[PATCH 04/14] KVM: Play nice with read-only memslots when querying host page size

Sean Christopherson
2020-01-08T20:27:45Z

Open code an equivalent of kvm_vcpu_gfn_to_memslot() when querying the
host page size to avoid the "writable" check in __gfn_to_hva_many(),
which will always fail on read-only memslots due to gfn_to_hva()
assuming writes. Functionally, this allows x86 to create large mappings
for read-only memslots that are backed by HugeTLB mappings.

Note, the changelog for commit 05da45583de9 ("KVM: MMU: large page
support") states "If the largepage contains write-protected pages, a
large pte is not used.", but "write-protected" refers to pages that are
temporarily read-only, e.g. read-only memslots didn't even exist at the
time.

Fixes: 4d8b81abc47b ("KVM: introduce readonly memslot")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
virt/kvm/kvm_main.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5f7f06824c2b..d9aced677ddd 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1418,15 +1418,23 @@ EXPORT_SYMBOL_GPL(kvm_is_visible_gfn);

unsigned long kvm_host_page_size(struct kvm_vcpu *vcpu, gfn_t gfn)
{
+ struct kvm_memory_slot *slot;
struct vm_area_struct *vma;
unsigned long addr, size;

size = PAGE_SIZE;

- addr = kvm_vcpu_gfn_to_hva(vcpu, gfn);
- if (kvm_is_error_hva(addr))
+ /*
+ * Manually do the equivalent of kvm_vcpu_gfn_to_hva() to avoid the
+ * "writable" check in __gfn_to_hva_many(), which will always fail on
+ * read-only memslots due to gfn_to_hva() assuming writes.
+ */
+ slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
+ if (!slot || slot->flags & KVM_MEMSLOT_INVALID)
return PAGE_SIZE;

+ addr = __gfn_to_hva_memslot(slot, gfn);
+
down_read(&current->mm->mmap_sem);
vma = find_vma(current->mm, addr);
if (!vma)
--
2.24.1