linux-kernel - [PATCH] cifs: don't dereference smb_file

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Message-Id: <20181101131430.28901-1-colin.king@canonical.com>
Date:   Thu,  1 Nov 2018 13:14:30 +0000
From:   Colin King <colin.king@...onical.com>
To:     Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
        samba-technical@...ts.samba.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] cifs: don't dereference smb_file_target before null check

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

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@...onical.com>
---
 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 e46af70153c0..db83938809a5 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -1003,8 +1003,8 @@ static loff_t cifs_remap_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;
 
@@ -1021,6 +1021,9 @@ static loff_t cifs_remap_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
-- 
2.19.1