Add a /proc/net magic authenticator.

This is a Linux-specific trick that I'm quite fond of: I've used it
before in 'agedu' and a lot of my unpublished personal scriptery.

Suppose you want to run a listening network server in such a way that
it can only accept connections from processes under your own control.
Often it's not convenient to do this by adding an authentication step
to the protocol itself (either because the password management gets
hairy or because the protocol is already well defined). The 'right'
answer is to switch from TCP to Unix-domain sockets, because then you
can use the file permissions on the path leading to the socket inode
to ensure that no other user id can connect to it - but that's often
inconvenient as well, because if any _client_ of the server is not
already prepared to speak AF_UNIX your control then you can only trick
it into connecting to an AF_UNIX socket instead of TCP by applying a
downstream patch or resorting to LD_PRELOAD shenanigans.

But on Linux, there's an alternative shenanigan available, in the form
of /proc/net/tcp (or tcp6), which lists every currently active TCP
endpoint known to the kernel, and for each one, lists an owning uid.
Listen on localhost only. Then, when a connection comes in, look up
the far end of it in that file and see if the owning uid is the right
one!

I've always vaguely wondered if there would be uses for this trick in
PuTTY. One potentially useful one might be to protect the listening
sockets created by local-to-remote port forwarding. But for the
moment, I'm only planning to use it for a less security-critical
purpose, which will appear in the next commit.

unix/procnet.c

@@ -0,0 +1,229 @@
+/*
+ * Locally authenticate a TCP socket via /proc/net.
+ *
+ * Obviously, if a TCP connection comes from a different host, there's
+ * no way to find out the identity of the thing at the other end (or
+ * even really to assign that concept a meaning) except by the usual
+ * method of speaking a protocol over the socket itself which involves
+ * some form of (preferably cryptographic) authentication exchange.
+ *
+ * But if the connection comes from localhost, then on at least some
+ * operating systems, you can do better. On Linux, /proc/net/tcp and
+ * /proc/net/tcp6 list the full set of active TCP connection
+ * endpoints, and they list an owning uid for each one. So once you've
+ * accepted a connection to a listening socket and found that the
+ * other end of it is a localhost address, you can look up the _other_
+ * endpoint in the right one of those files, and find out which uid
+ * owns it.
+ */
+
+#include <ctype.h>
+#include <stddef.h>
+#include <stdio.h>
+
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <arpa/inet.h>
+#include <netinet/in.h>
+
+#include "misc.h"
+
+static ptrlen get_space_separated_field(ptrlen *string)
+{
+    const char *p = string->ptr, *end = p + string->len;
+
+    while (p < end && isspace((unsigned char)*p))
+        p++;
+    if (p == end)
+        return PTRLEN_LITERAL("");
+
+    const char *start = p;
+    while (p < end && !isspace((unsigned char)*p))
+        p++;
+    *string = make_ptrlen(p, end - p);
+    return make_ptrlen(start, p - start);
+}
+
+enum { GOT_LOCAL_UID = 1, GOT_REMOTE_UID = 2 };
+
+/*
+ * Open a file formatted like /proc/net/tcp{,6}, and search it for
+ * both ends of a particular connection.
+ *
+ * The operands 'local' and 'remote' give the expected string
+ * representations of the local and remote addresses of the connection
+ * we're looking for.
+ *
+ * Return value is the bitwise OR of 1 if we found the local end of
+ * the connection and 2 if we found the remote. Each output uid_t
+ * parameter is filled in iff the corresponding bit is set in the
+ * return value.
+ */
+static int lookup_uids_in_procnet_file(
+    const char *path, ptrlen local, ptrlen remote,
+    uid_t *local_uid, uid_t *remote_uid)
+{
+    FILE *fp = NULL;
+    int toret = 0;
+    ptrlen line, field;
+
+    enum { GF_LOCAL = 1, GF_REMOTE = 2, GF_UID = 4 };
+
+    fp = fopen(path, "r");
+    if (!fp)
+        goto out;
+
+    /* Expected indices of fields in /proc/net/tcp* */
+    const int LOCAL_ADDR_INDEX = 1;
+    const int REMOTE_ADDR_INDEX = 2;
+    const int UID_INDEX = 7;
+
+    for (char *linez; (linez = chomp(fgetline(fp))) != NULL ;) {
+        line = ptrlen_from_asciz(linez);
+        int gotfields = 0;
+        ptrlen local_addr = PTRLEN_LITERAL("");
+        ptrlen remote_addr = PTRLEN_LITERAL("");
+        long uid = -1;
+
+        for (int i = 0; (field = get_space_separated_field(&line)).len != 0;
+             i++) {
+
+            if (i == LOCAL_ADDR_INDEX) {
+                gotfields |= GF_LOCAL;
+                local_addr = field;
+            } else if (i == REMOTE_ADDR_INDEX) {
+                gotfields |= GF_REMOTE;
+                remote_addr = field;
+            } else if (i == UID_INDEX) {
+                uid = 0;
+                for (const char *p = field.ptr, *end = p + field.len;
+                     p < end; p++) {
+                    if (!isdigit((unsigned char)*p)) {
+                        uid = -1;
+                        break;
+                    }
+                    int dval = *p - '0';
+                    if (uid > LONG_MAX/10) {
+                        uid = -1;
+                        break;
+                    }
+                    uid *= 10;
+                    if (uid > LONG_MAX - dval) {
+                        uid = -1;
+                        break;
+                    }
+                    uid += dval;
+                }
+
+                gotfields |= GF_UID;
+            }
+        }
+
+        if (gotfields == (GF_LOCAL | GF_REMOTE | GF_UID)) {
+            if (ptrlen_eq_ptrlen(local_addr, local) &&
+                ptrlen_eq_ptrlen(remote_addr, remote)) {
+                *local_uid = uid;
+                toret |= GOT_LOCAL_UID;
+            }
+            if (ptrlen_eq_ptrlen(local_addr, remote) &&
+                ptrlen_eq_ptrlen(remote_addr, local)) {
+                *remote_uid = uid;
+                toret |= GOT_REMOTE_UID;
+            }
+        }
+
+        sfree(linez);
+    }
+
+    fclose(fp);
+    fp = NULL;
+
+  out:
+    if (fp)
+        fclose(fp);
+    return toret;
+}
+
+static const char *procnet_path(int family)
+{
+    switch (family) {
+      case AF_INET: return "/proc/net/tcp";
+      case AF_INET6: return "/proc/net/tcp6";
+      default: return NULL;
+    }
+}
+
+static char *format_sockaddr(const void *addr, int family)
+{
+    if (family == AF_INET) {
+        const struct sockaddr_in *a = (const struct sockaddr_in *)addr;
+        assert(a->sin_family == family);
+        /* Linux /proc/net formats the IP address native-endian, so we
+         * don't use ntohl */
+        return dupprintf("%08X:%04X", a->sin_addr.s_addr, ntohs(a->sin_port));
+    } else if (family == AF_INET6) {
+        struct sockaddr_in6 *a = (struct sockaddr_in6 *)addr;
+        assert(a->sin6_family == family);
+
+        strbuf *sb = strbuf_new();
+
+        const uint32_t *addrwords = (const uint32_t *)a->sin6_addr.s6_addr;
+        for (int i = 0; i < 4; i++)
+            strbuf_catf(sb, "%08X", addrwords[i]);
+        strbuf_catf(sb, ":%04X", ntohs(a->sin6_port));
+
+        return strbuf_to_str(sb);
+    } else {
+        return NULL;
+    }
+}
+
+bool socket_peer_is_same_user(int fd)
+{
+    struct sockaddr_storage addr;
+    socklen_t addrlen;
+    int family;
+    bool toret = false;
+    char *local = NULL, *remote = NULL;
+    const char *path;
+
+    addrlen = sizeof(addr);
+    if (getsockname(fd, (struct sockaddr *)&addr, &addrlen) != 0)
+        goto out;
+    family = addr.ss_family;
+    if ((path = procnet_path(family)) == NULL)
+        goto out;
+    local = format_sockaddr(&addr, family);
+    if (!local)
+        goto out;
+
+    addrlen = sizeof(addr);
+    if (getpeername(fd, (struct sockaddr *)&addr, &addrlen) != 0)
+        goto out;
+    if (addr.ss_family != family)
+        goto out;
+    remote = format_sockaddr(&addr, family);
+    if (!remote)
+        goto out;
+
+    ptrlen locpl = ptrlen_from_asciz(local);
+    ptrlen rempl = ptrlen_from_asciz(remote);
+
+    /*
+     * Check that _both_ end of the socket are the uid we expect, as a
+     * sanity check on the /proc/net file being reasonable at all.
+     */
+    uid_t our_uid = getuid();
+    uid_t local_uid = -1, remote_uid = -1;
+    int got = lookup_uids_in_procnet_file(
+        path, locpl, rempl, &local_uid, &remote_uid);
+    if (got == (GOT_LOCAL_UID | GOT_REMOTE_UID) &&
+        local_uid == our_uid && remote_uid == our_uid)
+        toret = true;
+
+  out:
+    sfree(local);
+    sfree(remote);
+    return toret;
+}

