pipe: iovec: Fix memory corruption when retrying atomic copy as non-atomic

pipe_iov_copy_{from,to}_user() may be tried twice with the same iovec,
the first time atomically and the second time not.  The second attempt
needs to continue from the iovec position, pipe buffer offset and
remaining length where the first attempt failed, but currently the
pipe buffer offset and remaining length are reset.  This will corrupt
the piped data (possibly also leading to an information leak between
processes) and may also corrupt kernel memory.

This was fixed upstream by commits f0d1bec9d58d ("new helper:
copy_page_from_iter()") and 637b58c2887e ("switch pipe_read() to
copy_page_to_iter()"), but those aren't suitable for stable.  This fix
for older kernel versions was made by Seth Jennings for RHEL and I
have extracted it from their update.

CVE-2015-1805

Change-Id: I4a18eab0e46f35b1d72f178138d50e1b3eb867cd
References: https://bugzilla.redhat.com/show_bug.cgi?id=1202855


Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-repo: http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git


Git-commit: 14f81062f365fa9e3839bb2a16862217b71a553c
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>