linux-kernel - Re: [RFC][PATCH] s390, postinit-readonly: implement post-init RO

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Date:	Tue, 8 Mar 2016 13:56:12 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	Kees Cook <keescook@...omium.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	David Brown <david.brown@...aro.org>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Mathias Krause <minipli@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	PaX Team <pageexec@...email.hu>,
	Emese Revfy <re.emese@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [RFC][PATCH] s390, postinit-readonly: implement post-init RO

On Tue, Mar 08, 2016 at 12:43:15PM +0100, Heiko Carstens wrote:
> On Tue, Mar 08, 2016 at 09:51:05AM +0100, Christian Borntraeger wrote:
> > On 03/08/2016 01:41 AM, Kees Cook wrote:
> > 
> > >> --- a/arch/s390/kernel/vmlinux.lds.S
> > >> +++ b/arch/s390/kernel/vmlinux.lds.S
> > >> @@ -52,6 +52,12 @@ SECTIONS
> > >>
> > >>         RW_DATA_SECTION(0x100, PAGE_SIZE, THREAD_SIZE)
> > >>
> > >> +       . = ALIGN(PAGE_SIZE)
> > 
> > 
> > missing ";" ?
> > 
> > 
> > With that  and your fixes, this function claims to mark 0kB and 
> > lkdtm can still write. Reason is that _edata is 0xc11008 and start is
> > 0x0c11000.
> > 
> > making _edata page aligned as well, does now try to mark one page,

Yes, we do need that fixed.

> > but then
> > we run into the next issue, that 
> > 
> > static void change_page_attr(unsigned long addr, int numpages,
> >                              pte_t (*set) (pte_t))
> > {
> >         pte_t *ptep;
> >         int i;
> > 
> >         for (i = 0; i < numpages; i++) {
> >                 ptep = walk_page_table(addr);
> > 
> > triggers this
> >                 if (WARN_ON_ONCE(!ptep))
> >                         break;
> > 
> > because the kernel decided to map this with a large page. So we need
> > to fix this function to then break the large page into a smaller chunk....
> 
> Yes... however that's a rather large change. I'll try to come up with a
> patch that has less impact and implement the code that splits the kernel
> mapping later.
> Looking at our vmemmap code makes me realize that this code needs also to
> be improved.

Hm, that's going to be too ugly. So we'll go for the "real" solution, which
modifies the kernel mapping instead.