kunit_driver_create() accepts a name for the driver, but does not copy
it, so if that name is either on the stack, or otherwise freed, we end
up with a use-after-free when the driver is cleaned up.

Instead, strdup() the name, and manage it as another KUnit allocation.
As there was no existing kunit_kstrdup(), we add one. Further, add a
kunit_ variant of strdup_const() and kfree_const(), so we don't need to
allocate and manage the string in the majority of cases where it's a
constant.

However, these are inline functions, and is_kernel_rodata() only works
for built-in code. This causes problems in two cases:
- If kunit is built as a module, __{start,end}_rodata is not defined.
- If a kunit test using these functions is built as a module, it will
  suffer the same fate.

This fixes a KASAN splat with overflow.overflow_allocation_test, when
built as a module.

Restrict the is_kernel_rodata() case to when KUnit is built as a module,
which fixes the first case, at the cost of losing the optimisation.

Also, make kunit_{kstrdup,kfree}_const non-inline, so that other modules
using them will not accidentally depend on is_kernel_rodata(). If KUnit
is built-in, they'll benefit from the optimisation, if KUnit is not,
they won't, but the string will be properly duplicated.

Fixes: d03c720e ("kunit: Add APIs for managing devices")
Reported-by: Nico Pache <npache@redhat.com>
Closes: https://groups.google.com/g/kunit-dev/c/81V9b9QYON0Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>