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Date: Tue, 21 Jun 2016 12:47:28 -0700
From: Andy Lutomirski <>
To: Arnd Bergmann <>
Cc: Kees Cook <>, Andy Lutomirski <>, 
	"" <>, LKML <>, 
	linux-arch <>, Borislav Petkov <>, 
	Nadav Amit <>, Brian Gerst <>, 
	"" <>, 
	Linus Torvalds <>, Josh Poimboeuf <>, 
	Jann Horn <>, Heiko Carstens <>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

On Tue, Jun 21, 2016 at 12:47 PM, Arnd Bergmann <> wrote:
> On Tuesday, June 21, 2016 10:16:21 AM CEST Kees Cook wrote:
>> On Tue, Jun 21, 2016 at 2:24 AM, Arnd Bergmann <> wrote:
>> > On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote:
>> >>
>> >> On my laptop, this adds about 1.5┬Ás of overhead to task creation,
>> >> which seems to be mainly caused by vmalloc inefficiently allocating
>> >> individual pages even when a higher-order page is available on the
>> >> freelist.
>> >
>> > Would it help to have a fixed virtual address for the stack instead
>> > and map the current stack to that during a task switch, similar to
>> > how we handle fixmap pages?
>> >
>> > That would of course trade the allocation overhead for a task switch
>> > overhead, which may be better or worse. It would also give "current"
>> > a constant address, which may give a small performance advantage
>> > but may also introduce a new attack vector unless we randomize it
>> > again.
>> Right: we don't want a fixed address. That makes attacks WAY easier.
> Do we care about making the address more random then? When I look
> at /proc/vmallocinfo, I see that allocations are all using
> consecutive addresses, so if you can figure out the virtual
> address of the stack for one process that would give you a good
> chance of guessing the address for the next pid.

Quite possibly.  We should seriously consider at least randomizing the
*start* of the vmalloc area, at least on 64-bit architectures.


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