Re: [PATCH v8 24/29] KVM: s390: arm64: Implement required functions

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] kvm: arm64: guest-triggerable host infinite loop due to empty exception stubs
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commit d9fe4f13dd483ddf56413b5d5c4c9d6582209f75
Author: Steffen Eiden <seiden@linux.ibm.com>
KVM: s390: arm64: Implement required functions

This commit implements various stub functions and simple handlers required
by the shared ARM64 KVM code for the s390 architecture. It includes an
initial smccc handler to stop a vCPU and several exception injection stubs.

> diff --git a/arch/s390/kvm/arm64/handle_exit.c b/arch/s390/kvm/arm64/handle_exit.c
> --- /dev/null
> +++ b/arch/s390/kvm/arm64/handle_exit.c
[ ... ]
> +static int kvm_handle_unknown_ec(struct kvm_vcpu *vcpu)
> +{
> +	u64 esr = kvm_vcpu_get_esr(vcpu);
> +
> +	kvm_pr_unimpl("Unknown exception class: esr: %#016llx -- %s\n",
> +		      esr, esr_get_class_string(esr));
> +
> +	kvm_inject_undefined(vcpu);
> +	return 1;
> +}

[Severity: Medium]
Because kvm_inject_undefined() is currently an empty stub, does returning 1
here tell the run loop to resume the guest without modifying its state?

If the guest's PC is not advanced, this can cause the guest to immediately
re-trap the exact same instruction, leading to an infinite trap loop that
pegs the host VCPU thread at 100% and spams the kernel log.

> diff --git a/arch/s390/kvm/arm64/inject_fault.c b/arch/s390/kvm/arm64/inject_fault.c
> --- /dev/null
> +++ b/arch/s390/kvm/arm64/inject_fault.c
[ ... ]
> +void kvm_inject_undefined(struct kvm_vcpu *vcpu)
> +{
> +	/* Stub until s390 supports arm64 sysregs TODO sysregs*/
> +}

[Severity: Medium]
Will leaving this as an empty stub fail to advance the Program Counter or
modify guest registers to actually inject the exception?

Failing to modify the guest state before returning to execution means any
unprivileged guest executing an undefined instruction could trivially trigger
an infinite loop.

> +
> +int kvm_inject_sea(struct kvm_vcpu *vcpu, bool iabt, u64 addr)
> +{
> +	/* Stub until s390 supports arm64 sysregs TODO sysregs*/
> +	return 1;
> +}

[Severity: Medium]
Can returning 1 from this empty memory fault stub identically cause an
infinite loop for unimplemented memory faults?

Since the guest state isn't modified to inject the fault, resuming it will
simply re-execute the failing instruction over and over.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260918133107.1042730-1-seiden@linux.ibm.com?part=24