KVM: TDX: Reject INIT_MEM_REGION if number of bytes would overflow a u64

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]
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260806170602.4112602-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions