feat(kernel): derivation-tree revocation, region caps
- caps record parent (mint/delegate/ipc), CapRef w/ pid+slot gens - Revoke = delete subtree, keep self; drop/death re-parents children - CreateRegion/MapRegion; mapping lives only as long as its cap - drop Permission::Revoke + object-wide revoke - mint any cap (badge only ep/ntfn), process caps delegable - kani: descendants == subtree; model-based fuzz vs spec
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326
crates/kernel/src/tests/captree_model.rs
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326
crates/kernel/src/tests/captree_model.rs
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//! Randomized check of the derivation tree against an independent model.
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//! The model applies the rules of docs/ipc.md (derive records a parent,
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//! delete re-parents, revoke removes the subtree, death deletes everything
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//! a process holds) to its own data; after every step the kernel must hold
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//! exactly the capabilities the model says are alive.
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extern crate std;
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use std::collections::HashMap;
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use std::vec::Vec;
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use super::*;
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use crate::{SyscallOutcome, SyscallResult};
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use domain::syscall::Syscall;
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use types::address::{DataRef, VirtualAddress};
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use types::badge::Badge;
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use types::bounded::{CapSlot, PageCount};
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use types::id::ThreadId;
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use types::mapping::PagePerms;
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use types::permission::{Permission, PermissionSet};
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const STEPS: usize = 1500;
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struct Rng(u64);
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impl Rng {
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fn next(&mut self) -> u64 {
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self.0 ^= self.0 << 13;
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self.0 ^= self.0 >> 7;
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self.0 ^= self.0 << 17;
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self.0
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}
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fn below(&mut self, n: usize) -> usize {
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(self.next() % n as u64) as usize
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}
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fn pick<T: Copy>(&mut self, xs: &[T]) -> Option<T> {
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(!xs.is_empty()).then(|| xs[self.below(xs.len())])
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}
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}
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#[derive(Clone, Copy)]
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struct Node {
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parent: Option<usize>,
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is_region: bool,
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}
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/// The reference model: logical capabilities and where each one lives.
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#[derive(Default)]
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struct Model {
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nodes: Vec<Node>,
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alive: HashMap<usize, (ProcessId, CapSlot)>,
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}
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impl Model {
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fn add(&mut self, at: (ProcessId, CapSlot), parent: Option<usize>, is_region: bool) -> usize {
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self.nodes.push(Node { parent, is_region });
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let id = self.nodes.len() - 1;
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self.alive.insert(id, at);
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id
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}
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fn delete(&mut self, id: usize) {
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let grandparent = self.nodes[id].parent;
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for n in 0..self.nodes.len() {
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if self.alive.contains_key(&n) && self.nodes[n].parent == Some(id) {
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self.nodes[n].parent = grandparent;
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}
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}
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self.alive.remove(&id);
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}
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fn is_descendant(&self, mut n: usize, root: usize) -> bool {
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while let Some(p) = self.nodes[n].parent {
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if p == root {
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return true;
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}
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n = p;
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}
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false
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}
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fn revoke(&mut self, root: usize) {
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let doomed: Vec<usize> = self
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.alive
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.keys()
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.copied()
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.filter(|&n| self.is_descendant(n, root))
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.collect();
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for n in doomed {
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self.alive.remove(&n);
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}
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}
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fn held_by(&self, pid: ProcessId) -> Vec<(usize, CapSlot)> {
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self.alive
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.iter()
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.filter(|(_, (p, _))| *p == pid)
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.map(|(id, (_, s))| (*id, *s))
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.collect()
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}
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}
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fn all() -> PermissionSet {
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PermissionSet::empty()
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.with(Permission::Read)
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.with(Permission::Write)
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.with(Permission::Execute)
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.with(Permission::Delegate)
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}
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struct World {
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k: Kernel<MockPlatform>,
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m: Model,
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pids: [ProcessId; 3],
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tids: [ThreadId; 3],
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}
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impl World {
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fn new() -> Self {
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let (k, a) = kernel_with_process();
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let ta = k.current_thread().unwrap();
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let mut w = Self {
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k,
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m: Model::default(),
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pids: [a; 3],
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tids: [ta; 3],
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};
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// init's own boot capability (ProcessCreate) is a kernel root.
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let create = CapSlot::new(0).unwrap();
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w.m.add((a, create), None, false);
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for i in 1..3 {
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w.spawn(i);
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}
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for call in [Syscall::CreateEndpoint, Syscall::CreateNotification] {
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let s = w.slot(0, call);
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w.m.add((a, s), None, false);
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}
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let r = w.slot(
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0,
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Syscall::CreateRegion {
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pages: PageCount::new(1).unwrap(),
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},
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);
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w.m.add((a, r), None, true);
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w
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}
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fn slot(&mut self, who: usize, call: Syscall) -> CapSlot {
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self.k.switch_to_thread_for_test(self.tids[who]);
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match self.k.handle_syscall(call) {
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SyscallOutcome::Complete(SyscallResult::Slot(s)) => s,
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_ => panic!("expected a slot"),
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}
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}
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/// (Re)creates process `i` from init; init gets a root process cap.
