Messages in this thread | | | Date | Tue, 17 Aug 2021 21:54:21 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW |
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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
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