Fallback on invalid SSH key

backend/src/client.c

@@ -639,8 +639,9 @@ static void sink_accept(struct csink *s)
 
 /* Transparent source spoofing */
 
-// Runs ip(8) without a shell. Returns its exit status, -1 if it did not run.
-static int run_ip(const char *const argv[])
+// Runs a program without a shell, output discarded. Returns its exit status,
+// -1 if it did not run.
+static int run_cmd(const char *const argv[])
 {
     pid_t pid = fork();
     if (pid < 0)
@@ -651,7 +652,7 @@ static int run_ip(const char *const argv[])
             dup2(nul, STDOUT_FILENO);
             dup2(nul, STDERR_FILENO);
         }
-        execvp("ip", (char *const *)argv);
+        execvp(argv[0], (char *const *)argv);
         _exit(127);
     }
     int st;
@@ -676,16 +677,16 @@ static void spoof_routing(int add)
         // Remove copies left by an earlier run, then add exactly one.
         for (int k = 0; k < 16; k++) {
             const char *del[] = { "ip", fam, "rule", "del", "pref", tab, NULL };
-            if (run_ip(del) != 0)
+            if (run_cmd(del) != 0)
                 break;
         }
         const char *flush[] = { "ip", fam, "route", "flush", "table", tab, NULL };
-        run_ip(flush);
+        run_cmd(flush);
         if (!add)
             continue;
         const char *rule[] = { "ip", fam, "rule", "add", "pref", tab, "from", fams[i][1], "iif", "lo", "lookup", tab, NULL };
         const char *route[] = { "ip", fam, "route", "replace", "local", fams[i][2], "dev", "lo", "table", tab, NULL };
-        int ok = run_ip(rule) == 0 && run_ip(route) == 0;
+        int ok = run_cmd(rule) == 0 && run_cmd(route) == 0;
         if (i == 0)
             ok4 = ok;
         else if (!ok)
@@ -835,23 +836,86 @@ static int process_ctl_input(void)
 
 /* Session setup */
 
+// libssh2 before 1.11 signs RSA keys only with SHA-1 ("ssh-rsa"), which
+// OpenSSH 8.8+ rejects by default.
+static int rsa_usable(void)
+{
+    return libssh2_version(0x010b00) != NULL;
+}
+
+// Identifies RSA keys by the public key file or a PEM private key header.
+static int key_is_rsa(const char *path)
+{
+    char pub[300], head[64] = "";
+    snprintf(pub, sizeof(pub), "%s.pub", path);
+    FILE *f = fopen(pub, "r");
+    if (!f)
+        f = fopen(path, "r");
+    if (!f)
+        return 0;
+    size_t n = fread(head, 1, sizeof(head) - 1, f);
+    head[n] = '\0';
+    fclose(f);
+    return !strncmp(head, "ssh-rsa ", 8) || strstr(head, "BEGIN RSA PRIVATE KEY") != NULL;
+}
+
+// Creates PB_CLIENT_KEY with ssh-keygen unless it exists. Returns 0 if usable.
+static int ensure_own_key(void)
+{
+    if (access(PB_CLIENT_KEY, R_OK) == 0)
+        return 0;
+    char host[64], comment[96];
+    if (gethostname(host, sizeof(host)) < 0)
+        strcpy(host, "client");
+    host[sizeof(host) - 1] = '\0';
+    snprintf(comment, sizeof(comment), "patchbay@%s", host);
+    const char *argv[] = { "ssh-keygen", "-q", "-t", "ed25519", "-N", "", "-C", comment, "-f", PB_CLIENT_KEY, NULL };
+    if (run_cmd(argv) != 0 || access(PB_CLIENT_KEY, R_OK) != 0) {
+        log_err("cannot create %s with ssh-keygen (is openssh-client installed?)", PB_CLIENT_KEY);
+        return -1;
+    }
+    log_warn("created %s; add its public key in the web UI (patchbayd --pubkey)", PB_CLIENT_KEY);
+    return 0;
+}
+
 int client_identity(const struct pb_config *c, char *out, size_t outsz)
 {
-    const char *cands[] = { c->identity_file, "/root/.ssh/id_ed25519", "/root/.ssh/id_rsa",
-                            "/etc/ssh/ssh_host_ed25519_key", NULL };
-    for (int i = 0; cands[i]; i++) {
-        if (!cands[i][0])
-            continue;
-        if (access(cands[i], R_OK) == 0) {
-            snprintf(out, outsz, "%s", cands[i]);
-            return 0;
-        }
-        if (i == 0) {
-            log_err("IdentityFile %s is not readable", cands[i]);
+    if (c->identity_file[0]) {
+        if (access(c->identity_file, R_OK) != 0) {
+            log_err("IdentityFile %s is not readable", c->identity_file);
             return -1;
         }
+        if (key_is_rsa(c->identity_file) && !rsa_usable())
+            log_warn("IdentityFile %s is an RSA key, which libssh2 %s can only sign with SHA-1; "
+                     "the target will reject it. Use an ed25519 key or libssh2 1.11+", c->identity_file,
+                     libssh2_version(0));
+        snprintf(out, outsz, "%s", c->identity_file);
+        return 0;
     }
-    return -1;
+
+    const char *cands[] = { "/root/.ssh/id_ed25519", "/root/.ssh/id_rsa", "/etc/ssh/ssh_host_ed25519_key", NULL };
+    for (int i = 0; cands[i]; i++) {
+        if (access(cands[i], R_OK) != 0)
+            continue;
+        if (key_is_rsa(cands[i]) && !rsa_usable()) {
+            // A key the target rejects is worse than none: use our own instead.
+            static int told;
+            if (!told++)
+                log_warn("%s is an RSA key that libssh2 %s cannot sign acceptably; using %s instead", cands[i],
+                         libssh2_version(0), PB_CLIENT_KEY);
+            if (ensure_own_key() < 0)
+                return -1;
+            snprintf(out, outsz, "%s", PB_CLIENT_KEY);
+            return 0;
+        }
+        snprintf(out, outsz, "%s", cands[i]);
+        return 0;
+    }
+    // No usable key at all: create our own (or reuse one created earlier).
+    if (ensure_own_key() < 0)
+        return -1;
+    snprintf(out, outsz, "%s", PB_CLIENT_KEY);
+    return 0;
 }
 
 static int verify_host_key(void)

backend/src/config.h

@@ -4,6 +4,7 @@
 #define PB_DEFAULT_CONF "/etc/patchbay/patchbay.conf"
 #define PB_DEFAULT_DB   "/etc/patchbay/patchbay.db"
 #define PB_RUN_DIR      "/run/patchbay"
+#define PB_CLIENT_KEY   "/etc/patchbay/id_ed25519"   // created when root only has an unusable RSA key
 
 enum pb_role { ROLE_CLIENT, ROLE_TARGET };