Fix off-by-one in BN_rand

If BN_rand is called with |bits| set to 1 and |top| set to 1 then a 1 byte
buffer overflow can occur. There are no such instances within the OpenSSL at
the moment.

Thanks to Mateusz Kocielski (LogicalTrust), Marek Kroemeke, Filip Palian for
discovering and reporting this issue.
Reviewed-by: NKurt Roeckx <kurt@openssl.org>

crypto/bn/bn_rand.c

@@ -122,6 +122,11 @@ static int bnrand(int pseudorand, BIGNUM *rnd, int bits, int top, int bottom)
     int ret = 0, bit, bytes, mask;
     time_t tim;
 
+    if (bits < 0 || (bits == 1 && top > 0)) {
+        BNerr(BN_F_BNRAND, BN_R_BITS_TOO_SMALL);
+        return 0;
+    }
+
     if (bits == 0) {
         BN_zero(rnd);
         return 1;
@@ -168,7 +173,7 @@ static int bnrand(int pseudorand, BIGNUM *rnd, int bits, int top, int bottom)
         }
     }
 
-    if (top != -1) {
+    if (top >= 0) {
         if (top) {
             if (bit == 0) {
                 buf[0] = 1;

doc/crypto/BN_rand.pod

@@ -24,7 +24,8 @@ most significant bit of the random number can be zero. If B<top> is 0,
 it is set to 1, and if B<top> is 1, the two most significant bits of
 the number will be set to 1, so that the product of two such random
 numbers will always have 2*B<bits> length.  If B<bottom> is true, the
-number will be odd.
+number will be odd. The value of B<bits> must be zero or greater. If B<bits> is
+1 then B<top> cannot also be 1.
 
 BN_pseudo_rand() does the same, but pseudo-random numbers generated by
 this function are not necessarily unpredictable. They can be used for