fuzz: Add known_hosts file fuzzer

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/fuzz/CMakeLists.txt

@@ -23,3 +23,4 @@ fuzzer(ssh_client_fuzzer)
 fuzzer(ssh_server_fuzzer)
 fuzzer(ssh_client_config_fuzzer)
 fuzzer(ssh_bind_config_fuzzer)
+fuzzer(ssh_known_hosts_fuzzer)

tests/fuzz/ssh_known_hosts_fuzzer.c

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2022 Jakub Jelen <jjelen@redhat.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define LIBSSH_STATIC 1
+#include "libssh/libssh.h"
+#include "knownhosts.c"
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+    char *hostname = NULL;
+    const uint8_t *hostname_end = NULL;
+    size_t hostname_len = 0;
+    char filename[256];
+    struct ssh_list *entries = NULL;
+    struct ssh_iterator *it = NULL;
+    FILE *fp = NULL;
+
+    /* Interpret the first part of the string (until the first NULL byte)
+     * as a hostname we are searching for in the file */
+    hostname_end = memchr(data, '\0', size);
+    if (hostname_end == NULL) {
+        return 1;
+    }
+    hostname_len = hostname_end - data + 1;
+    if (hostname_len > 253) {
+        /* This is the maximum valid length of a hostname */
+        return 1;
+    }
+    hostname = malloc(hostname_len);
+    if (hostname == NULL) {
+        return 1;
+    }
+    memcpy(hostname, data, hostname_len);
+
+    snprintf(filename, sizeof(filename), "/tmp/libfuzzer.%d", getpid());
+    fp = fopen(filename, "wb");
+    if (!fp) {
+        free(hostname);
+        return 1;
+    }
+    fwrite(data + hostname_len, size - hostname_len, 1, fp);
+    fclose(fp);
+
+    ssh_init();
+
+    ssh_known_hosts_read_entries(hostname, filename, &entries);
+    for (it = ssh_list_get_iterator(entries);
+         it != NULL;
+         it = ssh_list_get_iterator(entries)) {
+        struct ssh_knownhosts_entry *entry = NULL;
+
+        entry = ssh_iterator_value(struct ssh_knownhosts_entry *, it);
+        ssh_knownhosts_entry_free(entry);
+        ssh_list_remove(entries, it);
+    }
+    ssh_list_free(entries);
+
+    ssh_finalize();
+
+    free(hostname);
+    unlink(filename);
+
+    return 0;
+}