ksmbd: fix null pointer dereference in compare_guid_key()

session_fd_check() walks the per-inode m_op_list during durable-handle
session teardown and sets op->conn = NULL for every opinfo whose conn
matched the closing session's connection. The matching opinfo, however,
stays linked in its per-ClientGuid lease_table_list entry's lb->lease_list
because destroy_lease_table() only runs on full TCP-connection teardown,
not on SESSION_LOGOFF.

If the same TCP connection then negotiates a fresh session with the
same ClientGuid (ClientGuid is bound to NEGOTIATE, not the session, and
is unchanged across LOGOFF + SETUP) and issues a SMB2 CREATE with a
lease context on a different inode, find_same_lease_key() walks
lb->lease_list, reaches the stale opinfo, and calls compare_guid_key(),
which unconditionally dereferences opinfo->conn->ClientGUID. The conn
pointer is NULL and the kernel panics.

Reproducer requires only a successful SMB2 SESSION_SETUP and a share
configured with 'durable handles = yes'. KASAN report on mainline
70390501d1:

  general protection fault, probably for non-canonical address
  0xdffffc0000000069: 0000 [#1] SMP KASAN PTI
  KASAN: null-ptr-deref in range [0x0000000000000348-0x000000000000034f]
  Workqueue: ksmbd-io handle_ksmbd_work
  RIP: 0010:bcmp+0x5b/0x230
  Call Trace:
   compare_guid_key+0x4b/0xd0
   find_same_lease_key+0x324/0x690
   smb2_open+0x6aea/0x8e60
   handle_ksmbd_work+0x796/0xee0
   ...

Faulting address 0x348 is the offset of ClientGUID within struct
ksmbd_conn, confirming opinfo->conn was NULL.

Read opinfo->conn once and bail out if it has been cleared by a
concurrent session_fd_check(). A half-detached opinfo cannot be the
owner of an active lease, so returning 0 is the correct match result.

Fixes: c8efcc7861 ("ksmbd: add support for durable handles v1/v2")
Cc: stable@vger.kernel.org
Signed-off-by: Jeremy Laratro <research@aradex.io>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/oplock.c

@@ -481,8 +481,12 @@ static inline int compare_guid_key(struct oplock_info *opinfo,
 				   const char *guid1, const char *key1)
 {
 	const char *guid2, *key2;
+	struct ksmbd_conn *conn;
 
-	guid2 = opinfo->conn->ClientGUID;
+	conn = READ_ONCE(opinfo->conn);
+	if (!conn)
+		return 0;
+	guid2 = conn->ClientGUID;
 	key2 = opinfo->o_lease->lease_key;
 	if (!memcmp(guid1, guid2, SMB2_CLIENT_GUID_SIZE) &&
 	    !memcmp(key1, key2, SMB2_LEASE_KEY_SIZE))