smb: client: fix potential UAF in cifs_debug_files_proc_show()

commit ca545b7 upstream.

Skip sessions that are being teared down (status == SES_EXITING) to
avoid UAF.

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
[ This patch removes lock/unlock operation in routine cifs_ses_exiting()
  for ses_lock is not present in v5.15 and not ported yet. ses->status
  is protected by a global lock, cifs_tcp_ses_lock, in v5.15. ]
Signed-off-by: Jianqi Ren <jianqi.ren.cn@windriver.com>
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Paulo Alcantara

fs/cifs/cifs_debug.c

@@ -183,6 +183,8 @@ static int cifs_debug_files_proc_show(struct seq_file *m, void *v)
 	list_for_each_entry(server, &cifs_tcp_ses_list, tcp_ses_list) {
 		list_for_each(tmp, &server->smb_ses_list) {
 			ses = list_entry(tmp, struct cifs_ses, smb_ses_list);
+			if (cifs_ses_exiting(ses))
+				continue;
 			list_for_each(tmp1, &ses->tcon_list) {
 				tcon = list_entry(tmp1, struct cifs_tcon, tcon_list);
 				spin_lock(&tcon->open_file_lock);

fs/cifs/cifsglob.h

@@ -2041,4 +2041,12 @@ static inline struct scatterlist *cifs_sg_set_buf(struct scatterlist *sg,
 	return sg;
 }
 
+static inline bool cifs_ses_exiting(struct cifs_ses *ses)
+{
+	bool ret;
+
+	ret = ses->status == CifsExiting;
+	return ret;
+}
+
 #endif	/* _CIFS_GLOB_H */