ui/vnc: Use gcrypto_random_bytes for start_auth_vnc

Use a better interface for random numbers than rand().
Fail gracefully if for some reason we cannot use the crypto system.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

ui/vnc.c

@@ -43,6 +43,7 @@
 #include "crypto/hash.h"
 #include "crypto/tlscredsanon.h"
 #include "crypto/tlscredsx509.h"
+#include "crypto/random.h"
 #include "qom/object_interfaces.h"
 #include "qemu/cutils.h"
 #include "io/dns-resolver.h"
@@ -2547,16 +2548,6 @@ static void authentication_failed(VncState *vs)
     vnc_client_error(vs);
 }
 
-static void make_challenge(VncState *vs)
-{
-    int i;
-
-    srand(time(NULL)+getpid()+getpid()*987654+rand());
-
-    for (i = 0 ; i < sizeof(vs->challenge) ; i++)
-        vs->challenge[i] = (int) (256.0*rand()/(RAND_MAX+1.0));
-}
-
 static int protocol_client_auth_vnc(VncState *vs, uint8_t *data, size_t len)
 {
     unsigned char response[VNC_AUTH_CHALLENGE_SIZE];
@@ -2628,7 +2619,16 @@ reject:
 
 void start_auth_vnc(VncState *vs)
 {
-    make_challenge(vs);
+    Error *err = NULL;
+
+    if (qcrypto_random_bytes(vs->challenge, sizeof(vs->challenge), &err)) {
+        trace_vnc_auth_fail(vs, vs->auth, "cannot get random bytes",
+                            error_get_pretty(err));
+        error_free(err);
+        authentication_failed(vs);
+        return;
+    }
+
     /* Send client a 'random' challenge */
     vnc_write(vs, vs->challenge, sizeof(vs->challenge));
     vnc_flush(vs);