Handle RDRAND failures.

I mistakenly believed that only RDSEED could fail. However, the Intel
manuals state that RDRAND can fail too.

This change cherry-picks the following BoringSSL changes:

2cac3506 – Handle RDRAND failures.
248abbd7 – Add missing comma in .type pragma for rdrand code.

Change-Id: Icdc56a50ce36e9c525063583882c676a5312d313

4 files changed, 75 insertions, 17 deletions

diff --git a/src/crypto/rand/asm/rdrand-x86_64.pl b/src/crypto/rand/asm/rdrand-x86_64.pl
index a917611..c32a55c 100644
--- a/src/crypto/rand/asm/rdrand-x86_64.pl
+++ b/src/crypto/rand/asm/rdrand-x86_64.pl
@@ -1,5 +1,19 @@
 #!/usr/bin/env perl
 
+# Copyright (c) 2015, Google Inc.
+#
+# Permission to use, copy, modify, and/or distribute this software for any
+# purpose with or without fee is hereby granted, provided that the above
+# copyright notice and this permission notice appear in all copies.
+#
+# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+# SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+# OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+# CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
 $flavour = shift;
 $output  = shift;
 if ($flavour =~ /\./) { $output = $flavour; undef $flavour; }
@@ -14,11 +28,47 @@ open OUT,"| \"$^X\" $xlate $flavour $output";
 print<<___;
 .text
 
+# CRYPTO_rdrand writes eight bytes of random data from the hardware RNG to
+# |out|. It returns one on success or zero on hardware failure.
+# int CRYPTO_rdrand(uint8_t out[8]);
 .globl	CRYPTO_rdrand
 .type	CRYPTO_rdrand,\@function,1
 .align	16
 CRYPTO_rdrand:
-	.byte 0x48, 0x0f, 0xc7, 0xf0
+	xorq %rax, %rax
+	# This is rdrand %rcx. It sets rcx to a random value and sets the carry
+	# flag on success.
+	.byte 0x48, 0x0f, 0xc7, 0xf1
+	# An add-with-carry of zero effectively sets %rax to the carry flag.
+	adcq %rax, %rax
+	movq %rcx, 0(%rdi)
+	retq
+
+# CRYPTO_rdrand_multiple8_buf fills |len| bytes at |buf| with random data from
+# the hardware RNG. The |len| argument must be a multiple of eight. It returns
+# one on success and zero on hardware failure.
+# int CRYPTO_rdrand_multiple8_buf(uint8_t *buf, size_t len);
+.globl CRYPTO_rdrand_multiple8_buf
+.type CRYPTO_rdrand_multiple8_buf,\@function,2
+.align 16
+CRYPTO_rdrand_multiple8_buf:
+	test %rsi, %rsi
+	jz .Lout
+	movq \$8, %rdx
+.Lloop:
+	# This is rdrand %rcx. It sets rcx to a random value and sets the carry
+	# flag on success.
+	.byte 0x48, 0x0f, 0xc7, 0xf1
+	jnc .Lerr
+	movq %rcx, 0(%rdi)
+	addq %rdx, %rdi
+	subq %rdx, %rsi
+	jnz .Lloop
+.Lout:
+	movq \$1, %rax
+	retq
+.Lerr:
+	xorq %rax, %rax
 	retq
 ___
 
diff --git a/src/crypto/rand/hwrand.c b/src/crypto/rand/hwrand.c
index 73d3de7..5f81f09 100644
--- a/src/crypto/rand/hwrand.c
+++ b/src/crypto/rand/hwrand.c
@@ -14,6 +14,7 @@
 
 #include <openssl/rand.h>
 
+#include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -26,21 +27,28 @@ int CRYPTO_have_hwrand(void) {
   return (OPENSSL_ia32cap_P[1] & (1u << 30)) != 0;
 }
 
-/* CRYPTO_rdrand is defined in asm/rdrand-x86_64.pl */
-extern uint64_t CRYPTO_rdrand(void);
+/* These functions are defined in asm/rdrand-x86_64.pl */
+extern int CRYPTO_rdrand(uint8_t out[8]);
+extern int CRYPTO_rdrand_multiple8_buf(uint8_t *buf, size_t len);
 
-void CRYPTO_hwrand(uint8_t *buf, size_t len) {
-  while (len >= 8) {
-    uint64_t rand = CRYPTO_rdrand();
-    memcpy(buf, &rand, sizeof(rand));
-    len -= sizeof(rand);
-    buf += sizeof(rand);
+int CRYPTO_hwrand(uint8_t *buf, size_t len) {
+  const size_t len_multiple8 = len & ~7;
+  if (!CRYPTO_rdrand_multiple8_buf(buf, len_multiple8)) {
+    return 0;
   }
+  len -= len_multiple8;
+
+  if (len != 0) {
+    assert(len < 8);
 
-  if (len > 0) {
-    uint64_t rand = CRYPTO_rdrand();
-    memcpy(buf, &rand, len);
+    uint8_t rand_buf[8];
+    if (!CRYPTO_rdrand(rand_buf)) {
+      return 0;
+    }
+    memcpy(buf + len_multiple8, rand_buf, len);
   }
+
+  return 1;
 }
 
 #else
diff --git a/src/crypto/rand/internal.h b/src/crypto/rand/internal.h
index 1cca7f3..5e6ea11 100644
--- a/src/crypto/rand/internal.h
+++ b/src/crypto/rand/internal.h
@@ -29,8 +29,9 @@ void CRYPTO_sysrand(uint8_t *buf, size_t len);
 int CRYPTO_have_hwrand(void);
 
 /* CRYPTO_hwrand fills |len| bytes at |buf| with entropy from the hardware.
- * This function can only be called if |CRYPTO_have_hwrand| returns one. */
-void CRYPTO_hwrand(uint8_t *buf, size_t len);
+ * This function can only be called if |CRYPTO_have_hwrand| returns one.
+ * It returns one on success or zero on hardware failure. */
+int CRYPTO_hwrand(uint8_t *buf, size_t len);
 
 
 #if defined(__cplusplus)
diff --git a/src/crypto/rand/rand.c b/src/crypto/rand/rand.c
index a647b6a..a96ac48 100644
--- a/src/crypto/rand/rand.c
+++ b/src/crypto/rand/rand.c
@@ -78,7 +78,8 @@ int RAND_bytes(uint8_t *buf, size_t len) {
     return 1;
   }
 
-  if (!CRYPTO_have_hwrand()) {
+  if (!CRYPTO_have_hwrand() ||
+      !CRYPTO_hwrand(buf, len)) {
     /* Without a hardware RNG to save us from address-space duplication, the OS
      * entropy is used directly. */
     CRYPTO_sysrand(buf, len);
@@ -108,8 +109,6 @@ int RAND_bytes(uint8_t *buf, size_t len) {
     state->partial_block_used = sizeof(state->partial_block);
   }
 
-  CRYPTO_hwrand(buf, len);
-
   if (len >= sizeof(state->partial_block)) {
     size_t remaining = len;
     while (remaining > 0) {