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

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Date:	Tue, 8 Mar 2016 09:51:05 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	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 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, 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....

Christian