fix: don't allow userspace copy to read kernel memory

fix: don't allow userspace copy to read kernel memory

This patch fixes a security issue which allows the root user to read
arbitrary kernel memory. Considering the security model used in LTTng
userspace tooling for kernel tracing, this bug also allows members of
the 'tracing' group to read arbitrary kernel memory.

Calls to __copy_from_user_inatomic() where wrongly enclosed in
set_fs(KERNEL_DS) defeating the access_ok() calls and allowing to read
from kernel memory if a kernel address is provided.

Remove all set_fs() calls around __copy_from_user_inatomic().

As a side effect this will allow us to support v5.10 which should remove
set_fs().

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I35e4562c835217352c012ed96a7b8f93e941381e