[Bug 1794067] Re: Bug in selinux on ubuntu 16.04 with kernel 4.15.0-34

This also affects me on Ubuntu 18.04, kernel 4.15.0-39.

This issue is causing update-initramfs to fail since many of the
initramfs-tools hooks use cp -a and are unable to complete.

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https://bugs.launchpad.net/bugs/1794067

Title:
Bug in selinux on ubuntu 16.04 with kernel 4.15.0-34

Status in linux package in Ubuntu:
In Progress
Status in linux source package in Bionic:
In Progress

Bug description:
Hello, just found a bug in selinux.

Whats going wrong:

Copy a link with "-a" option while selinux is on.

steps to reproduce:

~$ mkdir -p a/b
~$ ln -s b a/c
~$ cp -a a b
cp: failed to restore the default file creation context: Invalid argument

Results of my investigation:

The "cp" of coreutils is calling "setfscreatecon (NULL)" to restore
the default file creation context (coreutils-8.30/src/copy.c:1771) as
it is stated in the selinux api
(/libselinux/include/selinux/selinux.h:71).

As we see in the result of strace below, the kernel returns an -1 on
try to restore the default file creation context. So in my opinion is
the bug has to be in the selinux_setprocattr method in the
security/selinux/hooks.c file.

Part of "strace cp -a a b"

lgetxattr("a/c", "security.selinux", "system_u:object_r:user_home_dir_t:s0", 255) = 37
readlink("a/c", "b", 2) = 1
symlink("b", "b/a/c") = 0
open("/proc/self/task/2136/attr/fscreate", O_RDWR|O_CLOEXEC) = 3
write(3, NULL, 0) = -1 EINVAL (Invalid argument)
close(3) = 0
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2995, ...}) = 0
read(3, "# Locale name alias data base.\n#"..., 4096) = 2995
read(3, "", 4096) = 0
close(3)

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