[PATCH 0/2] KVM: Fix a lurking bug in kvm_clear_guest()

[Sean Christopherson <seanjc@xxxxxxxxxx>]

Fix a bug in kvm_clear_guest() where it would write beyond the target
page _if_ handed a gpa+len that would span multiple pages.  Luckily, the
bug is unhittable in the current code base as all users ensure the
gpa+len is bound to a single page.

Patch 2 hardens the underlying single page APIs to guard against a bad
offset+len, e.g. so that bugs like the one in kvm_clear_guest() are noisy
and don't escalate to an out-of-bounds access.

Verified and tested by hacking KVM to use kvm_clear_guest() when zeroing
all three pages used for KVM's hidden TSS.

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f18c2d8c7476..ce64e490e9c7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3872,14 +3872,17 @@ bool __vmx_guest_state_valid(struct kvm_vcpu *vcpu)
 
 static int init_rmode_tss(struct kvm *kvm, void __user *ua)
 {
-       const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
+       // const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
        u16 data;
-       int i;
+       // int i;
 
-       for (i = 0; i < 3; i++) {
-               if (__copy_to_user(ua + PAGE_SIZE * i, zero_page, PAGE_SIZE))
-                       return -EFAULT;
-       }
+       if (kvm_clear_guest(kvm, to_kvm_vmx(kvm)->tss_addr, PAGE_SIZE * 3))
+               return -EFAULT;
+
+       // for (i = 0; i < 3; i++) {
+       //      if (__copy_to_user(ua + PAGE_SIZE * i, zero_page, PAGE_SIZE))
+       //              return -EFAULT;
+       // }
 
        data = TSS_BASE_SIZE + TSS_REDIRECTION_SIZE;
        if (__copy_to_user(ua + TSS_IOPB_BASE_OFFSET, &data, sizeof(u16)))

Sean Christopherson (2):
  KVM: Write the per-page "segment" when clearing (part of) a guest page
  KVM: Harden guest memory APIs against out-of-bounds accesses

 virt/kvm/kvm_main.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)


base-commit: 15e1c3d65975524c5c792fcd59f7d89f00402261
-- 
2.46.0.469.g59c65b2a67-goog