KVM: TDX: Fix a benign off-by-one bug on the end GPA for INIT_MEM_REGION

When verifying that the incoming GPA rage for INIT_MEM_REGION doesn't wrap,
check the inclusive last GPA, not the exclusive last GPA.  Super duper
technically, it's ok if the very last GPA is -1ull.  In practice, the flaw
is benign as KVM x86 disallows memslots with GPAs that exceed MAXPHYADDR,
i.e. INIT_MEM_REGION would fail with -EINVAL anyways due to the memslot
check in kvm_gmem_populate().

Opportunistically use check_add_overflow() instead of manually checking for
wrap, mostly so that the inclusive math doesn't need to be copy+pasted in
the "is private" check.

Fixes: c846b451d3c5 ("KVM: TDX: Add an ioctl to create initial guest memory")
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://patch.msgid.link/20260806170602.4112602-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index d991996259a8..2a3acf493cb5 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3257,7 +3257,7 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
 	struct kvm_tdx_init_mem_region region;
 	struct tdx_gmem_post_populate_arg arg;
-	gpa_t nr_bytes;
+	gpa_t nr_bytes, end_gpa;
 	long gmem_ret;
 	int ret;
 
@@ -3279,9 +3279,9 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
 		return -EINVAL;
 
 	if (check_shl_overflow(region.nr_pages, PAGE_SHIFT, &nr_bytes) ||
-	    region.gpa + nr_bytes <= region.gpa ||
+	    check_add_overflow(region.gpa, nr_bytes - 1, &end_gpa) ||
 	    !vt_is_tdx_private_gpa(kvm, region.gpa) ||
-	    !vt_is_tdx_private_gpa(kvm, region.gpa + nr_bytes - 1))
+	    !vt_is_tdx_private_gpa(kvm, end_gpa))
 		return -EINVAL;
 
 	ret = 0;