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From:   Sasha Levin <sashal@kernel.org>
To:     stable@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Colin Ian King <colin.king@canonical.com>,
        Steve French <stfrench@microsoft.com>,
        Sasha Levin <sashal@kernel.org>, linux-cifs@vger.kernel.org
Subject: [PATCH AUTOSEL 4.18 02/39] cifs: don't dereference smb_file_target before null check
Date:   Tue, 13 Nov 2018 00:50:16 -0500
Message-Id: <20181113055053.78352-2-sashal@kernel.org>
In-Reply-To: <20181113055053.78352-1-sashal@kernel.org>
References: <20181113055053.78352-1-sashal@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Colin Ian King <colin.king@canonical.com>

[ Upstream commit 8c6c9bed8773375b1d54ccca2911ec892c59db5d ]

There is a null check on dst_file->private data which suggests
it can be potentially null. However, before this check, pointer
smb_file_target is derived from dst_file->private and dereferenced
in the call to tlink_tcon, hence there is a potential null pointer
deference.

Fix this by assigning smb_file_target and target_tcon after the
null pointer sanity checks.

Detected by CoverityScan, CID#1475302 ("Dereference before null check")

Fixes: 04b38d601239 ("vfs: pull btrfs clone API to vfs layer")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/cifs/cifsfs.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 69ec5427769c..9c194593270b 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -984,8 +984,8 @@ static int cifs_clone_file_range(struct file *src_file, loff_t off,
 	struct inode *src_inode = file_inode(src_file);
 	struct inode *target_inode = file_inode(dst_file);
 	struct cifsFileInfo *smb_file_src = src_file->private_data;
-	struct cifsFileInfo *smb_file_target = dst_file->private_data;
-	struct cifs_tcon *target_tcon = tlink_tcon(smb_file_target->tlink);
+	struct cifsFileInfo *smb_file_target;
+	struct cifs_tcon *target_tcon;
 	unsigned int xid;
 	int rc;
 
@@ -999,6 +999,9 @@ static int cifs_clone_file_range(struct file *src_file, loff_t off,
 		goto out;
 	}
 
+	smb_file_target = dst_file->private_data;
+	target_tcon = tlink_tcon(smb_file_target->tlink);
+
 	/*
 	 * Note: cifs case is easier than btrfs since server responsible for
 	 * checks for proper open modes and file type and if it wants
-- 
2.17.1