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Date: Tue, 26 Jul 2016 21:02:48 +0000
From: "Roberts, William C" <william.c.roberts@...el.com>
To: Nick Kralevich <nnk@...gle.com>
CC: "jason@...edaemon.net" <jason@...edaemon.net>, "linux-mm@...r.kernel.org"
<linux-mm@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>,
Greg KH <gregkh@...uxfoundation.org>, Jeffrey Vander Stoep
<jeffv@...gle.com>, "salyzyn@...roid.com" <salyzyn@...roid.com>, "Daniel
Cashman" <dcashman@...roid.com>
Subject: RE: [PATCH] [RFC] Introduce mmap randomization
> -----Original Message-----
> From: Nick Kralevich [mailto:nnk@...gle.com]
> Sent: Tuesday, July 26, 2016 1:41 PM
> To: Roberts, William C <william.c.roberts@...el.com>
> Cc: jason@...edaemon.net; linux-mm@...r.kernel.org; lkml <linux-
> kernel@...r.kernel.org>; kernel-hardening@...ts.openwall.com; Andrew
> Morton <akpm@...ux-foundation.org>; Kees Cook <keescook@...omium.org>;
> Greg KH <gregkh@...uxfoundation.org>; Jeffrey Vander Stoep
> <jeffv@...gle.com>; salyzyn@...roid.com; Daniel Cashman
> <dcashman@...roid.com>
> Subject: Re: [PATCH] [RFC] Introduce mmap randomization
>
> My apologies in advance if I misunderstand the purposes of this patch.
>
> IIUC, this patch adds a random gap between various mmap() mappings, with the
> goal of ensuring that both the mmap base address and gaps between pages are
> randomized.
>
> If that's the goal, please note that this behavior has caused significant
> performance problems to Android in the past. Specifically, random gaps between
> mmap()ed regions causes memory space fragmentation. After a program runs for
> a long time, the ability to find large contiguous blocks of memory becomes
> impossible, and mmap()s fail due to lack of a large enough address space.
Yes and fragmentation is definitely a problem here. Especially when the mmaps()
are not a consistent length for program life.
>
> This isn't just a theoretical concern. Android actually hit this on kernels prior to
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7dbaa46
> 6780a754154531b44c2086f6618cee3a8
> . Before that patch, the gaps between mmap()ed pages were randomized.
> See the discussion at:
>
> http://lists.infradead.org/pipermail/linux-arm-kernel/2011-
> November/073082.html
> http://marc.info/?t=132070957400005&r=1&w=2
>
> We ended up having to work around this problem in the following commits:
>
>
> https://android.googlesource.com/platform/dalvik/+/311886c6c6fcd3b531531f59
> 2d56caab5e2a259c
> https://android.googlesource.com/platform/art/+/51e5386
> https://android.googlesource.com/platform/art/+/f94b781
>
> If this behavior was re-introduced, it's likely to cause hard-to-reproduce
> problems, and I suspect Android based distributions would tend to disable this
> feature either globally, or for applications which make a large number of mmap()
> calls.
Yeah and this is the issue I want to see if we can overcome. I see the biggest benefit
being on libraries loaded by dl. Perhaps a random flag and modify to linkers. Im just
spit balling here and collecting the feedback, like this. Thanks for the detail, that
helps a lot.
>
> -- Nick
>
>
>
> On Tue, Jul 26, 2016 at 11:22 AM, <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.
> >
> > 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.
> >
> > Performance Measurements:
> > Using strace with -T option and filtering for mmap on the program ls
> > shows a slowdown of approximate 3.7%
> >
> > 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;
> > +
> > + slots = (end - start)/len;
> > + if (!slots)
> > + return 0;
> > +
> > + return PAGE_ALIGN(start + ((get_random_long() % slots) *
> > +len)); }
> > +
> > 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);
> > --
> > 1.9.1
> >
>
>
>
> --
> Nick Kralevich | Android Security | nnk@...gle.com | 650.214.4037
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