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Date: Wed, 17 Feb 2016 15:48:21 -0800
From: Kees Cook <>
To: David Brown <>
Cc: Russell King <>, 
	"" <>, 
	"" <>, Ingo Molnar <>, 
	Andy Lutomirski <>, "H. Peter Anvin" <>, 
	Michael Ellerman <>, Mathias Krause <>, 
	Thomas Gleixner <>, "" <>, Arnd Bergmann <>, 
	PaX Team <>, Emese Revfy <>, 
	LKML <>, linux-arch <>
Subject: Re: [PATCH] ARM: vdso: Mark vDSO code as read-only

On Wed, Feb 17, 2016 at 3:43 PM, David Brown <> wrote:
> On Wed, Feb 17, 2016 at 03:00:52PM -0800, Kees Cook wrote:
>> On Tue, Feb 16, 2016 at 9:20 PM, David Brown <>
>> 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 <>
>>>> 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
>>>>> 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.

Yeah, in the ARM case, that's true. I think we should probably keep it
marked "correctly" though.

> Is there a possible future consideration to perhaps make .rodata read
> only much earlier?

Yeah, this will likely be a future improvement. Some architectures
already mark .rodata before the mark_rodata_ro() call. Once we start
to have more use of postinit-readonly, I suspect we'll see more
clarification of when those things happen.


Kees Cook
Chrome OS & Brillo Security

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