Re: [RFC PATCH v3 04/18] fs: add ->s_master_keys to struct super_block

From: Eric Biggers <redacted>

Add an ->s_master_keys keyring to 'struct super_block'.  New fscrypt
ioctls will allow adding and removing encryption keys from this keyring.
This will enable solving multiple interrelated problems with how fscrypt
keys are provided and managed currently, including:

- Making the key status (which is currently per-process) match the
  filesystem-level status of which encrypted files are "unlocked".

- Supporting a proper API to remove encryption keys, "locking" the
  corresponding encrypted files.

- Caching an HMAC transform for each master key, allowing the use of
  HKDF while still retaining good performance.

- Preventing denial of service via keyctl_invalidate().

Similar to the existing ->s_cop, the keyring is added to the VFS-level
superblock struct rather than separately to the ext4, f2fs, and ubifs
superblock structs so that it can be used by the shared code in
fs/crypto/.  To minimize overhead, the keyring will only be allocated if
userspace actually adds a key; otherwise will stay NULL.

Signed-off-by: Eric Biggers <redacted>
---
 fs/super.c         | 3 +++
 include/linux/fs.h | 1 +
 2 files changed, 4 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 48e25eba8465..7ca05dda905c 100644
--- a/fs/super.c
+++ b/fs/super.c

@@ -291,6 +291,9 @@ static void __put_super(struct super_block *s)
                security_sb_free(s);
                put_user_ns(s->s_user_ns);
                kfree(s->s_subtype);
+#ifdef CONFIG_FS_ENCRYPTION
+               key_put(s->s_master_keys);
+#endif