unix/unix.h

@@ -369,6 +369,7 @@ bool init_ucs(struct unicode_data *ucsdata, char *line_codepage,
  * Spare functions exported directly from uxnet.c.
  */
 void *sk_getxdmdata(Socket *sock, int *lenp);
+int sk_net_get_fd(Socket *sock);
 SockAddr *unix_sock_addr(const char *path);
 Socket *new_unix_listener(SockAddr *listenaddr, Plug *plug);
 
@@ -421,6 +422,16 @@ extern char **pty_argv;
 char *gtk_askpass_main(const char *display, const char *wintitle,
                        const char *prompt, bool *success);
 
+/*
+ * procnet.c.
+ */
+bool socket_peer_is_same_user(int fd);
+static inline bool sk_peer_trusted(Socket *sock)
+{
+    int fd = sk_net_get_fd(sock);
+    return fd >= 0 && socket_peer_is_same_user(fd);
+}
+
 /*
  * uxsftpserver.c.
  */

unix/uxnet.c

@@ -1554,6 +1554,15 @@ static SocketPeerInfo *sk_net_peer_info(Socket *sock)
     return pi;
 }
 
+int sk_net_get_fd(Socket *sock)
+{
+    /* This function is not fully general: it only works on NetSocket */
+    if (sock->vt != &NetSocket_sockvt)
+        return -1;                     /* failure */
+    NetSocket *s = container_of(sock, NetSocket, sock);
+    return s->s;
+}
+
 static void uxsel_tell(NetSocket *s)
 {
     int rwx = 0;