[PATCH] ksmbd: refuse reauthentication of an expired session as another user

[Michail Andreas Michalopoulos <mike.michalo@gmail.com>]

When a Kerberos session expires, its owner can reauthenticate it. For an
NTLM reauthentication, ntlm_authenticate() only compares the new user with
sess->user when the session is still valid. An expired session is in
SMB2_SESSION_IN_PROGRESS during the reauthentication, so it takes the
other branch and simply sets sess->user to the new user:

- the session's open files, tree connects and other state, created under
  the original user's identity, now belong to whichever user just
  authenticated, and a later Kerberos reauthentication is compared
  against that user rather than the original one;
- the original struct ksmbd_user is overwritten without being freed.

krb5_authenticate() already refuses a different user in this case, and
the valid-session branch of ntlm_authenticate() does too. Do the same for
an expired session: compare with the existing user and refuse a different
one with -EKEYREJECTED, freeing the newly looked-up user either way.

An AI assistant was used to find and fix this and as an independent
reviewer.

Fixes: a47634cd729b ("ksmbd: expire SMB sessions when Kerberos tickets expire")
Cc: stable@vger.kernel.org
Signed-off-by: Michail Andreas Michalopoulos <mike.michalo@gmail.com>
---
 fs/smb/server/smb2pdu.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index b084cca8813cc..234439c1526f9 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -2365,6 +2365,17 @@ static int ntlm_authenticate(struct ksmbd_work *work,
 			return -EKEYREJECTED;
 		}
 		ksmbd_free_user(user);
+	} else if (sess->user) {
+		/*
+		 * Reauthentication of an expired session: as for a valid
+		 * session, and as krb5_authenticate() does, it must be by the
+		 * same user. Replacing sess->user would also leak the old one.
+		 */
+		if (!ksmbd_compare_user(sess->user, user)) {
+			ksmbd_free_user(user);
+			return -EKEYREJECTED;
+		}
+		ksmbd_free_user(user);
 	} else {
 		sess->user = user;
 	}
-- 
2.43.0