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Re: [PATCH] xen/sched: fix sched_move_domain() for domain without vcpus

On 06.09.21 13:14, Andrew Cooper wrote:
On 06/09/2021 12:00, Juergen Gross wrote:
In case a domain is created with a cpupool other than Pool-0 specified
it will be moved to that cpupool before any vcpus are allocated.

This will lead to a NULL pointer dereference in sched_move_domain().

Fix that by tolerating vcpus not being allocated yet.

Fixes: 70fadc41635b9b6 ("xen/cpupool: support moving domain between cpupools with 
different granularity")
Reported-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
  xen/common/sched/core.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 8d178baf3d..79c9100680 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -671,6 +671,10 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
for ( unit_idx = 0; unit_idx < n_units; unit_idx++ )
+        /* Special case for move at domain creation time. */
+        if ( !d->vcpu[unit_idx * gran] )
+            break;
          unit = sched_alloc_unit_mem();
          if ( unit )

I think the logic would be clearer if you wrap the entire for loop in if
( d->max_vcpus ).

No, d->max_vcpus is not 0 here, otherwise n_units would be 0.

This loop is only allocating units in the new
scheduler for existing vcpus, so there's no point entering the loop at
all during domain creation.

Also, this removes a non-speculatively-guarded d->vcpu[] deference.

I don't think this dereference is a real problem. In case you are
worried about it we should replace the one further below, too.


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