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ksmbd: fix tree connection use-after-free in smb2_tree_connect()

PATCH Applied Cen Zhang (Microsoft Security FORGE Labs) 2 participants 2 messages Sep 1 – Sep 3

Subsystems (3)
Stable branch SMB client (CIFS) Kernel SMB3 server (ksmbd)
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Cen Zhang (Microsoft Security FORGE Labs)

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PATCH 1/1 ksmbd: fix tree connection use-after-free in smb2_tree_connect()

ksmbd_tree_conn_connect() publishes a new tree connection in sess->tree_conns with a single reference and returns its pointer to smb2_tree_connect(). The handler continues to initialize the object and build the response after publication. A concurrent session logoff can erase the connection and drop that reference, freeing the object while the handler still uses it.

BUG: KASAN: slab-use-after-free in smb2_tree_connect+0xe3d/0xf90
  smb2_tree_connect (fs/smb/server/smb2pdu.c:2872)
  handle_ksmbd_work
  process_one_work
  worker_thread
  kthread

After xa_store() succeeds, take a second reference before releasing tree_conns_lock. The original reference belongs to the xarray entry and the second belongs to the creating smb2_tree_connect() handler.

Keep the references balanced in every path:

- On normal exit or an error after publication, smb2_tree_connect() drops its creator reference. Error cleanup also calls ksmbd_tree_conn_disconnect(), which drops the xarray reference only if it removes the exact entry.
- SMB2 TREE_DISCONNECT uses the same helper to remove the entry and drop its xarray reference. The request's existing lookup reference remains owned by the request and is released by the existing cleanup.
- Session LOGOFF removes each entry and drops its xarray reference. If it wins the race, later cleanup sees that the entry is gone and does not drop that reference again.

To enforce this ownership, claim the disconnected state and erase the exact entry atomically under tree_conns_lock. This guarantees one drop for the xarray reference and one drop by each in-flight user, regardless of which teardown path wins. If logoff removes the entry before initialization completes, fail the connect instead of marking the detached object TREE_CONNECTED.

Fixes 33b235a6e6eb ("ksmbd: fix race condition between tree conn lookup and disconnect")
Reported-by Xiang Mei (Microsoft) <[email protected]>
Cc [email protected]
Cc [email protected]
Signed-off-by Cen Zhang (Microsoft Security FORGE Labs) <[email protected]>
---
 fs/smb/server/mgmt/tree_connect.c |  8 ++++++++
 fs/smb/server/smb2pdu.c           | 28 +++++++++++++---------------
 2 files changed, 21 insertions(+), 15 deletions(-)
fs/smb/server/mgmt/tree_connect.c +8 -0
index 5f63e236267a..dd1db3554cae 100644
@@ -82,6 +82,8 @@ ksmbd_tree_conn_connect(struct ksmbd_work *work, const char *share_name)
 	down_write(&sess->tree_conns_lock);
 	ret = xa_err(xa_store(&sess->tree_conns, tree_conn->id, tree_conn,
 			      KSMBD_DEFAULT_GFP));
+	if (!ret)
+		atomic_inc(&tree_conn->refcount);
 	up_write(&sess->tree_conns_lock);
 	if (ret) {
 		status.ret = -ENOMEM;

@@ -129,6 +131,12 @@ int ksmbd_tree_conn_disconnect(struct ksmbd_session *sess,
 			       struct ksmbd_tree_connect *tree_conn)
 {
 	down_write(&sess->tree_conns_lock);
+	if (tree_conn->t_state == TREE_DISCONNECTED ||
+	    xa_load(&sess->tree_conns, tree_conn->id) != tree_conn) {
+		up_write(&sess->tree_conns_lock);
+		return -ENOENT;
+	}
+	tree_conn->t_state = TREE_DISCONNECTED;
 	xa_erase(&sess->tree_conns, tree_conn->id);
 	up_write(&sess->tree_conns_lock);
fs/smb/server/smb2pdu.c +13 -15
index 903ffffdd187..e7c965e7b068 100644
@@ -2804,6 +2804,7 @@ int smb2_tree_connect(struct ksmbd_work *work)
 	struct ksmbd_session *sess = work->sess;
 	char *treename = NULL, *name = NULL;
 	struct ksmbd_tree_conn_status status;
+	struct ksmbd_tree_connect *tree_conn = NULL;
 	struct ksmbd_share_config *share = NULL;
 	int rc = -EINVAL;
 

@@ -2831,6 +2832,7 @@ int smb2_tree_connect(struct ksmbd_work *work)
 
 	status = ksmbd_tree_conn_connect(work, name);
 	if (status.ret == KSMBD_TREE_CONN_STATUS_OK) {
+		tree_conn = status.tree_conn;
 		rsp->hdr.Id.SyncId.TreeId = cpu_to_le32(status.tree_conn->id);
 		share = status.tree_conn->share_conf;
 

