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Date:   Tue, 17 Aug 2021 21:54:21 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW
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References: <20210722205219.7934-1-yu-cheng.yu@intel.com>
 <20210722205219.7934-10-yu-cheng.yu@intel.com>
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On Tue, Aug 17, 2021 at 11:24:29AM -0700, Yu, Yu-cheng wrote:
> Indeed, this can be looked at in a few ways.  We can visualize pte_write()
> as 'CPU can write to it with MOV' or 'CPU can write to it with any opcodes'.
> Depending on whatever pte_write() is, copy-on-write code can be adjusted
> accordingly.

Can be?

I think you should exclude shadow stack pages from being writable
and treat them as read-only. How the CPU writes them is immaterial -
pte/pmd_write() is used by normal kernel code to query whether the page
is writable or not by any instruction - not by the CPU.

And since normal kernel code cannot write shadow stack pages, then for
that code those pages are read-only.

If special kernel code using shadow stack management insns needs
to modify a shadow stack, then it can check whether a page is
pte/pmd_shstk() but that code is special anyway.

Hell, a shadow stack page is (Write=0, Dirty=1) so calling it writable
			      ^^^^^^^
is simply wrong.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette