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

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

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 the last pCPU 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.

Treat a NULL cpupool like a cpupool with no remaining scheduling
resources and kill the timer, which is the intent of the existing
comment in move_repl_timer().

Fixes: b6f5334aeaca ("sched: fix cpu offlining with core scheduling")
Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@xxxxxxxx>
---

 xen/common/sched/rt.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/common/sched/rt.c b/xen/common/sched/rt.c
index f1feb4384e..49a9b76b89 100644
--- a/xen/common/sched/rt.c
+++ b/xen/common/sched/rt.c
@@ -766,8 +766,9 @@ rt_switch_sched(struct scheduler *new_ops, unsigned int cpu,
 
 static void move_repl_timer(struct rt_private *prv, unsigned int old_cpu)
 {
-    cpumask_t *online = get_sched_res(old_cpu)->cpupool->res_valid;
-    unsigned int new_cpu = cpumask_cycle(old_cpu, online);
+    const struct cpupool *c = get_sched_res(old_cpu)->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
-- 
2.43.0

base-commit: 959fa8800470e3d757b5a0bcd9394954ed7b923a
branch: amoi_sched_6