On 1/29/2021 2:53 PM, Dave Hansen wrote:
On 1/29/21 2:35 PM, Yu, Yu-cheng wrote:
Andy Cooper just mentioned on IRC about this nugget in the spec:
XRSTORS on CET state will do reserved bit and canonicality
checks on the state in similar manner as done by the WRMSR to
these state elements.
We're using copy_kernel_to_xregs_err(), so the #GP *should* be OK.
Could we prove this out in practice, please?
Do we want to verify that setting reserved bits in CET XSAVES states
triggers GP? Then, yes, I just verified it again. Thanks for
reminding. Do we have any particular case relating to this?
I want to confirm that it triggers #GP and kills userspace without the
kernel WARN'ing or otherwise being visibly unhappy.
For sigreturn, shadow stack pointer is checked against its restore token
and must be smaller than TASK_SIZE_MAX. Sigreturn cannot set any
MSR_IA32_U_CET reserved bits.
What about the return-to-userspace path after a ptracer writes content
to the CET fields? I don't see the same tolerance for errors in
__fpregs_load_activate(), for instance.
Good thought. I have not sent out my revised PTRACE patch, but values
from user will be checked for valid address and reserved bits.
--
Yu-cheng
