microblaze: prevent ptrace writes to MSR and pt_mode

microblaze: prevent ptrace writes to MSR and pt_mode

arch_ptrace() accepts PTRACE_POKEUSR for every aligned offset below
PT_SIZE. This includes the saved machine status register and pt_mode,
which are kernel-owned execution state rather than general registers.

On return from an exception, rtbd copies MSR_UMS to the live MSR_UM bit.
An unprivileged same-UID tracer can therefore clear MSR_UMS in its
tracee's saved register frame and make the tracee resume in supervisor
mode. pt_mode also controls whether later entry paths classify the frame
as kernel or user state.

Reject PTRACE_POKEUSR writes to both fields while retaining read access.

Fixes: 2b4384542691 ("microblaze_v8: ptrace support")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Link: https://patch.msgid.link/20260821103026.3627292-2-Jeremy.Jean@oss.cyber.gouv.fr
Signed-off-by: Michal Simek <michal.simek@amd.com>