fscrypt: simplify master key locking

The stated reasons for separating fscrypt_master_key::mk_secret_sem from
the standard semaphore contained in every 'struct key' no longer apply.

First, due to commit a992b20cd4ee ("fscrypt: add
fscrypt_prepare_new_inode() and fscrypt_set_context()"),
fscrypt_get_encryption_info() is no longer called from within a
filesystem transaction.

Second, due to commit d3ec10aa9581 ("KEYS: Don't write out to userspace
while holding key semaphore"), the semaphore for the "keyring" key type
no longer ranks above page faults.

That leaves performance as the only possible reason to keep the separate
mk_secret_sem.  Specifically, having mk_secret_sem reduces the
contention between setup_file_encryption_key() and
FS_IOC_{ADD,REMOVE}_ENCRYPTION_KEY.  However, these ioctls aren't
executed often, so this doesn't seem to be worth the extra complexity.

Therefore, simplify the locking design by just using key->sem instead of
mk_secret_sem.

Link: https://lore.kernel.org/r/20201117032626.320275-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 1 insertions, 7 deletions

diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
index d7ec52cb3d9a..0b3ffbb4faf4 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -347,7 +347,6 @@ static int add_new_master_key(struct fscrypt_master_key_secret *secret,
 	mk->mk_spec = *mk_spec;
 
 	move_master_key_secret(&mk->mk_secret, secret);
-	init_rwsem(&mk->mk_secret_sem);
 
 	refcount_set(&mk->mk_refcount, 1); /* secret is present */
 	INIT_LIST_HEAD(&mk->mk_decrypted_inodes);
@@ -427,11 +426,8 @@ static int add_existing_master_key(struct fscrypt_master_key *mk,
 	}
 
 	/* Re-add the secret if needed. */
-	if (rekey) {
-		down_write(&mk->mk_secret_sem);
+	if (rekey)
 		move_master_key_secret(&mk->mk_secret, secret);
-		up_write(&mk->mk_secret_sem);
-	}
 	return 0;
 }
 
@@ -975,10 +971,8 @@ static int do_remove_key(struct file *filp, void __user *_uarg, bool all_users)
 	/* No user claims remaining.  Go ahead and wipe the secret. */
 	dead = false;
 	if (is_master_key_secret_present(&mk->mk_secret)) {
-		down_write(&mk->mk_secret_sem);
 		wipe_master_key_secret(&mk->mk_secret);
 		dead = refcount_dec_and_test(&mk->mk_refcount);
-		up_write(&mk->mk_secret_sem);
 	}
 	up_write(&key->sem);
 	if (dead) {