linux-kernel - Re: [PATCH v5 18/39] mm: Handle faultless write upgrades for shstk

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Message-ID: <f337d3b0e401c210b67a6465bf35f66f6a46fc3d.camel@intel.com>
Date:   Tue, 31 Jan 2023 23:33:32 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v5 18/39] mm: Handle faultless write upgrades for shstk

On Tue, 2023-01-31 at 09:46 +0100, David Hildenbrand wrote:
> Sure ...
> 
> but I reconsidered :)
> 
> Maybe there is a cleaner way to do it and avoid the "NULL" argument.
> 
> What about having (while you're going over everything already):
> 
> pte_mkwrite(pte, vma)
> pte_mkwrite_kernel(pte)
> 
> The latter would only be used in that arch code where we're working
> on 
> kernel pgtables. We already have pte_offset_kernel() and 
> pte_alloc_kernel_track(), so it's not too weird.

Hmm, one downside is the "mk" part might lead people to guess
pte_mkwrite_kernel() would make it writable AND a kernel page (like
U/S=0 on x86). Instead of being a mkwrite() that's useful for setting
on kernel PTEs.

The other problem is that one of NULL passers is not for kernel memory.
huge_pte_mkwrite() calls pte_mkwrite(). Shadow stack memory can't be
created with MAP_HUGETLB, so it is not needed. Using
pte_mkwrite_kernel() would look weird in this case, but making
huge_pte_mkwrite() take a VMA would be for no reason. Maybe making
huge_pte_mkwrite() take a VMA is the better of those two options. Or
keep the NULL semantics...  Any thoughts?