Add U-mode task isolation and fix self-termination

[HeatCrab]

U-mode tasks could previously control other tasks and had no way to properly terminate themselves. This adds permission checks to restrict task control syscalls to self-only operations and enables safe self-termination through the existing zombie task mechanism.

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Summary by cubic

Locks down U‑mode task control to self-only and adds safe self-termination using the zombie task path. This prevents user tasks from affecting others and ensures proper cleanup.

• Bug Fixes
  • Enforce U‑mode isolation: tcancel/tsuspend/tpriority now only allow id == mo_task_id(); otherwise return -EPERM. tresume always returns -EPERM for U‑mode.
  • Fix self-termination: calling mo_task_cancel on self sets TASK_ZOMBIE and yields for scheduler cleanup; id == 0 still returns ERR_TASK_CANT_REMOVE.

^{Written for commit 8f6f132. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="kernel/task.c">

<violation number="1" location="kernel/task.c:1003">
P1: Marking the current task as TASK_ZOMBIE before _yield() lets dispatch() free the running task in task_cleanup_zombies(), then dereference kcb->task_current->data during context save/scheduling. This introduces a use-after-free on self-termination. Defer freeing the current task until after the context switch or skip cleanup of the current task.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P1: Marking the current task as TASK_ZOMBIE before _yield() lets dispatch() free the running task in task_cleanup_zombies(), then dereference kcb->task_current->data during context save/scheduling. This introduces a use-after-free on self-termination. Defer freeing the current task until after the context switch or skip cleanup of the current task.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At kernel/task.c, line 1003:

<comment>Marking the current task as TASK_ZOMBIE before _yield() lets dispatch() free the running task in task_cleanup_zombies(), then dereference kcb->task_current->data during context save/scheduling. This introduces a use-after-free on self-termination. Defer freeing the current task until after the context switch or skip cleanup of the current task.</comment>

<file context>
@@ -992,9 +992,20 @@ int32_t mo_task_spawn_user(void *task_entry, uint16_t stack_size)
+     */
+    if (id == mo_task_id()) {
+        tcb_t *self = kcb->task_current->data;
+        self->state = TASK_ZOMBIE;
+        _yield();
+        while (1)
</file context>