ksmbd: Fix race condition in RPC handle list access

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>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

diff --git a/fs/smb/server/mgmt/user_session.c b/fs/smb/server/mgmt/user_session.c
index 9dec4c2940bc0467def4c865504b52ca502a19f7..b36d0676dbe584631c053976c367e371ac4279c4 100644 (file)
--- 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)