Rewrite BN_bn2dec.

This is a more complete fix for CVE-2016-2182. The original commit
message was:
"If an oversize BIGNUM is presented to BN_bn2dec() it can cause
BN_div_word() to fail and not reduce the value of 't' resulting
in OOB writes to the bn_data buffer and eventually crashing.

Fix by checking return value of BN_div_word() and checking writes
don't overflow buffer.

Thanks to Shi Lei for reporting this bug."

BoringSSL's rewrite commit message:
"958aaf1ea1b481e8ef32970d5b0add80504be4b2, imported from upstream, had
an off-by-one error. Reproducing the failure is fairly easy as it can't
even serialize 1. See also upstream's
099e2968ed3c7d256cda048995626664082b1b30.

Rewrite the function completely with CBB and add a basic test.

BUG=chromium:639740"

CVE-2016-2182

Change-Id: I41a91514c4bb9e83854824ed5258ffe4e49d9491
Bug: 32096880
(cherry picked from commit 29b92ab938c1a17d4d1b3b039042a0f499f58b5d)
(cherry picked from commit 54bf62a81586d99d0a951ca3342d569b59e69b80
 with adaptations from <sultanxda@gmail.com>)

1 files changed, 42 insertions, 0 deletions

diff --git a/src/crypto/bn/bn_test.cc b/src/crypto/bn/bn_test.cc
index 6a7d48c..95b7bbb 100644
--- a/src/crypto/bn/bn_test.cc
+++ b/src/crypto/bn/bn_test.cc
@@ -122,6 +122,7 @@ static bool test_dec2bn(FILE *fp, BN_CTX *ctx);
 static bool test_hex2bn(FILE *fp, BN_CTX *ctx);
 static bool test_asc2bn(FILE *fp, BN_CTX *ctx);
 static bool test_rand();
+static bool TestBN2Dec();
 
 static const uint8_t kSample[] =
     "\xC6\x4F\x43\x04\x2A\xEA\xCA\x6E\x58\x36\x80\x5B\xE8\xC9"
@@ -341,6 +342,12 @@ int main(int argc, char *argv[]) {
   }
   flush_fp(bc_file.get());
 
+  message(bc_file.get(), "BN_bn2dec");
+  if (!TestBN2Dec()) {
+    return 1;
+  }
+  flush_fp(bc_file.get());
+
   printf("PASS\n");
   return 0;
 }
@@ -1628,3 +1635,38 @@ static bool test_rand() {
 
   return true;
 }
+
+static bool TestBN2Dec() {
+  static const char *kBN2DecTests[] = {
+      "0",
+      "1",
+      "-1",
+      "100",
+      "-100",
+      "123456789012345678901234567890",
+      "-123456789012345678901234567890",
+      "123456789012345678901234567890123456789012345678901234567890",
+      "-123456789012345678901234567890123456789012345678901234567890",
+  };
+
+  for (const char *test : kBN2DecTests) {
+    ScopedBIGNUM bn;
+    int ret = DecimalToBIGNUM(&bn, test);
+    if (ret == 0) {
+      return false;
+    }
+
+    ScopedOpenSSLString dec(BN_bn2dec(bn.get()));
+    if (!dec) {
+      fprintf(stderr, "BN_bn2dec failed on %s.\n", test);
+      return false;
+    }
+
+    if (strcmp(dec.get(), test) != 0) {
+      fprintf(stderr, "BN_bn2dec gave %s, wanted %s.\n", dec.get(), test);
+      return false;
+    }
+  }
+
+  return true;
+}