pam_fscrypt: decide cache dropping behavior automatically

Configuring whether pam_fscrypt drops caches or not isn't really
something the user should have to do, and it's also irrelevant for v2
encryption policies (the default on newer systems).  It's better to have
pam_fscrypt automatically decide whether it needs to drop caches or not.

Do this by making pam_fscrypt check whether any encryption policy keys
are being removed from a user keyring (rather than from a filesystem
keyring).  If so, it drops caches; otherwise it doesn't.  This
supersedes the "drop_caches" option, which won't do anything anymore.

1 files changed, 29 insertions, 13 deletions

diff --git a/pam_fscrypt/pam_fscrypt.go b/pam_fscrypt/pam_fscrypt.go
index a7582cc..195ba43 100644
--- a/pam_fscrypt/pam_fscrypt.go
+++ b/pam_fscrypt/pam_fscrypt.go
@@ -48,7 +48,12 @@ const (
 	// These flags are used to toggle behavior of the PAM module.
 	debugFlag = "debug"
 	lockFlag  = "lock_policies"
-	cacheFlag = "drop_caches"
+
+	// This option is accepted for compatibility with existing config files,
+	// but it no longer does anything.  pam_fscrypt now drops caches if and
+	// only if it is needed.  (Usually it is not needed anymore, as the
+	// FS_IOC_REMOVE_ENCRYPTION_KEY ioctl handles this automatically.)
+	dropCachesFlag = "drop_caches"
 )
 
 var (
@@ -213,15 +218,20 @@ func CloseSession(handle *pam.Handle, args map[string]bool) error {
 		return err
 	}
 
+	if args[dropCachesFlag] {
+		log.Print("ignoring deprecated 'drop_caches' option (now auto-detected)")
+	}
+
+	needDropCaches := false
 	var errLock, errCache error
 	// Don't automatically drop privileges, since we may need them to
 	// deprovision policies or to drop caches.
 	if args[lockFlag] {
 		log.Print("locking polices protected with login protector")
-		errLock = lockLoginPolicies(handle)
+		needDropCaches, errLock = lockLoginPolicies(handle)
 	}
 
-	if args[cacheFlag] {
+	if needDropCaches {
 		log.Print("dropping appropriate filesystem caches at session close")
 		errCache = security.DropFilesystemCache()
 	}
@@ -232,11 +242,14 @@ func CloseSession(handle *pam.Handle, args map[string]bool) error {
 	return errCache
 }
 
-// lockLoginPolicies deprovisions all policy keys that are protected by
-// the user's login protector.
-func lockLoginPolicies(handle *pam.Handle) error {
+// lockLoginPolicies deprovisions all policy keys that are protected by the
+// user's login protector.  It returns true if dropping filesystem caches will
+// be needed afterwards to completely lock the relevant directories.
+func lockLoginPolicies(handle *pam.Handle) (bool, error) {
+	needDropCaches := false
+
 	if err := handle.StartAsPamUser(); err != nil {
-		return err
+		return needDropCaches, err
 	}
 	defer handle.StopAsPamUser()
 
@@ -244,16 +257,16 @@ func lockLoginPolicies(handle *pam.Handle) error {
 	protector, err := loginProtector(handle)
 	if err != nil {
 		log.Printf("nothing to lock: %s", err)
-		return nil
+		return needDropCaches, nil
 	}
 	policies := policiesUsingProtector(protector)
 	if len(policies) == 0 {
 		log.Print("no policies to lock")
-		return nil
+		return needDropCaches, nil
 	}
 
 	if err = setupUserKeyringIfNeeded(handle, policies); err != nil {
-		return errors.Wrapf(err, "setting up user keyring")
+		return needDropCaches, errors.Wrapf(err, "setting up user keyring")
 	}
 
 	// We will try to deprovision all of the policies.
@@ -263,12 +276,15 @@ func lockLoginPolicies(handle *pam.Handle) error {
 				policy.Descriptor(), handle.PamUser.Username)
 			continue
 		}
+		if policy.NeedsUserKeyring() {
+			needDropCaches = true
+		}
 		if err := beginProvisioningOp(handle, policy); err != nil {
-			return err
+			return needDropCaches, err
 		}
 		deprovisionErr := policy.Deprovision(false)
 		if err := endProvisioningOp(handle, policy); err != nil {
-			return err
+			return needDropCaches, err
 		}
 		if deprovisionErr != nil {
 			log.Printf("deprovisioning policy %s: %s", policy.Descriptor(), deprovisionErr)
@@ -276,7 +292,7 @@ func lockLoginPolicies(handle *pam.Handle) error {
 		}
 		log.Printf("policy %s deprovisioned by %v", policy.Descriptor(), handle.PamUser.Username)
 	}
-	return nil
+	return needDropCaches, nil
 }
 
 // Chauthtok rewraps the login protector when the passphrase changes.