This is a note to let you know that I've just added the patch titled KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler to the 6.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: kvm-s390-diag-fix-racy-access-of-physical-cpu-number.patch and it can be found in the queue-6.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit bae0036b51963a467b99e02a574182c145afafaa Author: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx> Date: Mon May 15 10:42:34 2023 +0200 KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler [ Upstream commit 0bc380beb78aa352eadbc21d934dd9606fcee808 ] 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> Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/arch/s390/kvm/diag.c b/arch/s390/kvm/diag.c index 807fa9da1e721..3c65b8258ae67 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);