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ksmbd: add chann_lock to protect ksmbd_chann_list xarray

PATCH In review Kai Aizen 2 participants 4 messages May 1 – May 3

Subsystems (3)
Stable branch SMB client (CIFS) Kernel SMB3 server (ksmbd)
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Kai Aizen

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#0 lore raw

PATCH 1/1 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-6.6.y:
- File paths and surrounding context are identical to mainline at the time of the fix. The xa_store call sites in ntlm_authenticate / krb5_authenticate use the older form xa_store(..., GFP_KERNEL) rather than the newer
old = xa_store(..., KSMBD_DEFAULT_GFP); the lock-wrap is structurally identical.

CVE: CVE-2026-23226

Cc [email protected] # 6.6.y
Reported-by Igor Stepansky <[email protected]>
Signed-off-by Namjae Jeon <[email protected]>
Signed-off-by Steve French <[email protected]>
[backport for linux-6.6.y, verified 2026-05-01]
---
 fs/smb/server/mgmt/user_session.c |  5 +++++
 fs/smb/server/mgmt/user_session.h |  1 +
 fs/smb/server/smb2pdu.c           | 12 +++++++++++-
 3 files changed, 17 insertions(+), 1 deletion(-)
fs/smb/server/mgmt/user_session.c +5 -0
@@ -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,

@@ -218,7 +220,9 @@ 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;
 

@@ -451,6 +455,7 @@ static struct ksmbd_session *__session_create(int protocol)
 	rwlock_init(&sess->tree_conns_lock);
 	atomic_set(&sess->refcnt, 2);
 	init_rwsem(&sess->rpc_lock);
+	init_rwsem(&sess->chann_lock);
 
 	ret = __init_smb2_session(sess);
 	if (ret)
fs/smb/server/mgmt/user_session.h +1 -0
@@ -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;
fs/smb/server/smb2pdu.c +11 -1
@@ -80,7 +80,13 @@ 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;
 }
 
 /**

@@ -1559,7 +1565,9 @@ 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);
 		}
 	}
 

@@ -1652,7 +1660,9 @@ 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);
 		}
 	}

--
2.43.0

author
#1 lore raw

PATCH 1/1 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-6.1.y:
- The smb client/server consolidation has already happened by 6.1.y; file paths are fs/smb/server/, identical to mainline.
- The xa_store call sites use the older form xa_store(..., GFP_KERNEL) with no captured old, but the surrounding context for the lock-wrap is identical.

CVE: CVE-2026-23226

Cc [email protected] # 6.1.y
Reported-by Igor Stepansky <[email protected]>
Signed-off-by Namjae Jeon <[email protected]>
Signed-off-by Steve French <[email protected]>
[backport for linux-6.1.y, verified 2026-05-01]
---
 fs/smb/server/mgmt/user_session.c |  5 +++++
 fs/smb/server/mgmt/user_session.h |  1 +
 fs/smb/server/smb2pdu.c           | 12 +++++++++++-
 3 files changed, 17 insertions(+), 1 deletion(-)
fs/smb/server/mgmt/user_session.c +5 -0
@@ -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,

@@ -218,7 +220,9 @@ 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;
 

@@ -417,6 +421,7 @@ static struct ksmbd_session *__session_create(int protocol)
 	rwlock_init(&sess->tree_conns_lock);
 	atomic_set(&sess->refcnt, 2);
 	init_rwsem(&sess->rpc_lock);
+	init_rwsem(&sess->chann_lock);
 
 	ret = __init_smb2_session(sess);
 	if (ret)
fs/smb/server/mgmt/user_session.h +1 -0
@@ -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;
fs/smb/server/smb2pdu.c +11 -1
@@ -79,7 +79,13 @@ 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;
 }
 
 /**

@@ -1567,7 +1573,9 @@ 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);
 		}
 	}
 

@@ -1660,7 +1668,9 @@ 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);
 		}
 	}

--
2.43.0

author
#2 lore raw

PATCH 1/1 ksmbd: add chann_lock to protect ksmbd_chann_list xarray

In reply to Kai Aizen

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(-)
fs/ksmbd/mgmt/user_session.c +5 -0
@@ -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)
fs/ksmbd/mgmt/user_session.h +1 -0
@@ -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;
fs/ksmbd/smb2pdu.c +11 -1
@@ -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);
 		}
 	}

--
2.43.0

maintainer
#3 lore raw
In reply to Kai Aizen
Quoted 9 lines · Kai Aizen

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.

Thanks for the backport. Unfortunately I'm holding off on queuing this (and the 6.1.y / 5.15.y siblings) for now.

The backport is faithful to upstream, but on closer review the upstream commit 4f3a06cc5797 itself does not fully cover the race: there are xa_for_each() / xa_empty() / xa_load() call sites that remain unprotected after the patch. Shipping just this commit to the LTS trees would leave the same UAF window open.

--
Thanks,
Sasha