[GIT PULL 04/11] KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler

Janosch Frank
2023-06-21T15:35:15Z

From: Christian Borntraeger <borntraeger@linux.ibm.com>

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@linux.ibm.com>
Fixes: 87e28a15c42c ("KVM: s390: diag9c (directed yield) forwarding")
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
---
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);
--
2.41.0