linux-kernel - Re: [RFC PATCH v9 07/27] Add guard pages around a Shadow Stack.

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Date:   Tue, 25 Feb 2020 12:11:11 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 07/27] Add guard pages around a Shadow Stack.

On Wed, Feb 05, 2020 at 10:19:15AM -0800, Yu-cheng Yu wrote:
> INCSSPD/INCSSPQ instruction is used to unwind a Shadow Stack (SHSTK).  It
> performs 'pop and discard' of the first and last element from SHSTK in the
> range specified in the operand.  The maximum value of the operand is 255,
> and the maximum moving distance of the SHSTK pointer is 255 * 4 for
> INCSSPD, 255 * 8 for INCSSPQ.
> 
> Since SHSTK has a fixed size, creating a guard page above prevents
> INCSSP/RET from moving beyond.  Likewise, creating a guard page below
> prevents CALL from underflowing the SHSTK.

This commit log doesn't really explain why the code changes below are
needed? stack_guard_gap is configurable at boot, etc. This appears to be
limiting it? I don't follow this change...

-Kees

> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  include/linux/mm.h | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b5145fbe102e..75de07674649 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2464,9 +2464,15 @@ static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * m
>  static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
>  {
>  	unsigned long vm_start = vma->vm_start;
> +	unsigned long gap = 0;
>  
> -	if (vma->vm_flags & VM_GROWSDOWN) {
> -		vm_start -= stack_guard_gap;
> +	if (vma->vm_flags & VM_GROWSDOWN)
> +		gap = stack_guard_gap;
> +	else if (vma->vm_flags & VM_SHSTK)
> +		gap = PAGE_SIZE;
> +
> +	if (gap != 0) {
> +		vm_start -= gap;
>  		if (vm_start > vma->vm_start)
>  			vm_start = 0;
>  	}
> @@ -2476,9 +2482,15 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
>  static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
>  {
>  	unsigned long vm_end = vma->vm_end;
> +	unsigned long gap = 0;
> +
> +	if (vma->vm_flags & VM_GROWSUP)
> +		gap = stack_guard_gap;
> +	else if (vma->vm_flags & VM_SHSTK)
> +		gap = PAGE_SIZE;
>  
> -	if (vma->vm_flags & VM_GROWSUP) {
> -		vm_end += stack_guard_gap;
> +	if (gap != 0) {
> +		vm_end += gap;
>  		if (vm_end < vma->vm_end)
>  			vm_end = -PAGE_SIZE;
>  	}
> -- 
> 2.21.0
> 

-- 
Kees Cook