Re: [PATCH v19 4/8] fork: Add shadow stack support to clone3()

On Tue, 2025-08-19 at 17:21 +0100, Mark Brown wrote:

+int arch_shstk_validate_clone(struct task_struct *t,

• 	      struct vm_area_struct *vma,
  

• 	      struct page *page,
  

• 	      struct kernel_clone_args *args)
  

+{

• /*

• * SSP is aligned, so reserved bits and mode bit are a zero, just mark
  

• * the token 64-bit.
  

• */
  

What is this comment doing here? It doesn't make sense. It looks copied from create_rstor_token()?

• void *maddr = page_address(page);
• unsigned long token;
• int offset;
• u64 expected;
• token = args->shadow_stack_token;
• expected = (token + SS_FRAME_SIZE) | BIT(0);

Instead of the above comment, I think the important thing to say is that args-

shadow_stack_token is 8 byte aligned, so offset can't overflow out of the page.

Maybe?

/* kernel_clone_args verification assures token address is 8 byte aligned */

• offset = offset_in_page(token);
• if (!cmpxchg_to_user_page(vma, page, token, (unsigned long *)(maddr + offset),

• 		  expected, 0))
  

• return -EINVAL;
  

• set_page_dirty_lock(page);
• return 0;

+}

With those changes, for the series:

Reviewed-by: Rick Edgecombe rick.p.edgecombe@intel.com