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
