Re: [RFC 06/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory

Vitaly Kuznetsov
2020-05-25T15:08:51Z

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> New helpers copy_from_guest()/copy_to_guest() to be used if KVM memory
> protection feature is enabled.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> include/linux/kvm_host.h | 4 +++
> virt/kvm/kvm_main.c | 78 ++++++++++++++++++++++++++++++++++------
> 2 files changed, 72 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 131cc1527d68..bd0bb600f610 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -503,6 +503,7 @@ struct kvm {
> struct srcu_struct srcu;
> struct srcu_struct irq_srcu;
> pid_t userspace_pid;
> + bool mem_protected;
> };
>
> #define kvm_err(fmt, ...) \
> @@ -727,6 +728,9 @@ void kvm_set_pfn_dirty(kvm_pfn_t pfn);
> void kvm_set_pfn_accessed(kvm_pfn_t pfn);
> void kvm_get_pfn(kvm_pfn_t pfn);
>
> +int copy_from_guest(void *data, unsigned long hva, int len);
> +int copy_to_guest(unsigned long hva, const void *data, int len);
> +
> void kvm_release_pfn(kvm_pfn_t pfn, bool dirty, struct gfn_to_pfn_cache *cache);
> int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> int len);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 731c1e517716..033471f71dae 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2248,8 +2248,48 @@ static int next_segment(unsigned long len, int offset)
> return len;
> }
>
> +int copy_from_guest(void *data, unsigned long hva, int len)
> +{
> + int offset = offset_in_page(hva);
> + struct page *page;
> + int npages, seg;
> +
> + while ((seg = next_segment(len, offset)) != 0) {
> + npages = get_user_pages_unlocked(hva, 1, &page, 0);
> + if (npages != 1)
> + return -EFAULT;
> + memcpy(data, page_address(page) + offset, seg);
> + put_page(page);
> + len -= seg;
> + hva += seg;
> + offset = 0;
> + }
> +
> + return 0;
> +}
> +
> +int copy_to_guest(unsigned long hva, const void *data, int len)
> +{
> + int offset = offset_in_page(hva);
> + struct page *page;
> + int npages, seg;
> +
> + while ((seg = next_segment(len, offset)) != 0) {
> + npages = get_user_pages_unlocked(hva, 1, &page, FOLL_WRITE);
> + if (npages != 1)
> + return -EFAULT;
> + memcpy(page_address(page) + offset, data, seg);
> + put_page(page);
> + len -= seg;
> + hva += seg;
> + offset = 0;
> + }
> + return 0;
> +}
> +
> static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
> - void *data, int offset, int len)
> + void *data, int offset, int len,
> + bool protected)
> {
> int r;
> unsigned long addr;
> @@ -2257,7 +2297,10 @@ static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
> addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> - r = __copy_from_user(data, (void __user *)addr + offset, len);
> + if (protected)
> + r = copy_from_guest(data, addr + offset, len);
> + else
> + r = __copy_from_user(data, (void __user *)addr + offset, len);
> if (r)
> return -EFAULT;
> return 0;
> @@ -2268,7 +2311,8 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> {
> struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
>
> - return __kvm_read_guest_page(slot, gfn, data, offset, len);
> + return __kvm_read_guest_page(slot, gfn, data, offset, len,
> + kvm->mem_protected);
> }
> EXPORT_SYMBOL_GPL(kvm_read_guest_page);
>
> @@ -2277,7 +2321,8 @@ int kvm_vcpu_read_guest_page(struct kvm_vcpu *vcpu, gfn_t gfn, void *data,
> {
> struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>
> - return __kvm_read_guest_page(slot, gfn, data, offset, len);
> + return __kvm_read_guest_page(slot, gfn, data, offset, len,
> + vcpu->kvm->mem_protected);

Personally, I would've just added 'struct kvm' pointer to 'struct
kvm_memory_slot' to be able to extract 'mem_protected' info when
needed. This will make the patch much smaller.

> }
> EXPORT_SYMBOL_GPL(kvm_vcpu_read_guest_page);
>
> @@ -2350,7 +2395,8 @@ int kvm_vcpu_read_guest_atomic(struct kvm_vcpu *vcpu, gpa_t gpa,
> EXPORT_SYMBOL_GPL(kvm_vcpu_read_guest_atomic);
>
> static int __kvm_write_guest_page(struct kvm_memory_slot *memslot, gfn_t gfn,
> - const void *data, int offset, int len)
> + const void *data, int offset, int len,
> + bool protected)
> {
> int r;
> unsigned long addr;
> @@ -2358,7 +2404,11 @@ static int __kvm_write_guest_page(struct kvm_memory_slot *memslot, gfn_t gfn,
> addr = gfn_to_hva_memslot(memslot, gfn);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> - r = __copy_to_user((void __user *)addr + offset, data, len);
> +
> + if (protected)
> + r = copy_to_guest(addr + offset, data, len);
> + else
> + r = __copy_to_user((void __user *)addr + offset, data, len);

All users of copy_to_guest() will have to have the same 'if (protected)'
check, right? Why not move the check to copy_to/from_guest() then?

> if (r)
> return -EFAULT;
> mark_page_dirty_in_slot(memslot, gfn);
> @@ -2370,7 +2420,8 @@ int kvm_write_guest_page(struct kvm *kvm, gfn_t gfn,
> {
> struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
>
> - return __kvm_write_guest_page(slot, gfn, data, offset, len);
> + return __kvm_write_guest_page(slot, gfn, data, offset, len,
> + kvm->mem_protected);
> }
> EXPORT_SYMBOL_GPL(kvm_write_guest_page);
>
> @@ -2379,7 +2430,8 @@ int kvm_vcpu_write_guest_page(struct kvm_vcpu *vcpu, gfn_t gfn,
> {
> struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>
> - return __kvm_write_guest_page(slot, gfn, data, offset, len);
> + return __kvm_write_guest_page(slot, gfn, data, offset, len,
> + vcpu->kvm->mem_protected);
> }
> EXPORT_SYMBOL_GPL(kvm_vcpu_write_guest_page);
>
> @@ -2495,7 +2547,10 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> if (unlikely(!ghc->memslot))
> return kvm_write_guest(kvm, gpa, data, len);
>
> - r = __copy_to_user((void __user *)ghc->hva + offset, data, len);
> + if (kvm->mem_protected)
> + r = copy_to_guest(ghc->hva + offset, data, len);
> + else
> + r = __copy_to_user((void __user *)ghc->hva + offset, data, len);
> if (r)
> return -EFAULT;
> mark_page_dirty_in_slot(ghc->memslot, gpa >> PAGE_SHIFT);
> @@ -2530,7 +2585,10 @@ int kvm_read_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> if (unlikely(!ghc->memslot))
> return kvm_read_guest(kvm, ghc->gpa, data, len);
>
> - r = __copy_from_user(data, (void __user *)ghc->hva, len);
> + if (kvm->mem_protected)
> + r = copy_from_guest(data, ghc->hva, len);
> + else
> + r = __copy_from_user(data, (void __user *)ghc->hva, len);
> if (r)
> return -EFAULT;

--
Vitaly