runtime/secret: aggregated heap allocations can keep secret values alive longer than expected

[DanielMorsing]

$ go version
go version go1.26-devel_0d0d5c9a82 Fri Dec 5 17:31:30 2025 -0800 linux/amd64
$ cat secret.go
package main

import (
	"fmt"
	"math"
	"runtime"
	"runtime/secret"
	"unsafe"
)

var (
	xa *int
	xb *int
)

var addr uintptr

func main() {
	secret.Do(func() {
		a := math.MaxInt
		b := math.MaxInt
		xa = &a
		xb = &b
		addr = uintptr(unsafe.Pointer(&a))
	})
	xa = nil
	runtime.GC()
	x := *(*int)(unsafe.Pointer(addr))
	fmt.Printf("xa value after gc: %x\n", x)
	xb = nil
	runtime.GC()
	x = *(*int)(unsafe.Pointer(addr))
	fmt.Printf("xa value after xb unreachable: %x\n", x)
}
$ GOEXPERIMENT=runtimesecret go run secret.go
xa value after gc: 7fffffffffffffff
xa value after xb unreachable: 0

In this case, a secret value that should have been zeroed is kept "alive" by an adjacent value whos storage is aggregated together with it by the compiler (see cmd/compile/internal/ssagen/ssa.go:/newHeapaddr/)

I'm not sure what the right answer is here. This is a contrived example, but it does break the guarantees set out by the secret package. I think we can either:

1. ignore it and document that secret is best effort only and hope that users don't write code like this
2. make a new allocator function that knows about the structure of these aggregated values that will de-aggregate when running in secret mode.
3. disable this optimization. This would have to be globally since functions cannot know when they are running in secret mode.

[prattmic]

Is this just #76356 as it applies to runtime/secret, or is there more here?

cc @golang/runtime

[prattmic]

In this case, a secret value that should have been zeroed is kept "alive" by an adjacent value whos storage is aggregated together with it by the compiler (see cmd/compile/internal/ssagen/ssa.go:/newHeapaddr/)

Ah, I see, this is about aggregated heap allocations.

Though I imagine tiny allocations affect secret mode as well?

[DanielMorsing]

Tiny allocations are handled specially when we are in secret mode https://cs.opensource.google/go/go/+/master:src/runtime/malloc_stubs.go;l=61

[gopherbot]

Change https://go.dev/cl/728921 mentions this issue: runtime/secret: warn users about allocations, loosen gurantees.