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Date:   Fri, 9 Apr 2021 18:22:54 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Haitao Huang <haitao.huang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v24 17/30] mm: Add guard pages around a shadow stack.
Message-ID: <20210409152254.tmpfubsksrd2qbeq@box.shutemov.name>
References: <20210401221104.31584-1-yu-cheng.yu@intel.com>
 <20210401221104.31584-18-yu-cheng.yu@intel.com>
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In-Reply-To: <20210401221104.31584-18-yu-cheng.yu@intel.com>
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On Thu, Apr 01, 2021 at 03:10:51PM -0700, Yu-cheng Yu wrote:
> INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the
> first and the last elements in the range, effectively touches those memory
> areas.
> 
> The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and
> 255 * 4 = 1020 bytes by INCSSPD.  Both ranges are far from PAGE_SIZE.
> Thus, putting a gap page on both ends of a shadow stack prevents INCSSP,
> CALL, and RET from going beyond.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> v24:
> - Instead changing vm_*_gap(), create x86-specific versions.
> 
>  arch/x86/include/asm/page_types.h | 17 +++++++++++++++
>  arch/x86/mm/mmap.c                | 36 +++++++++++++++++++++++++++++++
>  include/linux/mm.h                |  4 ++++
>  3 files changed, 57 insertions(+)
> 
> diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
> index a506a411474d..3a5529bcfd76 100644
> --- a/arch/x86/include/asm/page_types.h
> +++ b/arch/x86/include/asm/page_types.h
> @@ -73,6 +73,23 @@ bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn);
>  
>  extern void initmem_init(void);
>  
> +/*
> + * 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.
> + */
> +#define SHADOW_STACK_GUARD_GAP PAGE_SIZE

The define is only used within arch/x86/mm/mmap.c. Maybe move it there?

Otherwise:

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov