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

[Binbin Wu <binbin.wu@linux.intel.com>]

On 8/7/2026 1:06 AM, Sean Christopherson wrote:
> 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>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>