Reject long inputs in c2i_ASN1_INTEGER.

Thanks to mlbrown for reporting this.

Bug: chromium:942269
Change-Id: Ie06970f25a6ab0e08a8861d604b2177c8fd1d1a8
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35326
Reviewed-by: Adam Langley <agl@google.com>

crypto/asn1/a_int.c

diff --git a/crypto/asn1/a_int.c b/crypto/asn1/a_int.c
index 0522e9f..7b483f2 100644
--- a/crypto/asn1/a_int.c
+++ b/crypto/asn1/a_int.c
@@ -195,6 +195,16 @@
     unsigned char *to, *s;
     int i;
 
+    /*
+     * This function can handle lengths up to INT_MAX - 1, but the rest of the
+     * legacy ASN.1 code mixes integer types, so avoid exposing it to
+     * ASN1_INTEGERS with larger lengths.
+     */
+    if (len < 0 || len > INT_MAX / 2) {
+        OPENSSL_PUT_ERROR(ASN1, ASN1_R_TOO_LONG);
+        return NULL;
+    }
+
     if ((a == NULL) || ((*a) == NULL)) {
         if ((ret = M_ASN1_INTEGER_new()) == NULL)
             return (NULL);