Avoid potential overflow to the sign bit when shifting left 24 places

Although there are platforms where int is 64 bit, 2GiB large BIGNUMs
instead of 4GiB should be "big enough for everybody".

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11857)

diff --git a/crypto/bn/bn_mpi.c b/crypto/bn/bn_mpi.c
index 504cddffecdec7a43c3abefcdb52b5659c8843e9..d2be44e2bdbe4e6a5848785d6843d1f4bbd0a802 100644 (file)
--- a/crypto/bn/bn_mpi.c
+++ b/crypto/bn/bn_mpi.c
@@ -45,7 +45,7 @@ BIGNUM *BN_mpi2bn(const unsigned char *d, int n, BIGNUM *ain)
     int neg = 0;
     BIGNUM *a = NULL;
 
-    if (n < 4) {
+    if (n < 4 || (d[0] & 0x80) != 0) {
         BNerr(BN_F_BN_MPI2BN, BN_R_INVALID_LENGTH);
         return NULL;
     }