Re: [PATCH 0/9] mm: Hardened usercopy

On Thu, Jul 7, 2016 at 3:30 AM, Christian Borntraeger
[off-list ref] wrote:

On 07/07/2016 12:25 AM, Kees Cook wrote:

quoted

Hi,

This is a start of the mainline port of PAX_USERCOPY[1]. After I started
writing tests (now in lkdtm in -next) for Casey's earlier port[2], I
kept tweaking things further and further until I ended up with a whole
new patch series. To that end, I took Rik's feedback and made a number
of other changes and clean-ups as well.

Based on my understanding, PAX_USERCOPY was designed to catch a few
classes of flaws around the use of copy_to_user()/copy_from_user(). These
changes don't touch get_user() and put_user(), since these operate on
constant sized lengths, and tend to be much less vulnerable. There
are effectively three distinct protections in the whole series,
each of which I've given a separate CONFIG, though this patch set is
only the first of the three intended protections. (Generally speaking,
PAX_USERCOPY covers what I'm calling CONFIG_HARDENED_USERCOPY (this) and
CONFIG_HARDENED_USERCOPY_WHITELIST (future), and PAX_USERCOPY_SLABS covers
CONFIG_HARDENED_USERCOPY_SPLIT_KMALLOC (future).)

This series, which adds CONFIG_HARDENED_USERCOPY, checks that objects
being copied to/from userspace meet certain criteria:
- if address is a heap object, the size must not exceed the object's
  allocated size. (This will catch all kinds of heap overflow flaws.)
- if address range is in the current process stack, it must be within the
  current stack frame (if such checking is possible) or at least entirely
  within the current process's stack. (This could catch large lengths that
  would have extended beyond the current process stack, or overflows if
  their length extends back into the original stack.)
- if the address range is part of kernel data, rodata, or bss, allow it.
- if address range is page-allocated, that it doesn't span multiple
  allocations.
- if address is within the kernel text, reject it.
- everything else is accepted

The patches in the series are:
- The core copy_to/from_user() checks, without the slab object checks:
      1- mm: Hardened usercopy
- Per-arch enablement of the protection:
      2- x86/uaccess: Enable hardened usercopy
      3- ARM: uaccess: Enable hardened usercopy
      4- arm64/uaccess: Enable hardened usercopy
      5- ia64/uaccess: Enable hardened usercopy
      6- powerpc/uaccess: Enable hardened usercopy
      7- sparc/uaccess: Enable hardened usercopy

Was there a reason why you did not change s390?

No reason -- just didn't have a good build setup for testing it.
(Everything but arm64 was already in grsecurity, and I was able to
build-test arm64 when I added it there.) I would love to include s390
too!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security