[PATCH v4 1/2] powerpc: fix KUAP warning in VMX usercopy path

[Ritesh Harjani (IBM)]

Sayali Patil <sayalip at linux.ibm.com> writes:

> 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 <sshegde at linux.ibm.com>
> Closes: https://lore.kernel.org/all/20260109064917.777587-2-sshegde@linux.ibm.com/
> Suggested-by: Christophe Leroy (CS GROUP) <chleroy at kernel.org>
> Reviewed-by: Christophe Leroy (CS GROUP) <chleroy at kernel.org>
> Co-developed-by: Aboorva Devarajan <aboorvad at linux.ibm.com>
> Signed-off-by: Aboorva Devarajan <aboorvad at linux.ibm.com>
> Signed-off-by: Sayali Patil <sayalip at linux.ibm.com>
> ---

Thanks Sayali & Christophe for the help on this. I liked the new
wrappers which removed the code duplication by calling into a common
function call raw_copy_tofrom_user().

The new changes looks good to me too. Please feel free to add:

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list at gmail.com>