[PATCH 4.4 061/143] cifs: check ntwrk_buf_start for NULL before dereferencing it

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

[ 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@xxxxxxxxxx>
Signed-off-by: Steve French <stfrench@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/cifs/readdir.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/cifs/readdir.c b/fs/cifs/readdir.c
index 57b039ebfb1f..43fa471c88d7 100644
--- a/fs/cifs/readdir.c
+++ b/fs/cifs/readdir.c
@@ -652,7 +652,14 @@ 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);
 
-- 
2.19.1