[release-branch.go1.19] path/filepath: do not Clean("a/../c:/b") into c:\b on Windows

Do not permit Clean to convert a relative path into one starting
with a drive reference. This change causes Clean to insert a .
path element at the start of a path when the original path does not
start with a volume name, and the first path element would contain
a colon.

This may introduce a spurious but harmless . path element under
some circumstances. For example, Clean("a/../b:/../c") becomes `.\c`.

This reverts CL 401595, since the change here supersedes the one
in that CL.

Thanks to RyotaK (https://twitter.com/ryotkak) for reporting this issue.

Updates #57274
Fixes #57275
Fixes CVE-2022-41722

Change-Id: I837446285a03aa74c79d7642720e01f354c2ca17
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1675249
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Julie Qiu <julieqiu@google.com>
TryBot-Result: Security TryBots <security-trybots@go-security-trybots.iam.gserviceaccount.com>
(cherry picked from commit 780dfa043ff5192c37de0d6fd1053a66b2b9f378)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1728206
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/468115
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Michael Pratt <mpratt@google.com>

3 files changed, 22 insertions, 14 deletions

diff --git a/src/path/filepath/path.go b/src/path/filepath/path.go
index de7a2c758b..9b1f5ed7c0 100644
--- a/src/path/filepath/path.go
+++ b/src/path/filepath/path.go
@@ -15,6 +15,7 @@ import (
 	"errors"
 	"io/fs"
 	"os"
+	"runtime"
 	"sort"
 	"strings"
 )
@@ -117,21 +118,9 @@ func Clean(path string) string {
 		case os.IsPathSeparator(path[r]):
 			// empty path element
 			r++
-		case path[r] == '.' && r+1 == n:
+		case path[r] == '.' && (r+1 == n || os.IsPathSeparator(path[r+1])):
 			// . element
 			r++
-		case path[r] == '.' && os.IsPathSeparator(path[r+1]):
-			// ./ element
-			r++
-
-			for r < len(path) && os.IsPathSeparator(path[r]) {
-				r++
-			}
-			if out.w == 0 && volumeNameLen(path[r:]) > 0 {
-				// When joining prefix "." and an absolute path on Windows,
-				// the prefix should not be removed.
-				out.append('.')
-			}
 		case path[r] == '.' && path[r+1] == '.' && (r+2 == n || os.IsPathSeparator(path[r+2])):
 			// .. element: remove to last separator
 			r += 2
@@ -157,6 +146,18 @@ func Clean(path string) string {
 			if rooted && out.w != 1 || !rooted && out.w != 0 {
 				out.append(Separator)
 			}
+			// If a ':' appears in the path element at the start of a Windows path,
+			// insert a .\ at the beginning to avoid converting relative paths
+			// like a/../c: into c:.
+			if runtime.GOOS == "windows" && out.w == 0 && out.volLen == 0 && r != 0 {
+				for i := r; i < n && !os.IsPathSeparator(path[i]); i++ {
+					if path[i] == ':' {
+						out.append('.')
+						out.append(Separator)
+						break
+					}
+				}
+			}
 			// copy element
 			for ; r < n && !os.IsPathSeparator(path[r]); r++ {
 				out.append(path[r])
diff --git a/src/path/filepath/path_test.go b/src/path/filepath/path_test.go
index a783d6be28..9a57920dd7 100644
--- a/src/path/filepath/path_test.go
+++ b/src/path/filepath/path_test.go
@@ -96,6 +96,13 @@ var wincleantests = []PathTest{
 	{`.\c:`, `.\c:`},
 	{`.\c:\foo`, `.\c:\foo`},
 	{`.\c:foo`, `.\c:foo`},
+
+	// Don't allow cleaning to move an element with a colon to the start of the path.
+	{`a/../c:`, `.\c:`},
+	{`a\..\c:`, `.\c:`},
+	{`a/../c:/a`, `.\c:\a`},
+	{`a/../../c:`, `..\c:`},
+	{`foo:bar`, `foo:bar`},
 }
 
 func TestClean(t *testing.T) {
diff --git a/src/path/filepath/path_windows_test.go b/src/path/filepath/path_windows_test.go
index 9e6c0ec81d..857f4d5c7c 100644
--- a/src/path/filepath/path_windows_test.go
+++ b/src/path/filepath/path_windows_test.go
@@ -542,7 +542,7 @@ func TestIssue52476(t *testing.T) {
 	}{
 		{`..\.`, `C:`, `..\C:`},
 		{`..`, `C:`, `..\C:`},
-		{`.`, `:`, `:`},
+		{`.`, `:`, `.\:`},
 		{`.`, `C:`, `.\C:`},
 		{`.`, `C:/a/b/../c`, `.\C:\a\c`},
 		{`.`, `\C:`, `.\C:`},