[PATCH] ksmbd: Fix race condition in RPC handle list access

[PATCH] ksmbd: Fix race condition in RPC handle list access

[Yunseong Kim]

The 'sess->rpc_handle_list' XArray manages RPC handles within a ksmbd
session. Access to this list is intended to be protected by
'sess->rpc_lock' (an rw_semaphore). However, the locking implementation was
flawed, leading to potential race conditions.

In ksmbd_session_rpc_open(), the code incorrectly acquired only a read lock
before calling xa_store() and xa_erase(). Since these operations modify
the XArray structure, a write lock is required to ensure exclusive access
and prevent data corruption from concurrent modifications.

Furthermore, ksmbd_session_rpc_method() accessed the list using xa_load()
without holding any lock at all. This could lead to reading inconsistent
data or a potential use-after-free if an entry is concurrently removed and
the pointer is dereferenced.

Fix these issues by:
1. Using down_write() and up_write() in ksmbd_session_rpc_open()
   to ensure exclusive access during XArray modification, and ensuring
   the lock is correctly released on error paths.
2. Adding down_read() and up_read() in ksmbd_session_rpc_method()
   to safely protect the lookup.

Fixes: a1f46c99d9ea ("ksmbd: fix use-after-free in ksmbd_session_rpc_open")
Fixes: b685757c7b08 ("ksmbd: Implements sess->rpc_handle_list as xarray")
Cc: stable@vger.kernel.org
Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
---
 fs/smb/server/mgmt/user_session.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/fs/smb/server/mgmt/user_session.c b/fs/smb/server/mgmt/user_session.c
index 9dec4c2940bc..b36d0676dbe5 100644
--- a/fs/smb/server/mgmt/user_session.c
+++ b/fs/smb/server/mgmt/user_session.c
@@ -104,29 +104,32 @@ int ksmbd_session_rpc_open(struct ksmbd_session *sess, char *rpc_name)
 	if (!entry)
 		return -ENOMEM;
 
-	down_read(&sess->rpc_lock);
 	entry->method = method;
 	entry->id = id = ksmbd_ipc_id_alloc();
 	if (id < 0)
 		goto free_entry;
+
+	down_write(&sess->rpc_lock);
 	old = xa_store(&sess->rpc_handle_list, id, entry, KSMBD_DEFAULT_GFP);
-	if (xa_is_err(old))
+	if (xa_is_err(old)) {
+		up_write(&sess->rpc_lock);
 		goto free_id;
+	}
 
 	resp = ksmbd_rpc_open(sess, id);
-	if (!resp)
-		goto erase_xa;
+	if (!resp) {
+		xa_erase(&sess->rpc_handle_list, entry->id);
+		up_write(&sess->rpc_lock);
+		goto free_id;
+	}
 
-	up_read(&sess->rpc_lock);
+	up_write(&sess->rpc_lock);
 	kvfree(resp);
 	return id;
-erase_xa:
-	xa_erase(&sess->rpc_handle_list, entry->id);
 free_id:
 	ksmbd_rpc_id_free(entry->id);
 free_entry:
 	kfree(entry);
-	up_read(&sess->rpc_lock);
 	return -EINVAL;
 }
 
@@ -144,9 +147,14 @@ void ksmbd_session_rpc_close(struct ksmbd_session *sess, int id)
 int ksmbd_session_rpc_method(struct ksmbd_session *sess, int id)
 {
 	struct ksmbd_session_rpc *entry;
+	int method;
 
+	down_read(&sess->rpc_lock);
 	entry = xa_load(&sess->rpc_handle_list, id);
-	return entry ? entry->method : 0;
+	method = entry ? entry->method : 0;
+	up_read(&sess->rpc_lock);
+
+	return method;
 }
 
 void ksmbd_session_destroy(struct ksmbd_session *sess)
-- 
2.51.0

Re: [PATCH] ksmbd: Fix race condition in RPC handle list access

[Namjae Jeon]

On Tue, Sep 16, 2025 at 7:44 AM Yunseong Kim <ysk@kzalloc.com> wrote:
>
> The 'sess->rpc_handle_list' XArray manages RPC handles within a ksmbd
> session. Access to this list is intended to be protected by
> 'sess->rpc_lock' (an rw_semaphore). However, the locking implementation was
> flawed, leading to potential race conditions.
>
> In ksmbd_session_rpc_open(), the code incorrectly acquired only a read lock
> before calling xa_store() and xa_erase(). Since these operations modify
> the XArray structure, a write lock is required to ensure exclusive access
> and prevent data corruption from concurrent modifications.
>
> Furthermore, ksmbd_session_rpc_method() accessed the list using xa_load()
> without holding any lock at all. This could lead to reading inconsistent
> data or a potential use-after-free if an entry is concurrently removed and
> the pointer is dereferenced.
>
> Fix these issues by:
> 1. Using down_write() and up_write() in ksmbd_session_rpc_open()
>    to ensure exclusive access during XArray modification, and ensuring
>    the lock is correctly released on error paths.
> 2. Adding down_read() and up_read() in ksmbd_session_rpc_method()
>    to safely protect the lookup.
>
> Fixes: a1f46c99d9ea ("ksmbd: fix use-after-free in ksmbd_session_rpc_open")
> Fixes: b685757c7b08 ("ksmbd: Implements sess->rpc_handle_list as xarray")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
Applied it to #ksmbd-for-next-next.
Thanks!