crypto: use /dev/[u]random as a final fallback random source

If neither gcrypt or gnutls are available to provide a
cryptographic random number generator, fallback to consuming
bytes directly from /dev/[u]random.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

crypto/Makefile.objs

@@ -12,6 +12,7 @@ crypto-obj-y += tlssession.o
 crypto-obj-y += secret.o
 crypto-obj-$(CONFIG_GCRYPT) += random-gcrypt.o
 crypto-obj-$(if $(CONFIG_GCRYPT),n,$(CONFIG_GNUTLS_RND)) += random-gnutls.o
+crypto-obj-$(if $(CONFIG_GCRYPT),n,$(if $(CONFIG_GNUTLS_RND),n,y)) += random-platform.o
 crypto-obj-y += pbkdf.o
 crypto-obj-$(CONFIG_NETTLE_KDF) += pbkdf-nettle.o
 crypto-obj-$(if $(CONFIG_NETTLE_KDF),n,$(CONFIG_GCRYPT_KDF)) += pbkdf-gcrypt.o
@@ -28,6 +29,5 @@ crypto-obj-y += block-luks.o
 # Let the userspace emulators avoid linking gnutls/etc
 crypto-aes-obj-y = aes.o
 
-stub-obj-y += random-stub.o
 stub-obj-y += pbkdf-stub.o
 stub-obj-y += hash-stub.o

crypto/random-platform.c

@@ -26,6 +26,39 @@ int qcrypto_random_bytes(uint8_t *buf G_GNUC_UNUSED,
                          size_t buflen G_GNUC_UNUSED,
                          Error **errp)
 {
-    error_setg(errp, "No random byte source provided in this build");
-    return -1;
+    int fd;
+    int ret = -1;
+    int got;
+
+    /* TBD perhaps also add support for BSD getentropy / Linux
+     * getrandom syscalls directly */
+    fd = open("/dev/urandom", O_RDONLY);
+    if (fd == -1 && errno == ENOENT) {
+        fd = open("/dev/random", O_RDONLY);
+    }
+
+    if (fd < 0) {
+        error_setg(errp, "No /dev/urandom or /dev/random found");
+        return -1;
+    }
+
+    while (buflen > 0) {
+        got = read(fd, buf, buflen);
+        if (got < 0) {
+            error_setg_errno(errp, errno,
+                             "Unable to read random bytes");
+            goto cleanup;
+        } else if (!got) {
+            error_setg(errp,
+                       "Unexpected EOF reading random bytes");
+            goto cleanup;
+        }
+        buflen -= got;
+        buf += got;
+    }
+
+    ret = 0;
+ cleanup:
+    close(fd);
+    return ret;
 }