Re: [PATCH] KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler

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On 23.05.23 16:05, Christian Borntraeger wrote:
We do check for target CPU == -1, but this might change at the time we
are going to use it. Hold the physical target CPU in a local variable to
avoid out-of-bound accesses to the cpu arrays.

Cc: Pierre Morel <pmorel@xxxxxxxxxxxxx>
Fixes: 87e28a15c42c ("KVM: s390: diag9c (directed yield) forwarding")
Reported-by: Marc Hartmayer <mhartmay@xxxxxxxxxxxxx>
Reviewed-by: Nico Boehr <nrb@xxxxxxxxxxxxx>
Reviewed-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
---
  arch/s390/kvm/diag.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/s390/kvm/diag.c b/arch/s390/kvm/diag.c
index 807fa9da1e72..3c65b8258ae6 100644
--- a/arch/s390/kvm/diag.c
+++ b/arch/s390/kvm/diag.c
@@ -166,6 +166,7 @@ static int diag9c_forwarding_overrun(void)
  static int __diag_time_slice_end_directed(struct kvm_vcpu *vcpu)
  {
  	struct kvm_vcpu *tcpu;
+	int tcpu_cpu;
  	int tid;
tid = vcpu->run->s.regs.gprs[(vcpu->arch.sie_block->ipa & 0xf0) >> 4];
@@ -181,14 +182,15 @@ static int __diag_time_slice_end_directed(struct kvm_vcpu *vcpu)
  		goto no_yield;
/* target guest VCPU already running */
-	if (READ_ONCE(tcpu->cpu) >= 0) {
+	tcpu_cpu = READ_ONCE(tcpu->cpu);
+	if (tcpu_cpu >= 0) {
  		if (!diag9c_forwarding_hz || diag9c_forwarding_overrun())
  			goto no_yield;
/* target host CPU already running */
-		if (!vcpu_is_preempted(tcpu->cpu))
+		if (!vcpu_is_preempted(tcpu_cpu))
  			goto no_yield;
-		smp_yield_cpu(tcpu->cpu);
+		smp_yield_cpu(tcpu_cpu);
  		VCPU_EVENT(vcpu, 5,
  			   "diag time slice end directed to %d: yield forwarded",
  			   tid);

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb




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