Re: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

Borislav Petkov
2023-03-08T09:23:36Z

On Mon, Feb 27, 2023 at 02:29:43PM -0800, Rick Edgecombe wrote:
> When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
^
in

> shadow stack memory. So for shadow stack memory this bit combination is
> valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
> write protected, the kernel has been taught to transition the Dirty=1
> bit to SavedDirty=1, to avoid inadvertently creating shadow stack
> memory. It does this inside pte_wrprotect() because it knows the PTE is
> not intended to be a writable shadow stack entry, it is supposed to be
> write protected.

>
> However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
> can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
> distinguish between the caller intending to create a shadow stack PTE or
> needing the SavedDirty shift.
>
> The kernel has been updated to not do this, and so Write=0,Dirty=1
> memory should only be created by the pte_mkfoo() helpers. Add a warning
> to make sure no new mk_pte() start doing this.

Might wanna add the note from below here:

"... start doing this, like, for example, set_memory_rox() did."

> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Kees Cook <keescook@chromium.org>
> Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>
> ---
> v6:
> - New patch (Note, this has already been a useful warning, it caught the
> newly added set_memory_rox() doing this)

Thx.

--
Regards/Gruss,
Boris.