cifs: check ntwrk_buf_start for NULL before dereferencing it

[ Upstream commit 59a63e479ce36a3f24444c3a36efe82b78e4a8e0 ]

RHBZ: 1021460

There is an issue where when multiple threads open/close the same directory
ntwrk_buf_start might end up being NULL, causing the call to smbCalcSize
later to oops with a NULL deref.

The real bug is why this happens and why this can become NULL for an
open cfile, which should not be allowed.
This patch tries to avoid a oops until the time when we fix the underlying
issue.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/fs/cifs/readdir.c b/fs/cifs/readdir.c
index ef24b4527459f354c35c339f9bd5bc32b05ac762..68183872bf8b66ae8244e69cc12a63a02fdc1ef4 100644 (file)
--- a/fs/cifs/readdir.c
+++ b/fs/cifs/readdir.c
@@ -655,7 +655,14 @@ find_cifs_entry(const unsigned int xid, struct cifs_tcon *tcon, loff_t pos,
                /* scan and find it */
                int i;
                char *cur_ent;
-               char *end_of_smb = cfile->srch_inf.ntwrk_buf_start +
+               char *end_of_smb;
+
+               if (cfile->srch_inf.ntwrk_buf_start == NULL) {
+                       cifs_dbg(VFS, "ntwrk_buf_start is NULL during readdir\n");
+                       return -EIO;
+               }
+
+               end_of_smb = cfile->srch_inf.ntwrk_buf_start +
                        server->ops->calc_smb_size(
                                        cfile->srch_inf.ntwrk_buf_start);