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Date:   Mon, 3 Oct 2022 11:30:59 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
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        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v2 18/39] mm: Add guard pages around a shadow stack.
Message-ID: <202210031127.C6CF796@keescook>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-19-rick.p.edgecombe@intel.com>
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On Thu, Sep 29, 2022 at 03:29:15PM -0700, Rick Edgecombe wrote:
> [...]
> +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).
> +	 * 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
> +	 */

Thank you for the details on how the size choice is made here! :)

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fef14ab3abcb..09458e77bf52 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2775,15 +2775,16 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr)
>  	return vma;
>  }
>  
> +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 9d780f415be3..f0d2e9143bd0 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -247,6 +247,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
>  	return origbrk;
>  }
>  

I feel like something could be done with this definitions to make them
inline, instead of __weak:

#ifndef stack_guard_start_gap
> +unsigned long __weak stack_guard_start_gap(struct vm_area_struct *vma)
> +{
> +	if (vma->vm_flags & VM_GROWSDOWN)
> +		return stack_guard_gap;
> +	return 0;
> +}
#endif

And then move the x86 stack_guard_start_gap to a header?

It's not exactly fast-path, but it feels a little weird. Regardlesss:

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook