cifs: check for NULL last_entry before calling cifs_save_resume_key

Prior to commit eaf35b1, cifs_save_resume_key had some NULL pointer
checks at the top. It turns out that at least one of those NULL
pointer checks is needed after all.

When the LastNameOffset in a FIND reply appears to be beyond the end of
the buffer, CIFSFindFirst and CIFSFindNext will set srch_inf.last_entry
to NULL. Since eaf35b1, the code will now oops in this situation.

Fix this by having the callers check for a NULL last entry pointer
before calling cifs_save_resume_key. No change is needed for the
call site in cifs_readdir as it's not reachable with a NULL
current_entry pointer.

This should fix:

    https://bugzilla.redhat.com/show_bug.cgi?id=750247

Cc: stable@vger.kernel.org
Cc: Christoph Hellwig <hch@infradead.org>
Reported-by: Adam G. Metzler <adamgmetzler@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/readdir.c

@@ -554,7 +554,10 @@ static int find_cifs_entry(const int xid, struct cifs_tcon *pTcon,
 				 rc);
 			return rc;
 		}
-		cifs_save_resume_key(cifsFile->srch_inf.last_entry, cifsFile);
+		/* FindFirst/Next set last_entry to NULL on malformed reply */
+		if (cifsFile->srch_inf.last_entry)
+			cifs_save_resume_key(cifsFile->srch_inf.last_entry,
+						cifsFile);
 	}
 
 	while ((index_to_find >= cifsFile->srch_inf.index_of_last_entry) &&
@@ -562,7 +565,10 @@ static int find_cifs_entry(const int xid, struct cifs_tcon *pTcon,
 		cFYI(1, "calling findnext2");
 		rc = CIFSFindNext(xid, pTcon, cifsFile->netfid,
 				  &cifsFile->srch_inf);
-		cifs_save_resume_key(cifsFile->srch_inf.last_entry, cifsFile);
+		/* FindFirst/Next set last_entry to NULL on malformed reply */
+		if (cifsFile->srch_inf.last_entry)
+			cifs_save_resume_key(cifsFile->srch_inf.last_entry,
+						cifsFile);
 		if (rc)
 			return -ENOENT;
 	}