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

[patchbot@xxxxxxx]

commit f294e3abe40cda37618d9fbad8cde86bbae0d1c6
Author:     Oleksii Moisieiev <oleksii_moisieiev@xxxxxxxx>
AuthorDate: Tue Jul 21 09:47:36 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jul 21 09:47:36 2026 +0200

    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: b6f5334aeaca ("sched: fix cpu offlining with core scheduling")
    Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@xxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/common/sched/rt.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/xen/common/sched/rt.c b/xen/common/sched/rt.c
index 75d65e28ad..744f214173 100644
--- a/xen/common/sched/rt.c
+++ b/xen/common/sched/rt.c
@@ -762,10 +762,16 @@ 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
@@ -792,7 +798,7 @@ 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);
 }
@@ -810,7 +816,7 @@ 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);
 }
--
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