fscrypt: stop using keyrings subsystem for fscrypt_master_key

The approach of fs/crypto/ internally managing the fscrypt_master_key
structs as the payloads of "struct key" objects contained in a
"struct key" keyring has outlived its usefulness.  The original idea was
to simplify the code by reusing code from the keyrings subsystem.
However, several issues have arisen that can't easily be resolved:

- When a master key struct is destroyed, blk_crypto_evict_key() must be
  called on any per-mode keys embedded in it.  (This started being the
  case when inline encryption support was added.)  Yet, the keyrings
  subsystem can arbitrarily delay the destruction of keys, even past the
  time the filesystem was unmounted.  Therefore, currently there is no
  easy way to call blk_crypto_evict_key() when a master key is
  destroyed.  Currently, this is worked around by holding an extra
  reference to the filesystem's request_queue(s).  But it was overlooked
  that the request_queue reference is *not* guaranteed to pin the
  corresponding blk_crypto_profile too; for device-mapper devices that
  support inline crypto, it doesn't.  This can cause a use-after-free.

- When the last inode that was using an incompletely-removed master key
  is evicted, the master key removal is completed by removing the key
  struct from the keyring.  Currently this is done via key_invalidate().
  Yet, key_invalidate() takes the key semaphore.  This can deadlock when
  called from the shrinker, since in fscrypt_ioctl_add_key(), memory is
  allocated with GFP_KERNEL under the same semaphore.

- More generally, the fact that the keyrings subsystem can arbitrarily
  delay the destruction of keys (via garbage collection delay, or via
  random processes getting temporary key references) is undesirable, as
  it means we can't strictly guarantee that all secrets are ever wiped.

- Doing the master key lookups via the keyrings subsystem results in the
  key_permission LSM hook being called.  fscrypt doesn't want this, as
  all access control for encrypted files is designed to happen via the
  files themselves, like any other files.  The workaround which SELinux
  users are using is to change their SELinux policy to grant key search
  access to all domains.  This works, but it is an odd extra step that
  shouldn't really have to be done.

The fix for all these issues is to change the implementation to what I
should have done originally: don't use the keyrings subsystem to keep
track of the filesystem's fscrypt_master_key structs.  Instead, just
store them in a regular kernel data structure, and rework the reference
counting, locking, and lifetime accordingly.  Retain support for
RCU-mode key lookups by using a hash table.  Replace fscrypt_sb_free()
with fscrypt_sb_delete(), which releases the keys synchronously and runs
a bit earlier during unmount, so that block devices are still available.

A side effect of this patch is that neither the master keys themselves
nor the filesystem keyrings will be listed in /proc/keys anymore.
("Master key users" and the master key users keyrings will still be
listed.)  However, this was mostly an implementation detail, and it was
intended just for debugging purposes.  I don't know of anyone using it.

This patch does *not* change how "master key users" (->mk_users) works;
that still uses the keyrings subsystem.  That is still needed for key
quotas, and changing that isn't necessary to solve the issues listed
above.  If we decide to change that too, it would be a separate patch.

I've marked this as fixing the original commit that added the fscrypt
keyring, but as noted above the most important issue that this patch
fixes wasn't introduced until the addition of inline encryption support.

Fixes: 22d94f4 ("fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl")
Signed-off-by: Eric Biggers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

fs/crypto/fscrypt_private.h

@@ -225,7 +225,7 @@ struct fscrypt_info {
 	 * will be NULL if the master key was found in a process-subscribed
 	 * keyring rather than in the filesystem-level keyring.
 	 */
-	struct key *ci_master_key;
+	struct fscrypt_master_key *ci_master_key;
 
 	/*
 	 * Link in list of inodes that were unlocked with the master key.
@@ -436,6 +436,40 @@ struct fscrypt_master_key_secret {
  */
 struct fscrypt_master_key {
 
