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Date:   Thu, 4 Feb 2021 12:24:09 -0800
From:   Kees Cook <keescook@chromium.org>
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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Subject: Re: [PATCH v19 16/25] mm: Add guard pages around a shadow stack.
Message-ID: <202102041224.5D300A122B@keescook>
References: <20210203225547.32221-1-yu-cheng.yu@intel.com>
 <20210203225547.32221-17-yu-cheng.yu@intel.com>
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On Wed, Feb 03, 2021 at 02:55:38PM -0800, 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>

Yay guard pages! :)

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

-- 
Kees Cook