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fn spawn(&mut self, i: usize) {
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let cap = self.slot(
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0,
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Syscall::CreateProcess {
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create_cap: CapSlot::new(0).unwrap(),
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image: DataRef::empty(),
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},
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);
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self.pids[i] = self.k.process_of_cap_for_test(cap);
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self.tids[i] =
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self.k
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.spawn_thread_for_test(self.pids[i], Priority::DEFAULT, TimeSlice::DEFAULT);
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self.m.add((self.pids[0], cap), None, false);
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}
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fn run(&mut self, who: usize, call: Syscall) -> bool {
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self.k.switch_to_thread_for_test(self.tids[who]);
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!matches!(
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self.k.handle_syscall(call),
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SyscallOutcome::Complete(SyscallResult::Error(_))
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)
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}
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fn step(&mut self, rng: &mut Rng) {
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let who = rng.below(3);
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let pid = self.pids[who];
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let held = self.m.held_by(pid);
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let Some((id, slot)) = rng.pick(&held) else {
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return;
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};
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match rng.below(10) {
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0..=2 => {
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// Badges only go on endpoints and notifications; the kernel
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// must refuse the rest, so try both.
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let badge = Badge::new(rng.below(2));
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let before = self.k.cap_slots_for_test(pid);
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let ok = self.run(
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who,
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Syscall::Mint {
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slot,
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badge,
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permissions: all(),
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},
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);
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if ok {
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let new = newly_used(before, self.k.cap_slots_for_test(pid));
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let is_region = self.m.nodes[id].is_region;
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self.m.add((pid, new), Some(id), is_region);
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}
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}
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3..=4 => {
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// Delegate through any process cap the caller holds.
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let proc_caps: Vec<(CapSlot, ProcessId)> = held
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.iter()
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.filter_map(|(_, s)| {
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self.k.process_cap_target_for_test(pid, *s).map(|t| (*s, t))
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})
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.collect();
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if let Some((to, target)) = rng.pick(&proc_caps) {
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let before = self.k.cap_slots_for_test(target);
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let ok = self.run(
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who,
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Syscall::Delegate {
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slot,
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to_endpoint: to,
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permissions: all(),
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},
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);
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if ok {
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let new = newly_used(before, self.k.cap_slots_for_test(target));
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let is_region = self.m.nodes[id].is_region;
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self.m.add((target, new), Some(id), is_region);
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}
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}
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}
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// init keeps its ProcessCreate cap so processes can be re-made.
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5 if !(who == 0 && slot == CapSlot::new(0).unwrap()) => {
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if self.run(who, Syscall::DropCap { slot }) {
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self.m.delete(id);
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}
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}
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6..=7 => {
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assert!(
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self.run(who, Syscall::Revoke { slot }),
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"revoking a held cap failed"
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);
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self.m.revoke(id);
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}
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8 => {
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let va = VirtualAddress::from_raw(0x30000 + rng.below(16) * 0x1000);
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let _ = self.run(
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who,
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Syscall::MapRegion {
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region: slot,
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va,
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perms: PagePerms::ReadWrite,
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},
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);
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}
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_ if who == 0 && rng.below(4) == 0 => {
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let victim = 1 + rng.below(2);
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let vp = self.pids[victim];
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// init's cap to the victim may already be revoked; kill it
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// directly, as a supervisor holding a cap would.
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for (n, _) in self.m.held_by(vp) {
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self.m.delete(n);
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}
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self.k.destroy_process(vp);
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self.spawn(victim);
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}
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_ => {}
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}
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}
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fn check(&self) {
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for &pid in &self.pids {
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let used = self.k.cap_slots_for_test(pid);
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let mut expected = [false; 64];
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for (_, s) in self.m.held_by(pid) {
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expected[s.value() as usize] = true;
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}
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assert_eq!(used, expected, "{pid:?} holds the wrong capabilities");
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}
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// Every capability-backed mapping is backed by a live capability
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// of the mapping process.
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for (_, region) in self.k.regions.iter() {
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for m in region.mappings() {
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if let Some(via) = m.via {
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assert!(self.k.resolves(via), "mapping outlived its capability");
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assert_eq!(via.process, m.process);
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}
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}
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}
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}
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}
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fn newly_used(before: [bool; 64], after: [bool; 64]) -> CapSlot {
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(0..64)
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.find(|&i| after[i] && !before[i])
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.and_then(|i| CapSlot::new(i as u8))
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.expect("a new slot")
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}
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#[test]
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fn derivation_tree_matches_the_model() {
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for seed in [5u64, 0xBADC0DE, 31337, 0x5EED, 2024] {
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let mut world = World::new();
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let mut rng = Rng(seed);
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let mut revokes = 0;
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for _ in 0..STEPS {
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let before = world.m.alive.len();
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world.step(&mut rng);
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if world.m.alive.len() < before {
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revokes += 1;
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}
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world.check();
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}
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assert!(
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revokes > 20,
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"seed {seed:#x}: too little deletion exercised"
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);
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assert!(
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world.m.nodes.len() > 100,
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"seed {seed:#x}: too little derivation"
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);
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}
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}
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