cmd/go: disallow -lto_library in LDFLAGS

The darwin linker allows setting the LTO library with the -lto_library
flag. This wasn't caught by our "safe linker flags" check because it
was covered by the -lx flag used for linking libraries. This change
adds a specific check for excluded flags which otherwise satisfy our
existing checks.

Loading a mallicious LTO library would allow an attacker to cause the
linker to execute abritrary code when "go build" was called.

Thanks to Juho Forsén of Mattermost for reporting this issue.

Fixes #67119
Fixes CVE-2024-24787

Change-Id: I77ac8585efbdbdfd5f39c39ed623b9408a0f9eaf
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/1380
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/583815
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/work/security.go

@@ -145,6 +145,12 @@ var validCompilerFlagsWithNextArg = []string{
 	"-x",
 }
 
+var invalidLinkerFlags = []*lazyregexp.Regexp{
+	// On macOS this means the linker loads and executes the next argument.
+	// Have to exclude separately because -lfoo is allowed in general.
+	re(`-lto_library`),
+}
+
 var validLinkerFlags = []*lazyregexp.Regexp{
 	re(`-F([^@\-].*)`),
 	re(`-l([^@\-].*)`),
@@ -234,12 +240,12 @@ var validLinkerFlagsWithNextArg = []string{
 
 func checkCompilerFlags(name, source string, list []string) error {
 	checkOverrides := true
-	return checkFlags(name, source, list, validCompilerFlags, validCompilerFlagsWithNextArg, checkOverrides)
+	return checkFlags(name, source, list, nil, validCompilerFlags, validCompilerFlagsWithNextArg, checkOverrides)
 }
 
 func checkLinkerFlags(name, source string, list []string) error {
 	checkOverrides := true
-	return checkFlags(name, source, list, validLinkerFlags, validLinkerFlagsWithNextArg, checkOverrides)
+	return checkFlags(name, source, list, invalidLinkerFlags, validLinkerFlags, validLinkerFlagsWithNextArg, checkOverrides)
 }
 
 // checkCompilerFlagsForInternalLink returns an error if 'list'
@@ -248,7 +254,7 @@ func checkLinkerFlags(name, source string, list []string) error {
 // external linker).
 func checkCompilerFlagsForInternalLink(name, source string, list []string) error {
 	checkOverrides := false
-	if err := checkFlags(name, source, list, validCompilerFlags, validCompilerFlagsWithNextArg, checkOverrides); err != nil {
+	if err := checkFlags(name, source, list, nil, validCompilerFlags, validCompilerFlagsWithNextArg, checkOverrides); err != nil {
 		return err
 	}
 	// Currently the only flag on the allow list that causes problems
@@ -261,7 +267,7 @@ func checkCompilerFlagsForInternalLink(name, source string, list []string) error
 	return nil
 }
 
-func checkFlags(name, source string, list []string, valid []*lazyregexp.Regexp, validNext []string, checkOverrides bool) error {
+func checkFlags(name, source string, list []string, invalid, valid []*lazyregexp.Regexp, validNext []string, checkOverrides bool) error {
 	// Let users override rules with $CGO_CFLAGS_ALLOW, $CGO_CFLAGS_DISALLOW, etc.
 	var (
 		allow    *regexp.Regexp
@@ -293,6 +299,11 @@ Args:
 		if allow != nil && allow.FindString(arg) == arg {
 			continue Args
 		}
+		for _, re := range invalid {
+			if re.FindString(arg) == arg { // must be complete match
+				goto Bad
+			}
+		}
 		for _, re := range valid {
 			if re.FindString(arg) == arg { // must be complete match
 				continue Args

src/cmd/go/testdata/script/darwin_lto_library_ldflag.txt

@@ -0,0 +1,17 @@
+[!GOOS:darwin] skip
+[!cgo] skip
+
+! go build
+stderr 'invalid flag in #cgo LDFLAGS: -lto_library'
+
+-- go.mod --
+module ldflag
+
+-- main.go --
+package main
+
+// #cgo CFLAGS: -flto
+// #cgo LDFLAGS: -lto_library bad.dylib
+import "C"
+
+func main() {}