Text 6732, 117 rader
Skriven 2005-08-26 20:44:52 av Mike '/m' (1:379/45)
Ärende: malloc() and free() redux
=================================
From: Mike '/m' <mike@barkto.com>


http://marc.theaimsgroup.com/?l=openbsd-misc&m=112475373731469&w=2

====
List:       openbsd-misc
Subject:    3.8 beta requests
From:       Theo de Raadt <deraadt () cvs ! openbsd ! org>
Date:       2005-08-22 23:33:40
Message-ID: 200508222333.j7MNXepH022873 () cvs ! openbsd ! org
...

We are heading towards making the real 3.8 release soonish.  I would like to
ask the community to do lots of testing over the next week if they can.

This release will bring a lot of new ideas from us.  One of them in particular
is somewhat risky.  I think it is time to talk about that one, and let people
know what is ahead on our road.

Traditionally, Unix malloc(3) has always just "extended the brk", which means
extending the traditional Unix process data segment to allocate more memory. 
malloc(3) would simply extend the data segment, and then calve off little
pieces to requesting callers as needed.  It also remembered which pieces were
which, so that free(3) could do it's job.

The way this was always done in Unix has had a number of consequences, some of
which we wanted to get rid of.  In particular, malloc & free have not been able
to provide strong protection against overflows or other corruption.

Our malloc implementation is a lot more resistant (than Linux) to "heap
overflows in the malloc arena", but we wanted to improve things even more.

Starting a few months ago, the following changes were made:

- We made the mmap(2) system call return random memory addresses.  As
well the kernel ensures that two objects are not mapped next to each
  other; in effect, this creates unallocated memory which we call a
  "guard page".

- We have changed malloc(3) to use mmap(2) instead of extending the data
  segment via brk()

- We also changed free(3) to return memory to the kernel, un-allocating
  them out of the process.

- As before, objects smaller than a page are allocated within shared
  pages that malloc(3) maintains.  But their allocation is now somewhat
  randomized as well.

- A number of other similar changes which are too dangerous for normal
  software or cause too much of a slowdown are available as malloc
  options as described in the manual page.  These are very powerful for
debugging buggy applications.

Other results:

- When you free an object that is >= 1 page in size, it is actually
  returned to the system.  Attempting to read or write to it after
  you free is no longer acceptable.  That memory is unmapped.  You get
  a SIGSEGV.

- For a decade and a bit, we have been fixing software for buffer
  overflows.  Now we are finding a lot of software that reads before the
start of the buffer, or reads too far off the end of the buffer.  You get a
SIGSEGV.

To some of you, this will sound like what the Electric Fence toolkit used to be
for.  But these features are enabled by default.  Electric Fence was also very
slow.  It took nearly 3 years to write these OpenBSD changes since performance
was a serious consideration.  (Early versions caused a nearly 50% slowdown).

Our changes have tremendous benefits, but until some bugs in external packages
are found and fixed, there are some risks as well.  Some software making
incorrect assumptions will be running into these new security technologies.

I discussed this in talks I have given before: I said that we were afraid to go
ahead with guard pages, because a lot of software is just written to such low
standards.  Applications over-read memory all the time, go 1 byte too far, read
1 byte too early, access memory after free, etc etc etc.

Oh well -- we've decided that we will try to ship with this protection
mechanism in any case, and try to solve the problems as we run into them.

Two examples:

Over the last two months, some OpenBSD users noticed that the X server was
crashing occasionally.  Two bugs have been diagnosed and fixed by us.  One was
a use-after-free bug in the X shared library linker.  The other was a
buffer-over-read bug deep down in the very lowest level fb* pixmap compositing
routines.  The latter bug in particular was very difficult to diagnose and fix,
and is about 10 years old.  We have found other bugs like this in other
external software, and even a few in the base OpenBSD tree (though those were
found a while back, even as we started experimenting with the new malloc code).

I would bet money that the X fb* bug has crashed Linux (and other) X servers
before.  It is just that it was very rare, and noone ever chased it.  The new
malloc we have just makes code get lucky less often, which lets us get to the
source of a bug easier.  As a programmer, I appreciate anything which makes
bugs easier to reproduce.

We expect that our malloc will find more bugs in software, and this might hurt
our user community in the short term.  We know that what this new malloc is
doing is perfectly legal, but that realistically some open source software is
of such low quality that it is just not ready for these things to happen.

We ask our users to help us uncover and fix more of these bugs in applications.
 Some will even be exploitable.  Instead of saying that OpenBSD is busted in
this regard, please realize that the software which is crashing is showing how
shoddily it was written.  Then help us fix it.  For everyone.. not just OpenBSD
users.
===

  /m

--- BBBS/NT v4.01 Flag-5
 * Origin: Barktopia BBS Site http://HarborWebs.com:8081 (1:379/45)