linux-hardening - Re: [RFC PATCH 2/6] powerpc/64s: Helpers to switch between linear and vmapped stack pointers

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Date:   Mon, 07 Nov 2022 22:38:06 +1000
From:   "Nicholas Piggin" <npiggin@...il.com>
To:     "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Andrew Donnellan" <ajd@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Cc:     "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "cmr@...escreens.de" <cmr@...escreens.de>
Subject: Re: [RFC PATCH 2/6] powerpc/64s: Helpers to switch between linear
 and vmapped stack pointers

On Sat Nov 5, 2022 at 6:00 PM AEST, Christophe Leroy wrote:
>
>
> Le 04/11/2022 à 18:27, Andrew Donnellan a écrit :
> > powerpc unfortunately has too many places where we run stuff in real mode.
> > 
> > With CONFIG_VMAP_STACK enabled, this means we need to be able to swap the
> > stack pointer to use the linear mapping when we enter a real mode section,
> > and back afterwards.
> > 
> > Store the top bits of the stack pointer in both the linear map and the
> > vmalloc space in the PACA, and add some helper macros/functions to swap
> > between them.
>
> That may work when pagesize is 64k because stack is on a single page, 
> but I doubt is works with 4k pages, because vmalloc may allocate non 
> contiguous pages.

Yeah. This could be a first-stage though, and depend on 64k page size
and stack size, or !KVM or whatever. When the real-mode code is solved,
that could be relaxed.

Thanks,
Nick