[release-branch.go1.25] cmd/go: disallow cgo trust boundary bypass

The cgo compiler implicitly trusts generated files
with 'cgo' prefixes; thus, SWIG files containing 'cgo'
in their names will cause bypass of the trust boundary,
leading to code smuggling or arbitrary code execution.

The cgo compiler will now produce an error if it
encounters any SWIG files containing this prefix.

Thanks to Juho Forsén of Mattermost for reporting this issue.

Fixes #78335
Fixes CVE-2026-27140

Change-Id: I44185a84e07739b3b347efdb86be7d8fa560b030
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3520
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3989
Reviewed-on: https://go-review.googlesource.com/c/go/+/763556
Reviewed-by: David Chase <drchase@google.com>
TryBot-Bypass: Gopher Robot <gobot@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/work/exec.go

diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index 6bfc83a..8c3bac5 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -3231,6 +3231,10 @@
 
 // Run SWIG on one SWIG input file.
 func (b *Builder) swigOne(a *Action, file, objdir string, pcCFLAGS []string, cxx bool, intgosize string) (outGo, outC string, err error) {
+	if strings.HasPrefix(file, "cgo") {
+		return "", "", errors.New("SWIG file must not use prefix 'cgo'")
+	}
+
 	p := a.Package
 	sh := b.Shell(a)