net/netip: allow only valid prefix digits in ParsePrefix

The prefix bits for a call to ParsePrefix are passed raw to
strconv.Atoi, this means that it can accept +- signs as well as leading
zeroes, which are not allowed prefix values following RFC 4632 Section
3.1 and RFC 4291 Section 2.3.

Validate non-digit characters as well as leading zeroes and return an
error accordingly.

Fixes golang#63850

Change-Id: I412a7e1cecc6ee9ea1582d4b04cb40d79ee714f1
GitHub-Last-Rev: 462d97f
GitHub-Pull-Request: golang#63859
Reviewed-on: https://go-review.googlesource.com/c/go/+/538860
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mauri870

src/net/netip/netip.go

@@ -1309,6 +1309,12 @@ func ParsePrefix(s string) (Prefix, error) {
 	}
 
 	bitsStr := s[i+1:]
+
+	// strconv.Atoi accepts a leading sign and leading zeroes, but we don't want that.
+	if len(bitsStr) > 1 && (bitsStr[0] < '1' || bitsStr[0] > '9') {
+		return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): bad bits after slash: " + strconv.Quote(bitsStr))
+	}
+
 	bits, err := strconv.Atoi(bitsStr)
 	if err != nil {
 		return Prefix{}, errors.New("netip.ParsePrefix(" + strconv.Quote(s) + "): bad bits after slash: " + strconv.Quote(bitsStr))

src/net/netip/netip_test.go

@@ -1456,7 +1456,7 @@ func TestParsePrefixError(t *testing.T) {
 		},
 		{
 			prefix: "1.1.1.0/-1",
-			errstr: "out of range",
+			errstr: "bad bits",
 		},
 		{
 			prefix: "1.1.1.0/33",
@@ -1475,6 +1475,22 @@ func TestParsePrefixError(t *testing.T) {
 			prefix: "2001:db8::%a/32",
 			errstr: "zones cannot be present",
 		},
+		{
+			prefix: "1.1.1.0/+32",
+			errstr: "bad bits",
+		},
+		{
+			prefix: "1.1.1.0/-32",
+			errstr: "bad bits",
+		},
+		{
+			prefix: "1.1.1.0/032",
+			errstr: "bad bits",
+		},
+		{
+			prefix: "1.1.1.0/0032",
+			errstr: "bad bits",
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.prefix, func(t *testing.T) {