Re: [PATCH v19 11/25] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW
From: Kees Cook <hidden>
Date: 2021-02-04 20:22:10
Also in:
linux-api, linux-arch, linux-mm, lkml
On Wed, Feb 03, 2021 at 02:55:33PM -0800, Yu-cheng Yu wrote:
When Shadow Stack is introduced, [R/O + _PAGE_DIRTY] PTE is reserved for shadow stack. Copy-on-write PTEs have [R/O + _PAGE_COW]. When a PTE goes from [R/W + _PAGE_DIRTY] to [R/O + _PAGE_COW], it could become a transient shadow stack PTE in two cases: The first case is that some processors can start a write but end up seeing a read-only PTE by the time they get to the Dirty bit, creating a transient shadow stack PTE. However, this will not occur on processors supporting Shadow Stack, and a TLB flush is not necessary. The second case is that when _PAGE_DIRTY is replaced with _PAGE_COW non- atomically, a transient shadow stack PTE can be created as a result. Thus, prevent that with cmpxchg. Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many insights to the issue. Jann Horn provided the cmpxchg solution. Signed-off-by: Yu-cheng Yu <redacted>
Reviewed-by: Kees Cook <redacted> -- Kees Cook