linux-kernel - [PATCH 4.20 257/352] cifs: check ntwrk_buf_start for NULL before dereferencing it

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Message-Id: <20190211141903.431384683@linuxfoundation.org>
Date:   Mon, 11 Feb 2019 15:18:04 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ronnie Sahlberg <lsahlber@...hat.com>,
        Steve French <stfrench@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.20 257/352] cifs: check ntwrk_buf_start for NULL before dereferencing it

4.20-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@...hat.com>
Signed-off-by: Steve French <stfrench@...rosoft.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 e169e1a5fd35..3925a7bfc74d 100644
--- 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,
 					server);
-- 
2.19.1