request);

    TRUE_OR_DIE(sendmsg(sock, &msg, 0) > 0);

    int tunFd = -1;
    DO_OR_DIE(tunFd = recv_fd(sock, 0));
    close(sock);
    return tunFd;
}

void cleanup(int signo, siginfo_t *info, void *_context) {
    for (int i = 0; i <= LAST_TEMP_PATH; ++i) {
        TRUE_OR_DIE(unlink(TEMP_PATHS[i]) != -1 || errno == ENOENT);
    }
    if (signo == SIGINT) {
        exit(EXIT_SUCCESS);
    }
    errx(EXIT_FAILURE, "Exiting with signal %d", signo);
}

int main(int argc, char **argv) {
    struct sigaction act = { 0 };
    act.sa_flags = SA_SIGINFO;
    act.sa_sigaction = cleanup;
    DO_OR_DIE(sigaction(SIGINT, &act, NULL));
    DO_OR_DIE(sigaction(SIGSEGV, &act, NULL));

    bool cmdServe = false;
    bool cmdPass = false;
    char *ifname = "vt%d";

    char **rest = argv + 1;
    char **end = argv + argc;

    TRUE_OR_DIE(argc > 1);
    if (strcmp(rest[0], "serve") == 0) {
        cmdServe = true;
        ++rest;
    } else if (strcmp(rest[0], "pass") == 0) {
        cmdPass = true;
        ++rest;
        for (; rest != end && rest[0][0] == '-'; ++rest) {
            if (strcmp(rest[0], "--")) { break; }
            else if (strncmp(rest[0], "--ifname=", sizeof("--ifname="))) {
                ifname = rest[0] + sizeof("--ifname=");
            }
        }
    } else {
        error(EINVAL, EINVAL, "no subcommand \"%s\"", rest[0]);
    }

    int nextArgc = argc - (rest - argv);
    char * const* nextArgv = rest;

    const char *patternsRaw = secure_getenv("TAPS_ALLOW");
    if (patternsRaw == NULL) {
        patternsRaw = "*";
    }

    struct allow_patterns patterns = { 0 };
    if (cmdServe) {
        PTR_OR_DIE((patterns = parsePatterns(patternsRaw)).patterns);
    }

    const char *servePath = secure_getenv("TAPS_SOCK");
    if (servePath == NULL) {
        servePath = "taps.sock";
    }

    if (cmdServe) {
        acceptRequests(servePath, &patterns);
    } else if (cmdPass) {
        TRUE_OR_DIE(nextArgc > 0);
        int fd = -1;
        DO_OR_DIE(fd = get(servePath, ifname, 0));
        if (fd != 3) {
            DO_OR_DIE(dup2(fd, 3));
            close(fd);
            fd = 3;
        }
        DO_OR_DIE(execvp(nextArgv[0], nextArgv));
    } else {
        error(EINVAL, EINVAL, "subcommand args");
    }

    return 0;
}