+	/*
+	 * Back-pointer to the super_block of the filesystem to which this
+	 * master key has been added.  Only valid if ->mk_active_refs > 0.
+	 */
+	struct super_block			*mk_sb;
+
+	/*
+	 * Link in ->mk_sb->s_master_keys->key_hashtable.
+	 * Only valid if ->mk_active_refs > 0.
+	 */
+	struct hlist_node			mk_node;
+
+	/* Semaphore that protects ->mk_secret and ->mk_users */
+	struct rw_semaphore			mk_sem;
+
+	/*
+	 * Active and structural reference counts.  An active ref guarantees
+	 * that the struct continues to exist, continues to be in the keyring
+	 * ->mk_sb->s_master_keys, and that any embedded subkeys (e.g.
+	 * ->mk_direct_keys) that have been prepared continue to exist.
+	 * A structural ref only guarantees that the struct continues to exist.
+	 *
+	 * There is one active ref associated with ->mk_secret being present,
+	 * and one active ref for each inode in ->mk_decrypted_inodes.
+	 *
+	 * There is one structural ref associated with the active refcount being
+	 * nonzero.  Finding a key in the keyring also takes a structural ref,
+	 * which is then held temporarily while the key is operated on.
+	 */
+	refcount_t				mk_active_refs;
+	refcount_t				mk_struct_refs;
+
+	struct rcu_head				mk_rcu_head;
+
 	/*
 	 * The secret key material.  After FS_IOC_REMOVE_ENCRYPTION_KEY is
 	 * executed, this is wiped and no new inodes can be unlocked with this
@@ -444,7 +478,10 @@ struct fscrypt_master_key {
 	 * FS_IOC_REMOVE_ENCRYPTION_KEY can be retried, or
 	 * FS_IOC_ADD_ENCRYPTION_KEY can add the secret again.
 	 *
-	 * Locking: protected by this master key's key->sem.
+	 * While ->mk_secret is present, one ref in ->mk_active_refs is held.
+	 *
+	 * Locking: protected by ->mk_sem.  The manipulation of ->mk_active_refs
+	 *	    associated with this field is protected by ->mk_sem as well.
 	 */
 	struct fscrypt_master_key_secret	mk_secret;
 
@@ -465,22 +502,12 @@ struct fscrypt_master_key {
 	 *
 	 * This is NULL for v1 policy keys; those can only be added by root.
 	 *
-	 * Locking: in addition to this keyring's own semaphore, this is
-	 * protected by this master key's key->sem, so we can do atomic
-	 * search+insert.  It can also be searched without taking any locks, but
-	 * in that case the returned key may have already been removed.
+	 * Locking: protected by ->mk_sem.  (We don't just rely on the keyrings
+	 * subsystem semaphore ->mk_users->sem, as we need support for atomic
+	 * search+insert along with proper synchronization with ->mk_secret.)
 	 */
 	struct key		*mk_users;
 
-	/*
-	 * Length of ->mk_decrypted_inodes, plus one if mk_secret is present.
-	 * Once this goes to 0, the master key is removed from ->s_master_keys.
-	 * The 'struct fscrypt_master_key' will continue to live as long as the
-	 * 'struct key' whose payload it is, but we won't let this reference
-	 * count rise again.
-	 */
-	refcount_t		mk_refcount;
-
 	/*
 	 * List of inodes that were unlocked using this key.  This allows the
 	 * inodes to be evicted efficiently if the key is removed.
@@ -506,10 +533,10 @@ static inline bool
 is_master_key_secret_present(const struct fscrypt_master_key_secret *secret)
 {
 	/*
-	 * The READ_ONCE() is only necessary for fscrypt_drop_inode() and
-	 * fscrypt_key_describe().  These run in atomic context, so they can't
-	 * take the key semaphore and thus 'secret' can change concurrently
-	 * which would be a data race.  But they only need to know whether the
+	 * The READ_ONCE() is only necessary for fscrypt_drop_inode().
+	 * fscrypt_drop_inode() runs in atomic context, so it can't take the key
+	 * semaphore and thus 'secret' can change concurrently which would be a
+	 * data race.  But fscrypt_drop_inode() only need to know whether the
 	 * secret *was* present at the time of check, so READ_ONCE() suffices.
 	 */
 	return READ_ONCE(secret->size) != 0;
@@ -538,7 +565,11 @@ static inline int master_key_spec_len(const struct fscrypt_key_specifier *spec)
 	return 0;
 }
 
-struct key *
+void fscrypt_put_master_key(struct fscrypt_master_key *mk);
+
+void fscrypt_put_master_key_activeref(struct fscrypt_master_key *mk);
+
+struct fscrypt_master_key *
 fscrypt_find_master_key(struct super_block *sb,
 			const struct fscrypt_key_specifier *mk_spec);
 

fs/crypto/hooks.c

@@ -5,8 +5,6 @@
  * Encryption hooks for higher-level filesystem operations.
  */
 
-#include <linux/key.h>
-
 #include "fscrypt_private.h"
 
 /**
@@ -142,7 +140,6 @@ int fscrypt_prepare_setflags(struct inode *inode,
 			     unsigned int oldflags, unsigned int flags)
 {
 	struct fscrypt_info *ci;
-	struct key *key;
 	struct fscrypt_master_key *mk;
 	int err;
 
@@ -158,14 +155,13 @@ int fscrypt_prepare_setflags(struct inode *inode,
 		ci = inode->i_crypt_info;
 		if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
 			return -EINVAL;
-		key = ci->ci_master_key;
-		mk = key->payload.data[0];
-		down_read(&key->sem);
+		mk = ci->ci_master_key;
+		down_read(&mk->mk_sem);
 		if (is_master_key_secret_present(&mk->mk_secret))
 			err = fscrypt_derive_dirhash_key(ci, mk);
 		else
 			err = -ENOKEY;
-		up_read(&key->sem);
+		up_read(&mk->mk_sem);
 		return err;
 	}
 	return 0;