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Date: Wed, 17 Feb 2016 16:43:06 -0700
From: David Brown <david.brown@...aro.org>
To: Kees Cook <keescook@...omium.org>
Cc: Russell King <linux@....linux.org.uk>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Ingo Molnar <mingo@...hat.com>,
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>,
LKML <linux-kernel@...r.kernel.org>,
linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] ARM: vdso: Mark vDSO code as read-only
On Wed, Feb 17, 2016 at 03:00:52PM -0800, Kees Cook wrote:
>On Tue, Feb 16, 2016 at 9:20 PM, David Brown <david.brown@...aro.org> wrote:
>> On Tue, Feb 16, 2016 at 01:52:33PM -0800, Kees Cook wrote:
>>>
>>> On Tue, Feb 16, 2016 at 1:36 PM, David Brown <david.brown@...aro.org>
>>> wrote:
>>>>
>>>> Although the arm vDSO is cleanly separated by code/data with the code
>>>> being read-only in userspace mappings, the code page is still writable
>>>> from the kernel. There have been exploits (such as
>>>> http://itszn.com/blog/?p=21) that take advantage of this on x86 to go
>>>> from a bad kernel write to full root.
>>>>
>>>> Prevent this specific exploit on arm by putting the vDSO code page in
>>>> post-init read-only memory as well.
>>>
>>>
>>> Is the vdso dynamically built at init time like on x86, or can this
>>> just use .rodata directly?
>>
>>
>> On ARM, it is patched during init. Arm64's is just plain read-only.
>
>Okay, great. I've added this to my postinit-readonly series (which I
>just refreshed and sent out again...)
However, this distinction between .rodata and .data..ro_after_init is
kind of fuzzy, anyway, since they both get made actually read-only at
the same time (post init). The patch actually does work fine with the
vDSO page in .rodata, since the patching happens during init.
Is there a possible future consideration to perhaps make .rodata read
only much earlier?
David
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