xen/sched: rt: fix NULL cpupool dereference in move_repl_timer()

schedule_cpu_rm() clears the cpupool pointer of the scheduling
resource before calling sched_deinit_pdata():

    sr->cpupool = NULL;
    ...
    sched_deinit_pdata(data->old_ops, data->ppriv_old, cpu);

For RTDS, rt_deinit_pdata() calls move_repl_timer() when the
replenishment timer lives on the cpu being removed, and
move_repl_timer() dereferences get_sched_res(old_cpu)->cpupool
without checking it for NULL. Removing a pCPU owning the timer from
an RTDS cpupool therefore dereferences NULL + 0x10 (the res_valid
member) and panics:

    (XEN) Data Abort Trap. Syndrome=0x1c28005
    (XEN) Walking Hypervisor VA 0x10 on CPU0 via TTBR ...
    (XEN) Xen call trace:
    (XEN)    [<...>] find_next_bit+0x74/0xa8 (PC)
    (XEN)    [<...>] rt.c#move_repl_timer+0xb8/0xec (LR)
    (XEN)
    (XEN) Panic on CPU 0:
    (XEN) CPU0: Unexpected Trap: Data Abort

Reproducer, on any host with at least 2 pCPUs and RTDS compiled in
(observed on arm64, but the path is common code):

    xl cpupool-create name="test" sched="rtds"
    xl cpupool-cpu-remove Pool-0 1
    xl cpupool-cpu-add test 1
    xl cpupool-cpu-remove test 1

The last command moves the RTDS replenishment timer to cpu1 (first
and only cpu of the pool) and then removes cpu1, hitting the NULL
dereference in the sched_deinit_pdata() callback.

Use the cpupool back-pointer of the scheduler instead of the one of
the scheduling resource. It is set by cpupool_create() before any
pCPU can be assigned to the pool and stays valid for the whole
lifetime of the scheduler, so it is still available when
rt_deinit_pdata() runs. Other schedulers already rely on it the same
way, e.g. credit2 in cpu_add_to_runqueue().

Fixes: b6f5334aea ("sched: fix cpu offlining with core scheduling")
Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
master commit: f294e3abe4
master date: 2026-07-21 09:47:36 +0200

xen/common/sched/rt.c

@@ -763,10 +763,16 @@ rt_switch_sched(struct scheduler *new_ops, unsigned int cpu,
     return &prv->lock;
 }
 
-static void move_repl_timer(struct rt_private *prv, unsigned int old_cpu)
+static void move_repl_timer(const struct scheduler *ops, unsigned int old_cpu)
 {
-    cpumask_t *online = get_sched_res(old_cpu)->cpupool->res_valid;
-    unsigned int new_cpu = cpumask_cycle(old_cpu, online);
+    struct rt_private *prv = rt_priv(ops);
+    /*
+     * Use the cpupool of the scheduler: the one of the scheduling resource
+     * is already cleared when this is called from rt_deinit_pdata().
+     */
+    const struct cpupool *c = ops->cpupool;
+    unsigned int new_cpu = c ? cpumask_cycle(old_cpu, c->res_valid)
+                             : nr_cpu_ids;
 
     /*
      * Make sure the timer run on one of the cpus that are still available
@@ -793,7 +799,7 @@ rt_deinit_pdata(const struct scheduler *ops, void *pcpu, int cpu)
     spin_lock_irqsave(&prv->lock, flags);
 
     if ( prv->repl_timer.cpu == cpu )
-        move_repl_timer(prv, cpu);
+        move_repl_timer(ops, cpu);
 
     spin_unlock_irqrestore(&prv->lock, flags);
 }
@@ -811,7 +817,7 @@ rt_move_timers(const struct scheduler *ops, struct sched_resource *sr)
     if ( prv->repl_timer.status != TIMER_STATUS_invalid &&
          prv->repl_timer.status != TIMER_STATUS_killed &&
          !cpumask_test_cpu(old_cpu, sr->cpupool->res_valid) )
-        move_repl_timer(prv, old_cpu);
+        move_repl_timer(ops, old_cpu);
 
     spin_unlock_irqrestore(&prv->lock, flags);
 }