[xen stable-4.18] xen/sched: fix adding offline cpu to cpupool

[patchbot@xxxxxxx]

commit 5ac87c8afd2ae2b1a9fd46a9b80d9152d650fb26
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Wed Dec 6 10:40:54 2023 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 6 10:40:54 2023 +0100

    xen/sched: fix adding offline cpu to cpupool
    
    Trying to add an offline cpu to a cpupool can crash the hypervisor,
    as the probably non-existing percpu area of the cpu is accessed before
    the availability of the cpu is being tested. This can happen in case
    the cpupool's granularity is "core" or "socket".
    
    Fix that by testing the cpu to be online.
    
    Fixes: cb563d7665f2 ("xen/sched: support core scheduling for moving cpus 
to/from cpupools")
    Reported-by: René Winther Højgaard <renewin@xxxxxxxxx>
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 06e8d65d33896aa90f5b6d9b2bce7f11433b33c9
    master date: 2023-12-05 09:57:38 +0100
---
 xen/common/sched/cpupool.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/common/sched/cpupool.c b/xen/common/sched/cpupool.c
index 2e094b0cfa..ad8f608462 100644
--- a/xen/common/sched/cpupool.c
+++ b/xen/common/sched/cpupool.c
@@ -892,6 +892,8 @@ int cpupool_do_sysctl(struct xen_sysctl_cpupool_op *op)
         if ( cpu >= nr_cpu_ids )
             goto addcpu_out;
         ret = -ENODEV;
+        if ( !cpu_online(cpu) )
+            goto addcpu_out;
         cpus = sched_get_opt_cpumask(c->gran, cpu);
         if ( !cpumask_subset(cpus, &cpupool_free_cpus) ||
              cpumask_intersects(cpus, &cpupool_locked_cpus) )
--
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