RE: [PATCH] [RFC] Introduce mmap randomization

[Roberts, William C]

> -----Original Message-----
> From: Jason Cooper [mailto:jason@xxxxxxxxxxxxxx]
> Sent: Tuesday, July 26, 2016 1:03 PM
> To: Roberts, William C <william.c.roberts@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kernel-
> hardening@xxxxxxxxxxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx;
> keescook@xxxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; nnk@xxxxxxxxxx;
> jeffv@xxxxxxxxxx; salyzyn@xxxxxxxxxxx; dcashman@xxxxxxxxxxx
> Subject: Re: [PATCH] [RFC] Introduce mmap randomization
> 
> Hi William!
> 
> On Tue, Jul 26, 2016 at 11:22:26AM -0700, william.c.roberts@xxxxxxxxx wrote:
> > From: William Roberts <william.c.roberts@xxxxxxxxx>
> >
> > 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.
> 
> Now I see how you found the randomize_range() fix. :-P
> 
> > 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.
> >
> > Using the test program mentioned here, that allocates fixed sized
> > blocks till exhaustion:
> > https://www.linux-mips.org/archives/linux-mips/2011-05/msg00252.html,
> > 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.
> 
> Did you test this with different allocation sizes?

No I didn't it. I wasn't sure the best way to test this, any ideas?

> 
> > Performance Measurements:
> > Using strace with -T option and filtering for mmap on the program ls
> > shows a slowdown of approximate 3.7%
> 
> I think it would be helpful to show the effect on the resulting object code.

Do you mean the maps of the process? I have some captures for whoopsie on my Ubuntu
system I can share.

One thing I didn't make clear in my commit message is why this is good. Right now, if you know
An address within in a process, you know all offsets done with mmap(). For instance, an offset
To libX can yield libY by adding/subtracting an offset. This is meant to make rops a bit harder, or
In general any mapping offset mmore difficult to find/guess.

> 
> > Signed-off-by: William Roberts <william.c.roberts@xxxxxxxxx>
> > ---
> >  mm/mmap.c | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index de2c176..7891272 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -43,6 +43,7 @@
> >  #include <linux/userfaultfd_k.h>
> >  #include <linux/moduleparam.h>
> >  #include <linux/pkeys.h>
> > +#include <linux/random.h>
> >
> >  #include <asm/uaccess.h>
> >  #include <asm/cacheflush.h>
> > @@ -1582,6 +1583,24 @@ unacct_error:
> >  	return error;
> >  }
> >
> > +/*
> > + * Generate a random address within a range. This differs from
> > +randomize_addr() by randomizing
> > + * on len sized chunks. This helps prevent fragmentation of the virtual
> memory map.
> > + */
> > +static unsigned long randomize_mmap(unsigned long start, unsigned
> > +long end, unsigned long len) {
> > +	unsigned long slots;
> > +
> > +	if ((current->personality & ADDR_NO_RANDOMIZE) ||
> !randomize_va_space)
> > +		return 0;
> 
> Couldn't we avoid checking this every time?  Say, by assigning a function pointer
> during init?

Yeah that could be done. I just copied the way others checked elsewhere in the kernel :-P

> 
> > +
> > +	slots = (end - start)/len;
> > +	if (!slots)
> > +		return 0;
> > +
> > +	return PAGE_ALIGN(start + ((get_random_long() % slots) * len)); }
> > +
> 
> Personally, I'd prefer this function noop out based on a configuration option.

Me too.

> 
> >  unsigned long unmapped_area(struct vm_unmapped_area_info *info)  {
> >  	/*
> > @@ -1676,6 +1695,8 @@ found:
> >  	if (gap_start < info->low_limit)
> >  		gap_start = info->low_limit;
> >
> > +	gap_start = randomize_mmap(gap_start, gap_end, length) ? :
> > +gap_start;
> > +
> >  	/* Adjust gap address to the desired alignment */
> >  	gap_start += (info->align_offset - gap_start) & info->align_mask;
> >
> > @@ -1775,6 +1796,9 @@ found:
> >  found_highest:
> >  	/* Compute highest gap address at the desired alignment */
> >  	gap_end -= info->length;
> > +
> > +	gap_end = randomize_mmap(gap_start, gap_end, length) ? : gap_end;
> > +
> >  	gap_end -= (gap_end - info->align_offset) & info->align_mask;
> >
> >  	VM_BUG_ON(gap_end < info->low_limit);
> 
> I'll have to dig into the mm code more before I can comment intelligently on this.
> 
> thx,
> 
> Jason.