cryptobyte: fix panic due to malformed ASN.1 inputs on 32-bit archs

When int is 32 bits wide (on 32-bit architectures like 386 and arm), an
overflow could occur, causing a panic, due to malformed ASN.1 being
passed to any of the ASN1 methods of String.

Tested on linux/386 and darwin/amd64.

This fixes CVE-2020-7919 and was found thanks to the Project Wycheproof
test vectors.

Change-Id: I8c9696a8bfad1b40ec877cd740dba3467d66ab54
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/645211
Reviewed-by: Katie Hockman <katiehockman@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/216677
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cryptobyte/string.go

@@ -24,7 +24,7 @@ type String []byte
 // read advances a String by n bytes and returns them. If less than n bytes
 // remain, it returns nil.
 func (s *String) read(n int) []byte {
-	if len(*s) < n {
+	if len(*s) < n || n < 0 {
 		return nil
 	}
 	v := (*s)[:n]
@@ -105,11 +105,6 @@ func (s *String) readLengthPrefixed(lenLen int, outChild *String) bool {
 		length = length << 8
 		length = length | uint32(b)
 	}
-	if int(length) < 0 {
-		// This currently cannot overflow because we read uint24 at most, but check
-		// anyway in case that changes in the future.
-		return false
-	}
 	v := s.read(int(length))
 	if v == nil {
 		return false