RE: [PATCH] [RFC] Introduce mmap randomization

["Roberts, William C" <william.c.roberts@intel.com>]

> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> Behalf Of Kirill A. Shutemov
> Sent: Tuesday, July 26, 2016 1:29 PM
> To: Roberts, William C <william.c.roberts@intel.com>
> Cc: linux-mm@kvack.org
> Subject: Re: [PATCH] [RFC] Introduce mmap randomization
> 
> On Tue, Jul 26, 2016 at 07:57:45PM +0000, Roberts, William C wrote:
> >
> >
> > > -----Original Message-----
> > > From: Kirill A. Shutemov [mailto:kirill@shutemov.name]
> > > Sent: Tuesday, July 26, 2016 12:26 PM
> > > To: Roberts, William C <william.c.roberts@intel.com>
> > > Cc: linux-mm@kvack.org
> > > Subject: Re: [PATCH] [RFC] Introduce mmap randomization
> > >
> > > On Tue, Jul 26, 2016 at 11:27:11AM -0700, william.c.roberts@intel.com wrote:
> > > > From: William Roberts <william.c.roberts@intel.com>
> > > >
> > > > This patch introduces the ability randomize mmap locations where
> > > > the address is not requested, for instance when ld is allocating
> > > > pages for shared libraries. It chooses to randomize based on the
> > > > current personality for ASLR.
> > > >
> > > > Currently, allocations are done sequentially within unmapped
> > > > address space gaps. This may happen top down or bottom up depending on
> scheme.
> > > >
> > > > For instance these mmap calls produce contiguous mappings:
> > > > int size = getpagesize();
> > > > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> > > 0x40026000
> > > > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> > > 0x40027000
> > > >
> > > > Note no gap between.
> > > >
> > > > After patches:
> > > > int size = getpagesize();
> > > > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> > > 0x400b4000
> > > > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> > > 0x40055000
> > > >
> > > > Note gap between.
> > >
> > > And why is it good?
> >
> > Currently if you get an info leak and discover, say the address to
> > libX It's just a matter of adding/subtracting a fixed offset to find
> > libY. This will make rop a bit harder if you're trying to rop into a
> > different library than what was leaked.
> >
> > This also has a benefit outside of just libraries in that it
> > randomizes all the Mappings done via mmap from run to run. So you
> > don't get consistent, known offsets to things within the memory space.
> >
> > >
> > > > Using the test program mentioned here, that allocates fixed sized
> > > > blocks till exhaustion:
> > > > https://www.linux-mips.org/archives/linux-mips/2011-05/msg00252.ht
> > > > ml, no difference was noticed in the number of allocations. Most
> > > > varied from run to run, but were always within a few allocations
> > > > of one another between patched and un-patched runs.
> > > >
> > > > Performance Measurements:
> > > > Using strace with -T option and filtering for mmap on the program
> > > > ls shows a slowdown of approximate 3.7%
> > >
> > > NAK.
> > >
> > > It's just too costly. And no obvious benefits.
> >
> > Sorry I used to have the explanation in the message, a carless edit
> > removed it.
> >
> > The cost does suck, perhaps something like personality + KConfig option....
> 
> Cost sucks even more than you've mentioned: you'll pay on every page fault, as
> find_vma() would have more vmas in the tree and vmacache will not be that
> effective. That's something people spend a lot time to tune.

Yes that is very true. Perhaps randomizing in some other manner is more
Prudent, yet another mmap() flag?

> 
> Taking this into account, I can't see any real-world application that would opt-in
> for this security feature.

Dynamic linker?

> 
> --
>  Kirill A. Shutemov
> 
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