Re: [PATCH 1/5] KVM: e500: retry if no memslot is found

[Sean Christopherson]

On Thu, Jan 09, 2025, Paolo Bonzini wrote:
> Avoid a NULL pointer dereference if the memslot table changes between the
> exit and the call to kvmppc_e500_shadow_map().
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/powerpc/kvm/e500_mmu_host.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/e500_mmu_host.c b/arch/powerpc/kvm/e500_mmu_host.c
> index e5a145b578a4..732335444d68 100644
> --- a/arch/powerpc/kvm/e500_mmu_host.c
> +++ b/arch/powerpc/kvm/e500_mmu_host.c
> @@ -349,6 +349,11 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
>  	 * pointer through from the first lookup.
>  	 */
>  	slot = gfn_to_memslot(vcpu_e500->vcpu.kvm, gfn);
> +	if (!slot) {
> +		ret = -EAGAIN;
> +		goto out;
> +	}

This is unnecessary, __gfn_to_hva_many() checks for a NULL @slot.

  static unsigned long __gfn_to_hva_many(const struct kvm_memory_slot *slot, gfn_t gfn,
				       gfn_t *nr_pages, bool write)
  {
	if (!slot || slot->flags & KVM_MEMSLOT_INVALID)
		return KVM_HVA_ERR_BAD;

	if (memslot_is_readonly(slot) && write)
		return KVM_HVA_ERR_RO_BAD;

	if (nr_pages)
		*nr_pages = slot->npages - (gfn - slot->base_gfn);

	return __gfn_to_hva_memslot(slot, gfn);
  }

  unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot,
					gfn_t gfn)
  {
	return gfn_to_hva_many(slot, gfn, NULL);
  }

Not checking the return value and doing a VMA lookup on hva=-1 when tlbsel==1 is
gross, but it should be functionally safe.

Returning -EAGAIN is nicer (kvmppc_e500_shadow_map() will inevitably return -EINVAL),
but in practice it doesn't matter because all callers ultimately ignore the return
value.

Since there's a ratelimited printk that yells if there's no slot, it's probably
best to let sleeping dogs lie.

	if (likely(!pfnmap)) {
		tsize_pages = 1UL << (tsize + 10 - PAGE_SHIFT);
		pfn = __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, NULL, &page);
		if (is_error_noslot_pfn(pfn)) {
			if (printk_ratelimit())
				pr_err("%s: real page not found for gfn %lx\n",
				       __func__, (long)gfn);
			return -EINVAL;
		}



> +
>  	hva = gfn_to_hva_memslot(slot, gfn);
>  
>  	if (tlbsel == 1) {
> -- 
> 2.47.1
>