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Message-ID: <476DC76E7D1DF2438D32BFADF679FC560125F272@ORSMSX103.amr.corp.intel.com>
Date: Tue, 26 Jul 2016 20:11:28 +0000
From: "Roberts, William C" <william.c.roberts@...el.com>
To: Jason Cooper <jason@...edaemon.net>
CC: "linux-mm@...r.kernel.org" <linux-mm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"keescook@...omium.org" <keescook@...omium.org>, "gregkh@...uxfoundation.org"
<gregkh@...uxfoundation.org>, "nnk@...gle.com" <nnk@...gle.com>,
"jeffv@...gle.com" <jeffv@...gle.com>, "salyzyn@...roid.com"
<salyzyn@...roid.com>, "dcashman@...roid.com" <dcashman@...roid.com>
Subject: RE: [PATCH] [RFC] Introduce mmap randomization
> -----Original Message-----
> From: Jason Cooper [mailto:jason@...edaemon.net]
> Sent: Tuesday, July 26, 2016 1:03 PM
> To: Roberts, William C <william.c.roberts@...el.com>
> Cc: linux-mm@...r.kernel.org; linux-kernel@...r.kernel.org; kernel-
> hardening@...ts.openwall.com; akpm@...ux-foundation.org;
> keescook@...omium.org; gregkh@...uxfoundation.org; nnk@...gle.com;
> jeffv@...gle.com; salyzyn@...roid.com; dcashman@...roid.com
> Subject: Re: [PATCH] [RFC] Introduce mmap randomization
>
> Hi William!
>
> On Tue, Jul 26, 2016 at 11:22:26AM -0700, william.c.roberts@...el.com wrote:
> > From: William Roberts <william.c.roberts@...el.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.
>
> 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@...el.com>
> > ---
> > 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.
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