Re: [PATCH v2] KVM: arm/arm64: Check memslot bounds before mapping hugepages

[Punit Agrawal]

Hi Lukas,

Lukas Braun <koomi@xxxxxxxxxxx> writes:

> Userspace can create a memslot with memory backed by (transparent)
> hugepages, but with bounds that do not align with hugepages.
> In that case, we cannot map the entire region in the guest as hugepages
> without exposing additional host memory to the guest and potentially
> interfering with other memslots.
> Consequently, this patch adds a bounds check when populating guest page
> tables and forces the creation of regular PTEs if mapping an entire
> hugepage would violate the memslots bounds.
>
> Signed-off-by: Lukas Braun <koomi@xxxxxxxxxxx>
> ---
>
> Hi everyone,
>
> for v2, in addition to writing the condition the way Marc suggested, I
> moved the whole check so it also catches the problem when the hugepage
> was allocated explicitly, not only for THPs.

Ok, that makes sense. Memslot bounds could exceed for hugetlbfs pages as
well.

> The second line is quite long, but splitting it up would make things
> rather ugly IMO, so I left it as it is.

Let's try to do better - user_mem_abort() is quite hard to follow as it
is.

>
>
> Regards,
> Lukas
>
>
>  virt/kvm/arm/mmu.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index ed162a6c57c5..ba77339e23ec 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1500,7 +1500,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		return -EFAULT;
>  	}
>  
> -	if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
> +	if ((fault_ipa & S2_PMD_MASK) < (memslot->base_gfn << PAGE_SHIFT) ||
> +	    ALIGN(fault_ipa, S2_PMD_SIZE) >= ((memslot->base_gfn + memslot->npages) << PAGE_SHIFT)) {
> +		/* PMD entry would map something outside of the memslot */
> +		force_pte = true;
> +	} else if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
>  		hugetlb = true;
>  		gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
>  	} else {

For the purpose of this fix, using a helper to check whether the mapping
fits in the memslot makes things clearer (imo) (untested patch below) -

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index ed162a6c57c5..8bca141eb45e 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1466,6 +1466,18 @@ static void kvm_send_hwpoison_signal(unsigned long address,
        send_sig_info(SIGBUS, &info, current);
 }
 
+static bool mapping_in_memslot(struct kvm_memory_slot *memslot,
+                        phys_addr_t fault_ipa, unsigned long mapping_size)
+{
+ gfn_t start_gfn = (fault_ipa & ~(mapping_size - 1)) >> PAGE_SHIFT;
+ gfn_t end_gfn = ALIGN(fault_ipa, mapping_size) >> PAGE_SHIFT;
+
+ WARN_ON(!is_power_of_2(mapping_size));
+
+ return memslot->base_gfn <= start_gfn &&
+         end_gfn < memslot->base_gfn + memslot->npages;
+}
+
 static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
                          struct kvm_memory_slot *memslot, unsigned long hva,
                          unsigned long fault_status)
@@ -1480,7 +1492,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
        kvm_pfn_t pfn;
        pgprot_t mem_type = PAGE_S2;
        bool logging_active = memslot_is_logging(memslot);
-   unsigned long flags = 0;
+ unsigned long vma_pagesize, flags = 0;
 
        write_fault = kvm_is_write_fault(vcpu);
        exec_fault = kvm_vcpu_trap_is_iabt(vcpu);
@@ -1500,7 +1512,15 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
                return -EFAULT;
        }
 
-   if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
+ vma_pagesize = vma_kernel_pagesize(vma);
+ /* Is the mapping contained in the memslot? */
+ if (!mapping_in_memslot(memslot, fault_ipa, vma_pagesize)) {
+         /* memslot should be aligned to page size */
+         vma_pagesize = PAGE_SIZE;
+         force_pte = true;
+ }
+
+ if (vma_pagesize == PMD_SIZE && !logging_active) {
                hugetlb = true;
                gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
        } else {

Thoughts?