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Date:   Fri, 9 Apr 2021 18:12:24 +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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        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
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        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
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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 14/30] x86/mm: Shadow Stack page fault error checking
Message-ID: <20210409151224.4tlklypbjpxcztpt@box.shutemov.name>
References: <20210401221104.31584-1-yu-cheng.yu@intel.com>
 <20210401221104.31584-15-yu-cheng.yu@intel.com>
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In-Reply-To: <20210401221104.31584-15-yu-cheng.yu@intel.com>
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On Thu, Apr 01, 2021 at 03:10:48PM -0700, Yu-cheng Yu wrote:
> Shadow stack accesses are those that are performed by the CPU where it
> expects to encounter a shadow stack mapping.  These accesses are performed
> implicitly by CALL/RET at the site of the shadow stack pointer.  These
> accesses are made explicitly by shadow stack management instructions like
> WRUSSQ.
> 
> Shadow stacks accesses to shadow-stack mapping can see faults in normal,
> valid operation just like regular accesses to regular mappings.  Shadow
> stacks need some of the same features like delayed allocation, swap and
> copy-on-write.
> 
> Shadow stack accesses can also result in errors, such as when a shadow
> stack overflows, or if a shadow stack access occurs to a non-shadow-stack
> mapping.
> 
> In handling a shadow stack page fault, verify it occurs within a shadow
> stack mapping.  It is always an error otherwise.  For valid shadow stack
> accesses, set FAULT_FLAG_WRITE to effect copy-on-write.  Because clearing
> _PAGE_DIRTY (vs. _PAGE_RW) is used to trigger the fault, shadow stack read
> fault and shadow stack write fault are not differentiated and both are
> handled as a write access.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>

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

-- 
 Kirill A. Shutemov