Re: [PATCH v4 08/39] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

On Fri, Dec 02, 2022 at 04:35:35PM -0800, Rick Edgecombe wrote:

From: Yu-cheng Yu <redacted>

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 it the memory gets written to. This
creates addiontal work for the CPU. So tradional 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, removed Dirty=1 from any kernel Write=0 PTEs.

Tested-by: Pengfei Xu <redacted>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Yu-cheng Yu <redacted>

Reviewed-by: Kees Cook <redacted>

-- 
Kees Cook