[efi] Retry calls to GetRNG() as needed

The UEFI specification allows GetRNG() to return EFI_NOT_READY, which
is not a particularly helpful error status since there is nothing that
can sensibly be done except to retry immediately.

Retry failed calls to GetRNG() up to a maximum number of attempts.

Debugged-by: Stoo Davies <sdavies@nvidia.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/interface/efi/efi_rng.c b/src/interface/efi/efi_rng.c
index b76a6fc0d3252003303e5517b0eb0b0aeafa5a4e..058f0ee7d3d6aa7adeb48399d3c9c1f18da65b34 100644 (file)
--- a/src/interface/efi/efi_rng.c
+++ b/src/interface/efi/efi_rng.c
@@ -54,6 +54,15 @@ EFI_REQUEST_PROTOCOL ( EFI_RNG_PROTOCOL, &efirng );
  */
 #define EFIRNG_LEN 32
 
+/** Maximum number of times to attempting requesting data from RNG
+ *
+ * The UEFI spec allows GetRNG() to return EFI_NOT_READY, which is not
+ * a particularly helpful error status since there is nothing that can
+ * sensibly be done except to retry immediately.  We retry failed
+ * calls to GetRNG() (for any reason) up to this number of times.
+ */
+#define EFIRNG_MAX_RETRY 16
+
 /**
  * Enable entropy gathering
  *
@@ -85,29 +94,35 @@ static int efirng_enable ( void ) {
  */
 static int efirng_get_noise ( noise_sample_t *noise ) {
        uint8_t buf[EFIRNG_LEN];
+       unsigned int i;
        EFI_STATUS efirc;
        int rc;
 
        /* Sanity check */
        assert ( efirng != NULL );
 
-       /* Get the minimum allowed number of random bytes */
-       if ( ( efirc = efirng->GetRNG ( efirng, NULL, sizeof ( buf ),
-                                       buf ) ) != 0 ) {
-               rc = -EEFI ( efirc );
-               DBGC ( &efirng, "ENTROPY could not read from RNG: %s\n",
-                      strerror ( rc ) );
-               return rc;
+       /* Get random bytes, retrying if needed */
+       for ( i = 0 ; i < EFIRNG_MAX_RETRY ; i++ ) {
+
+               /* Get the minimum allowed number of random bytes */
+               if ( ( efirc = efirng->GetRNG ( efirng, NULL, sizeof ( buf ),
+                                               buf ) ) != 0 ) {
+                       rc = -EEFI ( efirc );
+                       continue;
+               }
+
+               /* Reduce random bytes to a single noise sample.  This
+                * seems like overkill, but we have no way of knowing
+                * how much entropy is actually present in the bytes
+                * returned by the RNG protocol.
+                */
+               *noise = crc32_le ( 0, buf, sizeof ( buf ) );
+               return 0;
        }
 
-       /* Reduce random bytes to a single noise sample.  This seems
-        * like overkill, but we have no way of knowing how much
-        * entropy is actually present in the bytes returned by the
-        * RNG protocol.
-        */
-       *noise = crc32_le ( 0, buf, sizeof ( buf ) );
-
-       return 0;
+       DBGC ( &efirng, "ENTROPY could not read from RNG: %s\n",
+              strerror ( rc ) );
+       return rc;
 }
 
 /** EFI random number generator protocol entropy source */