rtslib: safely call shutil.copy()
rtslib: safely call shutil.copy()

Previously we had to replace shutil.copyfile() with shutil.copy(),
because we want to copy the file permissions to the destination file
along with the data.

It appears that shutil.copy() is opening the destination file with
wide access (0666) first, and then it starts copying the data and
at the end it is copying the permissions from source file to destination.

If we closely notice there appears a window between destination file
is opened vs permissions are set on the destination file, which could
allow a user to get the contents of the file when opening it at the
right time.

The behavior is a bit unsteady here, it is noticed that, when
saveconfig.json file exists, then on shutil.copy(), destination file is
opened and a mask 0600 is applied on the file, in case shutil.copy() had
to open a new destination saveconfig.json file, then mask 0644 is applied.

Thanks and Credits to 'Stefan Cornelius <scorneli@redhat.com>' for
reporting this, here is the strace he shared from RHEL-7/python-2.7.5
env:

Case 1: When /etc/target/saveconfig.json doesn't exist:

open("/etc/target/saveconfig.json.temp", O_RDONLY) = 3
open("/etc/target/saveconfig.json", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
fstat(3, {st_mode=S_IFREG|0600, st_size=71, ...}) = 0
fstat(4, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
[...]
chmod("/etc/target/saveconfig.json", 0600) = 0}")")}")

Case 2: When /etc/target/saveconfig.json already exist:

open("/etc/target/saveconfig.json.temp", O_RDONLY) = 3
open("/etc/target/saveconfig.json", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
fstat(3, {st_mode=S_IFREG|0600, st_size=71, ...}) = 0
fstat(4, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
[...]
chmod("/etc/target/saveconfig.json", 0600) = 0}")")}")

Signed-off-by: Prasanna Kumar Kalever <a78d8f98557446023f5b3ba9944d7be259a978a3@redhat.com>

cvubrugier/rtslib-fb