powerpc: fix KUAP warning in VMX usercopy path

On powerpc with PREEMPT_FULL or PREEMPT_LAZY and function tracing enabled, KUAP warnings can be triggered from the VMX usercopy path under memory stress workloads.

KUAP requires that no subfunctions are called once userspace access has been enabled. The existing VMX copy implementation violates this requirement by invoking enter_vmx_usercopy() from the assembly path after userspace access has already been enabled. If preemption occurs in this window, the AMR state may not be preserved correctly, leading to unexpected userspace access state and resulting in KUAP warnings.

Fix this by restructuring the VMX usercopy flow so that VMX selection and VMX state management are centralized in raw_copy_tofrom_user(), which is invoked by the raw_copy_{to,from,in}_user() wrappers.

The new flow is:

- raw_copy_{to,from,in}_user() calls raw_copy_tofrom_user()
- raw_copy_tofrom_user() decides whether to use the VMX path based on size and CPU capability
- Call enter_vmx_usercopy() before enabling userspace access
- Enable userspace access as per the copy direction and perform the VMX copy
- Disable userspace access as per the copy direction
- Call exit_vmx_usercopy()
- Fall back to the base copy routine if the VMX copy faults

With this change, the VMX assembly routines no longer perform VMX state management or call helper functions; they only implement the copy operations.
The previous feature-section based VMX selection inside __copy_tofrom_user_power7() is removed, and a dedicated __copy_tofrom_user_power7_vmx() entry point is introduced.

This ensures correct KUAP ordering, avoids subfunction calls while KUAP is unlocked, and eliminates the warnings while preserving the VMX fast path.

Fixes de78a9c42a79 ("powerpc: Add a framework for Kernel Userspace Access Protection")

Reported-by Shrikanth Hegde <[email protected]>

Closes https://lore.kernel.org/all/[email protected]/ open here

Suggested-by Christophe Leroy (CS GROUP) <[email protected]>

Reviewed-by Christophe Leroy (CS GROUP) <[email protected]>

Co-developed-by Aboorva Devarajan <[email protected]>

Signed-off-by Aboorva Devarajan <[email protected]>

Signed-off-by Sayali Patil <[email protected]>

---