Re: [PATCH v4 24/39] mm: Warn on shadow stack memory in wrong vma

[Kees Cook]

On Fri, Dec 02, 2022 at 04:35:51PM -0800, Rick Edgecombe wrote:
> The x86 Control-flow Enforcement Technology (CET) feature includes a new
> type of memory called shadow stack. This shadow stack memory has some
> unusual properties, which requires some core mm changes to function
> properly.
> 
> One sharp edge is that PTEs that are both Write=0 and Dirty=1 are
> treated as shadow by the CPU, but this combination used to be created by
> the kernel on x86. Previous patches have changed the kernel to now avoid
> creating these PTEs unless they are for shadow stack memory. In case any
> missed corners of the kernel are still creating PTEs like this for
> non-shadow stack memory, and to catch any re-introductions of the logic,
> warn if any shadow stack PTEs (Write=0, Dirty=1) are found in non-shadow
> stack VMAs when they are being zapped. This won't catch transient cases
> but should have decent coverage. It will be compiled out when shadow
> stack is not configured.
> 
> In order to check if a pte is shadow stack in core mm code, add default
> implmentations for pte_shstk() and pmd_shstk().
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook