This is still work-in-progress, but putting it up in case it is
helpful to people working in this area and for early comments.
Add a set of tests to validate the expected security properties
of threads created with K_USER when CONFIG_USERSPACE=y. This can
be used as a regression test for architectures that already implement
this support and as a validation test for others.
I considered incorporating these tests into the existing protection
test, but decided against it since protection does not enable or rely
upon CONFIG_USERSPACE for its existing tests and passes on everything
that provides MPU or MMU support, even without full userspace support.
I also considered incorporating these tests into the existing
obj_validation test, but decided against it since obj_validation only
tests the object validation/permission logic, does not run any user
mode threads (or strictly depend on that support), and passes
on both x86 and arm today, unlike these tests. That said, I have no
strong objections if it would be preferable to fold these into it
(and perhaps rename it to be more general).
The current tests implemented in this test program verify the following
for a thread created with K_USER:
is_usermode: is running in usermode
priv_insn: cannot invoke privileged insns directly
write_control: cannot write to control registers
disable_mmu_mpu: cannot disable memory protections (MMU/MPU)
read_kernram: cannot read from kernel RAM
write_kernram: cannot write to kernel RAM
write_kernro: cannot write to kernel rodata
write_kerntext: cannot write to kernel text
read_kernel_data: cannot read __kernel-marked data
write_kernel_data: cannot write __kernel-marked data
read_kernel_stack: cannot read the kernel/privileged stack
write_kernel_stack: cannot write the kernel/privileged stack
pass_user_object: cannot pass a non-kernel object to a syscall
pass_noperms_object: cannot pass an object to a syscall without a grant
start_kernel_thread: cannot start a kernel (non-user) thread
Some of the tests overlap and could possibly be dropped, but it
seems harmless to retain them. The particular targets of read/write
tests are arbitrary other than meeting the test criteria and can be
changed (e.g. in data, rodata, or text) if desired to avoid coupling
to kernel implementation details that may change in the future.
On qemu_x86, all of the tests pass. And, if you replace all
occurrences of ztest_user_unit_test() with ztest_unit_test(), then
all of the tests fail (i.e. when the tests are run in kernel mode,
they all fail as expected). On frdm_k64f presently (w/o the arm
userspace patches), all of the tests fail except for write_kernro and
write_kerntext, as expected.
ToDo:
- Verify that a user thread cannot access data in another memory domain.
- Verify that a user thread cannot access another thread's stack.
- Verify that a user thread cannot access another thread's kobject.
- Verify that k_thread_user_mode_enter() transitions correctly.
- Verify that k_object_access_revoke() is enforced.
- Verify that syscalls return to user mode upon completion.
- Verify that a user thread cannot abuse other svc calls (ARM-specific).
- Other suggested properties we should be testing?
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Stephen Smalley