[release-branch.go1.19] net/url: consistently remove ../ elements in JoinPath

JoinPath would fail to remove relative elements from the start of
the path when the first path element is "".

In addition, JoinPath would return the original path unmodified
when provided with no elements to join, violating the documented
behavior of always cleaning the resulting path.

Correct both these cases.

    JoinPath("http://go.dev", "../go")
    // before: http://go.dev/../go
    // after:  http://go.dev/go

    JoinPath("http://go.dev/../go")
    // before: http://go.dev/../go
    // after:  http://go.dev/go

For #54385.
Fixes #54635.
Fixes CVE-2022-32190.

Change-Id: I6d22cd160d097c50703dd96e4f453c6c118fd5d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/423514
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
(cherry picked from commit 0765da5884adcc8b744979303a36a27092d8fc51)
Reviewed-on: https://go-review.googlesource.com/c/go/+/425357
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 72 insertions, 11 deletions

diff --git a/src/net/url/url.go b/src/net/url/url.go
index e82ae6aeef..d7d2d54a0d 100644
--- a/src/net/url/url.go
+++ b/src/net/url/url.go
@@ -1191,17 +1191,23 @@ func (u *URL) UnmarshalBinary(text []byte) error {
 // any existing path and the resulting path cleaned of any ./ or ../ elements.
 // Any sequences of multiple / characters will be reduced to a single /.
 func (u *URL) JoinPath(elem ...string) *URL {
-	url := *u
-	if len(elem) > 0 {
-		elem = append([]string{u.EscapedPath()}, elem...)
-		p := path.Join(elem...)
-		// path.Join will remove any trailing slashes.
-		// Preserve at least one.
-		if strings.HasSuffix(elem[len(elem)-1], "/") && !strings.HasSuffix(p, "/") {
-			p += "/"
-		}
-		url.setPath(p)
+	elem = append([]string{u.EscapedPath()}, elem...)
+	var p string
+	if !strings.HasPrefix(elem[0], "/") {
+		// Return a relative path if u is relative,
+		// but ensure that it contains no ../ elements.
+		elem[0] = "/" + elem[0]
+		p = path.Join(elem...)[1:]
+	} else {
+		p = path.Join(elem...)
 	}
+	// path.Join will remove any trailing slashes.
+	// Preserve at least one.
+	if strings.HasSuffix(elem[len(elem)-1], "/") && !strings.HasSuffix(p, "/") {
+		p += "/"
+	}
+	url := *u
+	url.setPath(p)
 	return &url
 }
 
diff --git a/src/net/url/url_test.go b/src/net/url/url_test.go
index 263eddffcf..577cf631c8 100644
--- a/src/net/url/url_test.go
+++ b/src/net/url/url_test.go
@@ -2082,6 +2082,26 @@ func TestJoinPath(t *testing.T) {
 		},
 		{
 			base: "https://go.googlesource.com/",
+			elem: []string{"../go"},
+			out:  "https://go.googlesource.com/go",
+		},
+		{
+			base: "https://go.googlesource.com",
+			elem: []string{"../go"},
+			out:  "https://go.googlesource.com/go",
+		},
+		{
+			base: "https://go.googlesource.com",
+			elem: []string{"../go", "../../go", "../../../go"},
+			out:  "https://go.googlesource.com/go",
+		},
+		{
+			base: "https://go.googlesource.com/../go",
+			elem: nil,
+			out:  "https://go.googlesource.com/go",
+		},
+		{
+			base: "https://go.googlesource.com/",
 			elem: []string{"./go"},
 			out:  "https://go.googlesource.com/go",
 		},
@@ -2112,7 +2132,7 @@ func TestJoinPath(t *testing.T) {
 		{
 			base: "https://go.googlesource.com",
 			elem: nil,
-			out:  "https://go.googlesource.com",
+			out:  "https://go.googlesource.com/",
 		},
 		{
 			base: "https://go.googlesource.com/",
@@ -2130,10 +2150,45 @@ func TestJoinPath(t *testing.T) {
 			out:  "https://go.googlesource.com/a%2fb/c%2fd",
 		},
 		{
+			base: "https://go.googlesource.com/a/b",
+			elem: []string{"/go"},
+			out:  "https://go.googlesource.com/a/b/go",
+		},
+		{
 			base: "/",
 			elem: nil,
 			out:  "/",
 		},
+		{
+			base: "a",
+			elem: nil,
+			out:  "a",
+		},
+		{
+			base: "a",
+			elem: []string{"b"},
+			out:  "a/b",
+		},
+		{
+			base: "a",
+			elem: []string{"../b"},
+			out:  "b",
+		},
+		{
+			base: "a",
+			elem: []string{"../../b"},
+			out:  "b",
+		},
+		{
+			base: "",
+			elem: []string{"a"},
+			out:  "a",
+		},
+		{
+			base: "",
+			elem: []string{"../a"},
+			out:  "a",
+		},
 	}
 	for _, tt := range tests {
 		wantErr := "nil"