[PATCH 5.15.y] ksmbd: add chann_lock to protect ksmbd_chann_list xarray

From: Namjae Jeon <[email protected]>

[ Upstream commit 4f3a06cc57976cafa8c6f716646be6c79a99e485 ]

ksmbd_chann_list xarray lacks synchronization, allowing use-after-free in
multi-channel sessions (between lookup_chann_list() and ksmbd_chann_del).

Adds rw_semaphore chann_lock to struct ksmbd_session and protects
all xa_load/xa_store/xa_erase accesses.

Backport notes for linux-5.15.y:
  - 5.15 still uses fs/ksmbd/ paths (the smb client/server consolidation
    landed in 6.1).
  - struct ksmbd_session does not yet have rpc_lock; init_rwsem for
    chann_lock is added directly after the refcnt initialisation.
  - Function bodies for free_channel_list, ksmbd_chann_del,
    lookup_chann_list, and the two xa_store sites in
    ntlm_authenticate / krb5_authenticate are byte-identical to
    mainline at the time of the fix.

CVE: CVE-2026-23226
Cc: [email protected] # 5.15.y
Reported-by: Igor Stepansky <[email protected]>
Signed-off-by: Namjae Jeon <[email protected]>
Signed-off-by: Steve French <[email protected]>
[backport for linux-5.15.y, verified 2026-05-01]
---
 fs/ksmbd/mgmt/user_session.c |  5 +++++
 fs/ksmbd/mgmt/user_session.h |  1 +
 fs/ksmbd/smb2pdu.c           | 12 +++++++++++-
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/fs/ksmbd/mgmt/user_session.c b/fs/ksmbd/mgmt/user_session.c
--- a/fs/ksmbd/mgmt/user_session.c
+++ b/fs/ksmbd/mgmt/user_session.c
@@ -30,12 +30,14 @@ static void free_channel_list(struct ksmbd_session *sess)
 	struct channel *chann;
 	unsigned long index;

+	down_write(&sess->chann_lock);
 	xa_for_each(&sess->ksmbd_chann_list, index, chann) {
 		xa_erase(&sess->ksmbd_chann_list, index);
 		kfree(chann);
 	}

 	xa_destroy(&sess->ksmbd_chann_list);
+	up_write(&sess->chann_lock);
 }

 static void __session_rpc_close(struct ksmbd_session *sess,
@@ -201,7 +203,9 @@ static int ksmbd_chann_del(struct ksmbd_conn *conn, struct ksmbd_session *sess)
 {
 	struct channel *chann;

+	down_write(&sess->chann_lock);
 	chann = xa_erase(&sess->ksmbd_chann_list, (long)conn);
+	up_write(&sess->chann_lock);
 	if (!chann)
 		return -ENOENT;

@@ -395,6 +399,7 @@ static struct ksmbd_session *__session_create(int protocol)
 	sess->sequence_number = 1;
 	rwlock_init(&sess->tree_conns_lock);
 	atomic_set(&sess->refcnt, 1);
+	init_rwsem(&sess->chann_lock);

 	ret = __init_smb2_session(sess);
 	if (ret)
diff --git a/fs/ksmbd/mgmt/user_session.h b/fs/ksmbd/mgmt/user_session.h
--- a/fs/ksmbd/mgmt/user_session.h
+++ b/fs/ksmbd/mgmt/user_session.h
@@ -48,6 +48,7 @@ struct ksmbd_session {
 	char				sess_key[CIFS_KEY_SIZE];

 	struct hlist_node		hlist;
+	struct rw_semaphore		chann_lock;
 	struct xarray			ksmbd_chann_list;
 	struct xarray			tree_conns;
 	struct ida			tree_conn_ida;
diff --git a/fs/ksmbd/smb2pdu.c b/fs/ksmbd/smb2pdu.c
--- a/fs/ksmbd/smb2pdu.c
+++ b/fs/ksmbd/smb2pdu.c
@@ -78,7 +78,13 @@ static inline bool check_session_id(struct ksmbd_conn *conn, u64 id)

 struct channel *lookup_chann_list(struct ksmbd_session *sess, struct ksmbd_conn *conn)
 {
-	return xa_load(&sess->ksmbd_chann_list, (long)conn);
+	struct channel *chann;
+
+	down_read(&sess->chann_lock);
+	chann = xa_load(&sess->ksmbd_chann_list, (long)conn);
+	up_read(&sess->chann_lock);
+
+	return chann;
 }

 /**
@@ -1570,7 +1576,9 @@ static int ntlm_authenticate(struct ksmbd_work *work,
 				return -ENOMEM;

 			chann->conn = conn;
+			down_write(&sess->chann_lock);
 			xa_store(&sess->ksmbd_chann_list, (long)conn, chann, GFP_KERNEL);
+			up_write(&sess->chann_lock);
 		}
 	}

@@ -1667,7 +1675,9 @@ static int krb5_authenticate(struct ksmbd_work *work,
 				return -ENOMEM;

 			chann->conn = conn;
+			down_write(&sess->chann_lock);
 			xa_store(&sess->ksmbd_chann_list, (long)conn, chann, GFP_KERNEL);
+			up_write(&sess->chann_lock);
 		}
 	}