Re: [PATCH v3 18/37] mm: Add guard pages around a shadow stack.

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 04, 2022 at 03:35:45PM -0700, Rick Edgecombe wrote:

> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index c90c20904a60..66da1f3298b0 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -248,3 +248,26 @@ bool pfn_modify_allowed(unsigned long pfn, pgprot_t prot)
>  		return false;
>  	return true;
>  }
> +
> +unsigned long stack_guard_start_gap(struct vm_area_struct *vma)
> +{
> +	if (vma->vm_flags & VM_GROWSDOWN)
> +		return stack_guard_gap;
> +
> +	/*
> +	 * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D).

Can we perhaps write this like:	INCSPP[QD] ? The () notation makes it
look like a function.

> +	 * INCSSPQ moves shadow stack pointer up to 255 * 8 = ~2 KB
> +	 * (~1KB for INCSSPD) and touches the first and the last element
> +	 * in the range, which triggers a page fault if the range is not
> +	 * in a shadow stack. Because of this, creating 4-KB guard pages
> +	 * around a shadow stack prevents these instructions from going
> +	 * beyond.
> +	 *
> +	 * Creation of VM_SHADOW_STACK is tightly controlled, so a vma
> +	 * can't be both VM_GROWSDOWN and VM_SHADOW_STACK
> +	 */
> +	if (vma->vm_flags & VM_SHADOW_STACK)
> +		return PAGE_SIZE;
> +
> +	return 0;
> +}
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5d9536fa860a..0a3f7e2b32df 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2832,15 +2832,16 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr)
>  	return mtree_load(&mm->mm_mt, addr);
>  }
>  
> +unsigned long stack_guard_start_gap(struct vm_area_struct *vma);
> +
>  static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
>  {
> +	unsigned long gap = stack_guard_start_gap(vma);
>  	unsigned long vm_start = vma->vm_start;
>  
> -	if (vma->vm_flags & VM_GROWSDOWN) {
> -		vm_start -= stack_guard_gap;
> -		if (vm_start > vma->vm_start)
> -			vm_start = 0;
> -	}
> +	vm_start -= gap;
> +	if (vm_start > vma->vm_start)
> +		vm_start = 0;
>  	return vm_start;
>  }
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2def55555e05..f67606fbc464 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -281,6 +281,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
>  	return origbrk;
>  }
>  
> +unsigned long __weak stack_guard_start_gap(struct vm_area_struct *vma)
> +{
> +	if (vma->vm_flags & VM_GROWSDOWN)
> +		return stack_guard_gap;
> +	return 0;
> +}

I'm thinking perhaps this wants to be an inline function?