util: optimize key rotation flow for cryptsetup

This patch adds the missing memory limits to cryptsetup `AddKey`.
If this limit is missing `cryptsetup` will determine its own limits
in order to satisfy the KDF, which can lead to serious resource
contentions, IOW, more power the system has, more resources will be
used (this is intentional).

`Open` and `RemoveKey` use the memory limits(set when format/adding) from
the existing headers on the device and hence do not require manual limits.

A limit on active threads is also enforced to run KDF on a single
thread, reducing CPU contention when a lot of processes are active
at the same time.

Additionally, use the `--test-passphrase` with `open` for verification
instead of calling `changeKey` with same passphrase to save
on unnecessary derivations and disk IO (test-passphrase is read only).

Signed-off-by: Niraj Yadav <niryadav@redhat.com>

Author: black-dragon74

internal/util/cryptsetup/cryptsetup.go

@@ -39,6 +39,10 @@ const (
 	// Maximum time to wait for cryptsetup commands to complete.
 	ExecutionTimeout = 2*time.Minute + 30*time.Second
 
+	// Limit PBKDF parallel threads to 1 to reduce CPU contention
+	// when many cryptsetup processes run concurrently.
+	cryptsetupPBKDFParallelLimit = 1
+
 	// Limit memory used by Argon2i PBKDF to 32 MiB.
 	cryptsetupPBKDFMemoryLimit = 32 << 10 // 32768 KiB
 	luks2MetadataSize          = 32 << 7  // 4096 KiB
@@ -580,6 +584,8 @@ func (l *luksWrapper) Format(devicePath, passphrase string, cipherOptions *Encry
 		strconv.Itoa(luks2KeySlotsSize)+"k",
 		"--pbkdf-memory",
 		strconv.Itoa(cryptsetupPBKDFMemoryLimit),
+		"--pbkdf-parallel",
+		strconv.Itoa(cryptsetupPBKDFParallelLimit),
 		devicePath,
 		"-d",
 		"-")
@@ -635,6 +641,10 @@ func (l *luksWrapper) AddKey(devicePath, passphrase, newPassphrase, slot string)
 		"--verbose",
 		"--key-file="+passFile.Name(),
 		"--key-slot="+slot,
+		"--pbkdf-memory",
+		strconv.Itoa(cryptsetupPBKDFMemoryLimit),
+		"--pbkdf-parallel",
+		strconv.Itoa(cryptsetupPBKDFParallelLimit),
 		"luksAddKey",
 		devicePath,
 		newPassFile.Name(),
@@ -720,14 +730,17 @@ func (l *luksWrapper) VerifyKey(devicePath, passphrase, slot string) (bool, erro
 	}
 	defer os.Remove(keyFile.Name()) //nolint:errcheck // failed to delete temp file :-(
 
+	// Use "open --test-passphrase" instead of "luksChangeKey" to avoid
+	// an unnecessary PBKDF write derivation. --test-passphrase only
+	// derives the key once (read-only) to verify it matches the slot.
 	_, stderr, err := l.execCryptsetupCommand(
 		nil,
 		"--verbose",
 		"--key-file="+keyFile.Name(),
 		"--key-slot="+slot,
-		"luksChangeKey",
+		"open",
+		"--test-passphrase",
 		devicePath,
-		keyFile.Name(),
 	)
 	if err != nil {
 		// If the passphrase doesn't match the key in given slot