[PATCH v3 12/16] fscrypt: save session key credentials for extent infos

[Sweet Tea Dorminy <sweettea-kernel@dorminy.me>]

For v1 encryption policies using per-session keys, the thread which
opens the inode and therefore initializes the encryption info is part of
the session, so it can get the key from the session keyring. However,
for extent encryption, the extent infos are likely loaded from a
different thread, which does not have access to the session keyring.
This change saves the credentials of the inode opening thread and reuses
those credentials temporarily when dealing with extent infos, allowing
finding the encryption key correctly.

v1 encryption policies using per-session keys should probably not exist
for new usages such as extent encryption, but this makes more tests
work without change; maybe the right answer is to disallow v1 session
keys plus extent encryption and deal with editing tests to not use v1
session encryption so much.

Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
---
 fs/crypto/fscrypt_private.h |  8 ++++++++
 fs/crypto/keysetup.c        | 14 ++++++++++++++
 fs/crypto/keysetup_v1.c     |  1 +
 3 files changed, 23 insertions(+)

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index c6bf0bd0259a..cd29c71b4349 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -231,6 +231,14 @@ struct fscrypt_info {
 	 */
 	bool ci_inlinecrypt;
 #endif
+	/* Credential struct from the thread which created this info. This is
+	 * only used in v1 session keyrings with extent encryption; it allows
+	 * the thread creating extents for an inode to join the session
+	 * keyring temporarily, since otherwise the thread is usually part of
+	 * kernel writeback and therefore unrelated to the thread with the
+	 * right session key.
+	 */
+	struct cred *ci_session_creds;
 
 	/*
 	 * Encryption mode used for this inode.  It corresponds to either the
diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index 0076c7f2af3d..8d50716bdf11 100644
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -649,6 +649,8 @@ static void put_crypt_info(struct fscrypt_info *ci)
 
 		fscrypt_put_master_key_activeref(ci->ci_sb, mk);
 	}
+	if (ci->ci_session_creds)
+		abort_creds(ci->ci_session_creds);
 	memzero_explicit(ci, sizeof(*ci));
 	kmem_cache_free(fscrypt_info_cachep, ci);
 }
@@ -665,6 +667,7 @@ fscrypt_setup_encryption_info(struct inode *inode,
 	struct fscrypt_master_key *mk = NULL;
 	int res;
 	bool info_for_extent = !!info_ptr;
+	const struct cred *creds = NULL;
 
 	if (!info_ptr)
 		info_ptr = &inode->i_crypt_info;
@@ -707,7 +710,18 @@ fscrypt_setup_encryption_info(struct inode *inode,
 	if (res)
 		goto out;
 
+	if (info_for_extent && inode->i_crypt_info->ci_session_creds) {
+		/*
+		 * The inode this is being created for is using a session key,
+		 * so we have to join this thread to that session temporarily
+		 * in order to be able to find the right key...
+		 */
+		creds = override_creds(inode->i_crypt_info->ci_session_creds);
+	}
+
 	res = fscrypt_setup_file_key(crypt_info, mk);
+	if (creds)
+		revert_creds(creds);
 	if (res)
 		goto out;
 
diff --git a/fs/crypto/keysetup_v1.c b/fs/crypto/keysetup_v1.c
index 41d317f08aeb..4f2be2377dfa 100644
--- a/fs/crypto/keysetup_v1.c
+++ b/fs/crypto/keysetup_v1.c
@@ -318,6 +318,7 @@ int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci)
 		return PTR_ERR(key);
 
 	err = fscrypt_setup_v1_file_key(ci, payload->raw);
+	ci->ci_session_creds = prepare_creds();
 	up_read(&key->sem);
 	key_put(key);
 	return err;
-- 
2.41.0