Re: [RFC PATCH 6/6] KVM: SVM: Pin SEV pages in MMU during sev_launch_update_data()

Maciej S. Szmigiero
2022-01-18T15:28:53Z

Hi Nikunj,

On 18.01.2022 12:06, Nikunj A Dadhania wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Pin the memory for the data being passed to launch_update_data()
> because it gets encrypted before the guest is first run and must
> not be moved which would corrupt it.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> [ * Changed hva_to_gva() to take an extra argument and return gpa_t.
> * Updated sev_pin_memory_in_mmu() error handling.
> * As pinning/unpining pages is handled within MMU, removed
> {get,put}_user(). ]
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
> arch/x86/kvm/svm/sev.c | 122 ++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 119 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 14aeccfc500b..1ae714e83a3c 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -22,6 +22,7 @@
> #include <asm/trapnr.h>
> #include <asm/fpu/xcr.h>
>
> +#include "mmu.h"
> #include "x86.h"
> #include "svm.h"
> #include "svm_ops.h"
> @@ -490,6 +491,110 @@ static unsigned long get_num_contig_pages(unsigned long idx,
> return pages;
> }
>
> +#define SEV_PFERR_RO (PFERR_USER_MASK)
> +#define SEV_PFERR_RW (PFERR_WRITE_MASK | PFERR_USER_MASK)
> +
> +static struct kvm_memory_slot *hva_to_memslot(struct kvm *kvm,
> + unsigned long hva)
> +{
> + struct kvm_memslots *slots = kvm_memslots(kvm);
> + struct kvm_memory_slot *memslot;
> + int bkt;
> +
> + kvm_for_each_memslot(memslot, bkt, slots) {
> + if (hva >= memslot->userspace_addr &&
> + hva < memslot->userspace_addr +
> + (memslot->npages << PAGE_SHIFT))
> + return memslot;
> + }
> +
> + return NULL;
> +}

We have kvm_for_each_memslot_in_hva_range() now, please don't do a linear
search through memslots.
You might need to move the aforementioned macro from kvm_main.c to some
header file, though.

> +static gpa_t hva_to_gpa(struct kvm *kvm, unsigned long hva, bool *ro)
> +{
> + struct kvm_memory_slot *memslot;
> + gpa_t gpa_offset;
> +
> + memslot = hva_to_memslot(kvm, hva);
> + if (!memslot)
> + return UNMAPPED_GVA;
> +
> + *ro = !!(memslot->flags & KVM_MEM_READONLY);
> + gpa_offset = hva - memslot->userspace_addr;
> + return ((memslot->base_gfn << PAGE_SHIFT) + gpa_offset);
> +}
> +
> +static struct page **sev_pin_memory_in_mmu(struct kvm *kvm, unsigned long addr,
> + unsigned long size,
> + unsigned long *npages)
> +{
> + struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> + struct kvm_vcpu *vcpu;
> + struct page **pages;
> + unsigned long i;
> + u32 error_code;
> + kvm_pfn_t pfn;
> + int idx, ret = 0;
> + gpa_t gpa;
> + bool ro;
> +
> + pages = sev_alloc_pages(sev, addr, size, npages);
> + if (IS_ERR(pages))
> + return pages;
> +
> + vcpu = kvm_get_vcpu(kvm, 0);
> + if (mutex_lock_killable(&vcpu->mutex)) {
> + kvfree(pages);
> + return ERR_PTR(-EINTR);
> + }
> +
> + vcpu_load(vcpu);
> + idx = srcu_read_lock(&kvm->srcu);
> +
> + kvm_mmu_load(vcpu);
> +
> + for (i = 0; i < *npages; i++, addr += PAGE_SIZE) {
> + if (signal_pending(current)) {
> + ret = -ERESTARTSYS;
> + break;
> + }
> +
> + if (need_resched())
> + cond_resched();
> +
> + gpa = hva_to_gpa(kvm, addr, &ro);
> + if (gpa == UNMAPPED_GVA) {
> + ret = -EFAULT;
> + break;
> + }

This function is going to have worst case O(n²) complexity if called with
the whole VM memory (or O(n * log(n)) when hva_to_memslot() is modified
to use kvm_for_each_memslot_in_hva_range()).

That's really bad for something that can be done in O(n) time - look how
kvm_for_each_memslot_in_gfn_range() does it over gfns.

Thanks,
Maciej