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

Kirill A. Shutemov
2020-05-25T15:18:01Z

On Mon, May 25, 2020 at 05:08:43PM +0200, Vitaly Kuznetsov wrote:
> "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.

Okay, can do.

Other thing I tried is to have per-slot flag to indicate that it's
protected. But Sean pointed that it's all-or-nothing feature and having
the flag in the slot would be misleading.

> > }
> > 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?

Good point.

> > 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
>
>

--
Kirill A. Shutemov