Read secret environment variables once per command, then unset them (closes #60)

init and vault create put the mnemonic into the process environment for
vault.CreateVault to read back, so every program they ran, gpg included,
inherited it, and SB_SECRET_MNEMONIC and SB_UNLOCK_PASSPHRASE were read
at 13 places and never unset. Each command that may need them now reads
both once, in its RunE, into locked buffers on the CLI Instance, and
unsets them at once. The buffers are passed down: vault.CreateVault
takes the mnemonic, a Vault carries Mnemonic and UnlockPassphrase, and
the PGP, keychain and Secure Enclave unlocker constructors take both;
CreatePGPUnlocker sets them on the vault it loads through SetMnemonic
and SetUnlockPassphrase, new in VaultInterface. README warns against
both variables.

Model: opus-5-5

internal/cli/cli.go

@@ -3,8 +3,10 @@ package cli
 
 import (
 	"fmt"
+	"os"
 
 	"git.eeqj.de/sneak/secret/internal/secret"
+	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
 	"github.com/spf13/cobra"
 )
@@ -14,6 +16,11 @@ type Instance struct {
 	fs       afero.Fs
 	stateDir string
 	cmd      *cobra.Command
+	// Mnemonic and UnlockPassphrase hold the values of SB_SECRET_MNEMONIC
+	// and SB_UNLOCK_PASSPHRASE that readSecretEnv read, or nil when it found
+	// none.
+	Mnemonic         *memguard.LockedBuffer
+	UnlockPassphrase *memguard.LockedBuffer
 }
 
 // NewCLIInstance creates a new CLI instance with the real filesystem
@@ -68,3 +75,43 @@ func (cli *Instance) SetStateDir(stateDir string) {
 func (cli *Instance) GetStateDir() string {
 	return cli.stateDir
 }
+
+// readSecretEnv reads SB_SECRET_MNEMONIC into cli.Mnemonic and
+// SB_UNLOCK_PASSPHRASE into cli.UnlockPassphrase. A command that may need
+// either calls it once, before anything else, and passes the buffers on
+// from there: each variable is unset as soon as it is read, so that the
+// processes this one starts, gpg among them, do not inherit it, and a
+// second read would find nothing. The returned function destroys both
+// buffers.
+func (cli *Instance) readSecretEnv() func() {
+	cli.Mnemonic = readAndUnsetEnv(secret.EnvMnemonic)
+	cli.UnlockPassphrase = readAndUnsetEnv(secret.EnvUnlockPassphrase)
+
+	mnemonic, passphrase := cli.Mnemonic, cli.UnlockPassphrase
+
+	return func() {
+		if mnemonic != nil {
+			mnemonic.Destroy()
+		}
+
+		if passphrase != nil {
+			passphrase.Destroy()
+		}
+	}
+}
+
+// readAndUnsetEnv returns the value of the environment variable name in a
+// locked buffer, or nil when it is unset or empty, and unsets the variable.
+// Unsetting does not erase the value: it stays in this process's memory,
+// and in /proc/<pid>/environ, which shows the environment the process
+// started with. The caller must destroy the returned buffer.
+func readAndUnsetEnv(name string) *memguard.LockedBuffer {
+	value := os.Getenv(name)
+	_ = os.Unsetenv(name)
+
+	if value == "" {
+		return nil
+	}
+
+	return memguard.NewBufferFromBytes([]byte(value))
+}