Re: [PATCH RFC 0/9] PKS write protected page tables

Kees Cook
2021-05-05T18:38:45Z

On Wed, May 05, 2021 at 10:37:29AM +0200, Peter Zijlstra wrote:
> On Tue, May 04, 2021 at 11:25:31PM -0700, Kees Cook wrote:
>
> > It looks like PKS-protected page tables would be much like the
> > RO-protected text pages in the sense that there is already code in
> > the kernel to do things to make it writable, change text, and set it
> > read-only again (alternatives, ftrace, etc).
>
> We don't actually modify text by changing the mapping at all. We modify
> through a writable (but not executable) temporary alias on the page (on
> x86).
>
> Once a mapping is RX it will *never* be writable again (until we tear it
> all down).

Yes, quite true. I was trying to answer the concern about "is it okay
that there is a routine in the kernel that can write to page tables
(via temporary disabling of PKS)?" by saying "yes, this is fine -- we
already have similar routines in the kernel that bypass memory
protections, and that's okay because the defense is primarily about
blocking flaws that allow attacker-controlled writes to be used to
leverage greater control over kernel state, of which the page tables are
pretty central. :)

--
Kees Cook