[PATCH] KVM: nSVM: Ignore nCR3[4:0] when loading PDPTEs from memory

[Sean Christopherson <seanjc@xxxxxxxxxx>]

Ignore nCR3[4:0] when loading PDPTEs from memory for nested SVM, as bits
4:0 of CR3 are ignored when PAE paging is used, and thus VMRUN doesn't
enforce 32-byte alignment of nCR3.

In the absolute worst case scenario, failure to ignore bits 4:0 can result
in an out-of-bounds read, e.g. if the target page is at the end of a
memslot, and the VMM isn't using guard pages.

Per the APM:

  The CR3 register points to the base address of the page-directory-pointer
  table. The page-directory-pointer table is aligned on a 32-byte boundary,
  with the low 5 address bits 4:0 assumed to be 0.

And the SDM's much more explicit:

  4:0    Ignored

Note, KVM gets this right when loading PDPTRs, it's only the nSVM flow
that is broken.

Fixes: e4e517b4be01 ("KVM: MMU: Do not unconditionally read PDPTE from guest memory")
Reported-by: Kirk Swidowski <swidowski@xxxxxxxxxx>
Cc: Andy Nguyen <theflow@xxxxxxxxxx>
Cc: 3pvd <3pvd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-ID: <20241009140838.1036226-1-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index d5314cb7dff4..cf84103ce38b 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -63,8 +63,12 @@ static u64 nested_svm_get_tdp_pdptr(struct kvm_vcpu *vcpu, int index)
 	u64 pdpte;
 	int ret;
 
+	/*
+	 * Note, nCR3 is "assumed" to be 32-byte aligned, i.e. the CPU ignores
+	 * nCR3[4:0] when loading PDPTEs from memory.
+	 */
 	ret = kvm_vcpu_read_guest_page(vcpu, gpa_to_gfn(cr3), &pdpte,
-				       offset_in_page(cr3) + index * 8, 8);
+				       (cr3 & GENMASK(11, 5)) + index * 8, 8);
 	if (ret)
 		return 0;
 	return pdpte;