Re: [PATCH v5 08/39] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

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On Thu, Jan 19, 2023 at 01:22:46PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> New processors that support Shadow Stack regard Write=0,Dirty=1 PTEs as
> shadow stack pages.
> 
> In normal cases, it can be helpful to create Write=1 PTEs as also Dirty=1
> if HW dirty tracking is not needed, because if the Dirty bit is not already
> set the CPU has to set Dirty=1 when the memory gets written to. This
> creates additional work for the CPU. So traditional wisdom was to simply
> set the Dirty bit whenever you didn't care about it. However, it was never
> really very helpful for read-only kernel memory.
> 
> When CR4.CET=1 and IA32_S_CET.SH_STK_EN=1, some instructions can write to
> such supervisor memory. The kernel does not set IA32_S_CET.SH_STK_EN, so
> avoiding kernel Write=0,Dirty=1 memory is not strictly needed for any
> functional reason. But having Write=0,Dirty=1 kernel memory doesn't have
> any functional benefit either, so to reduce ambiguity between shadow stack
> and regular Write=0 pages, remove Dirty=1 from any kernel Write=0 PTEs.
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook



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