@@ -2874,8 +2876,15 @@ int smb2_tree_connect(struct ksmbd_work *work)
 		status.tree_conn->posix_extensions = true;
 
 	down_write(&sess->tree_conns_lock);
-	status.tree_conn->t_state = TREE_CONNECTED;
+	if (status.tree_conn->t_state == TREE_DISCONNECTED) {
+		status.ret = KSMBD_TREE_CONN_STATUS_ERROR;
+		share = NULL;
+	} else {
+		status.tree_conn->t_state = TREE_CONNECTED;
+	}
 	up_write(&sess->tree_conns_lock);
+	if (status.ret != KSMBD_TREE_CONN_STATUS_OK)
+		goto out_err1;
 	rsp->StructureSize = cpu_to_le16(16);
 out_err1:
 	/*

@@ -2902,9 +2911,6 @@ int smb2_tree_connect(struct ksmbd_work *work)
 	rc = ksmbd_iov_pin_rsp(work, rsp, sizeof(struct smb2_tree_connect_rsp));
 	if (rc) {
 		if (status.ret == KSMBD_TREE_CONN_STATUS_OK) {
-			down_write(&sess->tree_conns_lock);
-			status.tree_conn->t_state = TREE_DISCONNECTED;
-			up_write(&sess->tree_conns_lock);
 			ksmbd_tree_conn_disconnect(sess, status.tree_conn);
 			status.tree_conn = NULL;
 		}

@@ -2945,6 +2951,9 @@ int smb2_tree_connect(struct ksmbd_work *work)
 	if (status.ret != KSMBD_TREE_CONN_STATUS_OK)
 		smb2_set_err_rsp(work);
 
+	if (tree_conn)
+		ksmbd_tree_connect_put(tree_conn);
+
 	return rc;
 }
 

@@ -3048,17 +3057,6 @@ int smb2_tree_disconnect(struct ksmbd_work *work)
 
 	ksmbd_close_tree_conn_fds(work);
 
-	down_write(&sess->tree_conns_lock);
-	if (tcon->t_state == TREE_DISCONNECTED) {
-		up_write(&sess->tree_conns_lock);
-		rsp->hdr.Status = STATUS_NETWORK_NAME_DELETED;
-		err = -ENOENT;
-		goto err_out;
-	}
-
-	tcon->t_state = TREE_DISCONNECTED;
-	up_write(&sess->tree_conns_lock);
-
 	err = ksmbd_tree_conn_disconnect(sess, tcon);
 	if (err) {
 		rsp->hdr.Status = STATUS_NETWORK_NAME_DELETED;

base-commit: f7e5381fe0e2956317075a6d24aad1f56d6ce382

Signature · 1 line
2.55.0
maintainer Applied
#1 lore raw
Quoted 39 lines · Cen Zhang (Microsoft Security FORGE Labs)

ksmbd_tree_conn_connect() publishes a new tree connection in sess->tree_conns with a single reference and returns its pointer to smb2_tree_connect(). The handler continues to initialize the object and build the response after publication. A concurrent session logoff can erase the connection and drop that reference, freeing the object while the handler still uses it.

BUG: KASAN: slab-use-after-free in smb2_tree_connect+0xe3d/0xf90
  smb2_tree_connect (fs/smb/server/smb2pdu.c:2872)
  handle_ksmbd_work
  process_one_work
  worker_thread
  kthread

After xa_store() succeeds, take a second reference before releasing tree_conns_lock. The original reference belongs to the xarray entry and the second belongs to the creating smb2_tree_connect() handler.

Keep the references balanced in every path:

- On normal exit or an error after publication, smb2_tree_connect() drops its creator reference. Error cleanup also calls ksmbd_tree_conn_disconnect(), which drops the xarray reference only if it removes the exact entry.
- SMB2 TREE_DISCONNECT uses the same helper to remove the entry and drop its xarray reference. The request's existing lookup reference remains owned by the request and is released by the existing cleanup.
- Session LOGOFF removes each entry and drops its xarray reference. If it wins the race, later cleanup sees that the entry is gone and does not drop that reference again.

To enforce this ownership, claim the disconnected state and erase the exact entry atomically under tree_conns_lock. This guarantees one drop for the xarray reference and one drop by each in-flight user, regardless of which teardown path wins. If logoff removes the entry before initialization completes, fail the connect instead of marking the detached object TREE_CONNECTED.

Quoted 5 lines
Fixes 33b235a6e6eb ("ksmbd: fix race condition between tree conn lookup and disconnect")
Reported-by Xiang Mei (Microsoft) <[email protected]>
Cc [email protected]
Cc [email protected]
Signed-off-by Cen Zhang (Microsoft Security FORGE Labs) <[email protected]>

Applied it to #ksmbd-for-next.
Thanks!