Code:  static int nfs_can_extend_write(struct file *file, struct page *page, struct inode *inode)
 {
 	if (file->f_flags & O_DSYNC)
 		return 0;
 	if (NFS_PROTO(inode)->have_delegation(inode, FMODE_WRITE))
 		return 1;
	if (nfs_write_pageuptodate(page, inode) && (inode->i_flock == NULL ||
			(inode->i_flock->fl_start == 0 &&
 			inode->i_flock->fl_end == OFFSET_MAX &&
			inode->i_flock->fl_type != F_RDLCK)))
 		return 1;
 	return 0;
 }

Vulnerability Type: +Info 
CWE ID: CWE-20
Summary: The nfs_can_extend_write function in fs/nfs/write.c in the Linux kernel before 3.13.3 relies on a write delegation to extend a write operation without a certain up-to-date verification, which allows local users to obtain sensitive information from kernel memory in opportunistic circumstances by writing to a file in an NFS filesystem and then reading the same file.
Commit Message: nfs: always make sure page is up-to-date before extending a write to cover the entire page

We should always make sure the cached page is up-to-date when we're
determining whether we can extend a write to cover the full page -- even
if we've received a write delegation from the server.

Commit c7559663 added logic to skip this check if we have a write
delegation, which can lead to data corruption such as the following
scenario if client B receives a write delegation from the NFS server:

Client A:
    # echo 123456789 > /mnt/file

Client B:
    # echo abcdefghi >> /mnt/file
    # cat /mnt/file
    0�D0�abcdefghi

Just because we hold a write delegation doesn't mean that we've read in
the entire page contents.

Cc: <stable@vger.kernel.org> # v3.11+
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>