[PATCH v2 1/2] KVM: TDX: Reject INIT_MEM_REGION if number of bytes would overflow a u64

[Sean Christopherson <seanjc@google.com>]

From: Haotian Jiang <jianghaotian.sunday@gmail.com>

The nr_pages field in struct kvm_tdx_init_mem_region is a u64 that comes
directly from userspace via copy_from_user().  The current validation
uses a manual overflow check:

  region.gpa + (region.nr_pages << PAGE_SHIFT) <= region.gpa

When nr_pages >= 2^52, the shift (nr_pages << PAGE_SHIFT) wraps around
to a small value, bypassing the wrap check.  While downstream protections
(gfn_to_memslot() returning NULL for GFNs outside any memslot, and
kvm_slot_has_gmem() checking for NULL) prevent any actual out-of-bounds
access, the overflow itself is a real bug that should be caught at the
validation layer.

Replace the manual overflow check with check_shl_overflow() to correctly
detect the wrap-around.

Note, the manual wrap-around check on the gpa+size technically has a benign
off-by-one bug, and can also use check_add_overflow().  Those flaws will be
addressed shortly.

Opportunistically separate the initial sanity checks from the more involved
checks to try and make the code easier to read.

Fixes: c846b451d3c5 ("KVM: TDX: Add an ioctl to create initial guest memory")
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260630214952.98C851F000E9@smtp.kernel.org
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Binbin Wu <binbin.wu@linux.intel.com>
Cc: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Haotian Jiang <jianghaotian.sunday@gmail.com>
[sean: use gpa_t, isolate check_shl_overflow() change, tweak changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/vmx/tdx.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b272c20586a7..929115aeb9ec 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3219,6 +3219,7 @@ 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;
 	long gmem_ret;
 	int ret;
 
@@ -3236,10 +3237,13 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
 		return -EFAULT;
 
 	if (!PAGE_ALIGNED(region.source_addr) || !region.source_addr ||
-	    !PAGE_ALIGNED(region.gpa) || !region.nr_pages ||
-	    region.gpa + (region.nr_pages << PAGE_SHIFT) <= region.gpa ||
+	    !PAGE_ALIGNED(region.gpa) || !region.nr_pages)
+		return -EINVAL;
+
+	if (check_shl_overflow(region.nr_pages, PAGE_SHIFT, &nr_bytes) ||
+	    region.gpa + nr_bytes <= region.gpa ||
 	    !vt_is_tdx_private_gpa(kvm, region.gpa) ||
-	    !vt_is_tdx_private_gpa(kvm, region.gpa + (region.nr_pages << PAGE_SHIFT) - 1))
+	    !vt_is_tdx_private_gpa(kvm, region.gpa + nr_bytes - 1))
 		return -EINVAL;
 
 	ret = 0;
-- 
2.55.0.679.g6767b8d81c-goog