[0/9] Function Granular Kernel Address Space Layout Randomization

Message

Kristen Carlson Accardi April 15, 2020, 9:04 p.m. UTC

This patch set is an implementation of finer grained kernel address space
randomization. It rearranges your kernel code at load time 
on a per-function level granularity, with only around a second added to
boot time.

Background
----------
KASLR was merged into the kernel with the objective of increasing the
difficulty of code reuse attacks. Code reuse attacks reused existing code
snippets to get around existing memory protections. They exploit software bugs
which expose addresses of useful code snippets to control the flow of
execution for their own nefarious purposes. KASLR moves the entire kernel
code text as a unit at boot time in order to make addresses less predictable.
The order of the code within the segment is unchanged - only the base address
is shifted. There are a few shortcomings to this algorithm.

1. Low Entropy - there are only so many locations the kernel can fit in. This
   means an attacker could guess without too much trouble.
2. Knowledge of a single address can reveal the offset of the base address,
   exposing all other locations for a published/known kernel image.
3. Info leaks abound.

Finer grained ASLR has been proposed as a way to make ASLR more resistant
to info leaks. It is not a new concept at all, and there are many variations
possible. Function reordering is an implementation of finer grained ASLR
which randomizes the layout of an address space on a function level
granularity. We use the term "fgkaslr" in this document to refer to the
technique of function reordering when used with KASLR, as well as finer grained
KASLR in general.

Proposed Improvement
--------------------
This patch set proposes adding function reordering on top of the existing
KASLR base address randomization. The over-arching objective is incremental
improvement over what we already have, as well as something that can be
merged and deployed with as little disruption to our existing kernel/ecosystem
as possible. It is designed to work with the existing solution, although it
can be used independently (not sure why you would do that though...). The
implementation is really pretty simple, and there are 2 main area where
changes occur:

* Build time

GCC has an option to place functions into individual .text sections. We can
use this option to implement function reordering at load time. The final
compiled vmlinux retains all the section headers, which can be used to help
us find the address ranges of each function. Using this information and
an expanded table of relocation addresses, we can shuffle the individual
text sections immediately after decompression. You are probably asking
yourself how this could possibly work given the number of tables of
addresses that exist inside the kernel today for features such as exception
handling and kprobes. Most of these tables generate relocations and
require a simple update, and some tables that have assumptions about order
require sorting after the update. In order to modify these tables, we
preserve a few key symbols from the objcopy symbol stripping process for use
after shuffling the text segments.

Some highlights from the build time changes to look for:

The top level kernel Makefile was modified to add the gcc flag if it
is supported. Currently, I am applying this flag to everything it is
possible to randomize. Future work could turn off this flags for selected
files or even entire subsystems, although obviously at the cost of security.

The relocs tool is updated to add relative relocations. This information
previously wasn't included because it wasn't necessary when moving the
entire .text segment as a unit. 

A new file was created to contain a list of symbols that objcopy should
keep. We use those symbols at load time as described below.

* Load time

The boot kernel was modified to parse the vmlinux elf file after
decompression to check for our interesting symbols that we kept, and to
look for any .text.* sections to randomize. We then shuffle the sections
and update any tables that need to be updated or resorted. The existing
code which updated relocation addresses was modified to account for not
just a fixed delta from the load address, but the offset that the function
section was moved to. This requires inspection of each address to see if
it was impacted by a randomization. We use a bsearch to make this less
horrible on performance.

In order to hide our new layout, symbols reported through /proc/kallsyms
will be sorted by name alphabetically rather than by address.

Security Considerations
-----------------------
The objective of this patch set is to improve a technology that is already
merged into the kernel (KASLR). Obviously, this code is not a one stop
shopping place for blocking code reuse attacks, but should instead be
considered as one of several tools that can be used. In particular, this
code is meant to make KASLR more effective in the presence of info leaks.
A key point to note is that we are basically accepting that there are
many and various ways to leak address today and in the future, and rather
than assume that we can stop them all, we should find a method which makes
individual info leaks less important. How much entropy we are adding to
the existing entropy of standard KASLR will depend on a few variables.
Firstly and most obviously, the number of functions you randomize matters.
This implementation keeps the existing .text section for code that cannot be
randomized - for example, because it was assembly code, or we opted out of
randomization for performance reasons. The less sections to randomize, the
less entropy. In addition, due to alignment (16 bytes for x86_64), the number
of bits in a address that the attacker needs to guess is reduced, as the lower
bits are identical.

Performance Impact
------------------
There are two areas where function reordering can impact performance: boot
time latency, and run time performance.

* Boot time latency
This implementation of finer grained KASLR impacts the boot time of the kernel
in several places. It requires additional parsing of the kernel ELF file to
obtain the section headers of the sections to be randomized. It calls the
random number generator for each section to be randomized to determine that
section's new memory location. It copies the decompressed kernel into a new
area of memory to avoid corruption when laying out the newly randomized
sections. It increases the number of relocations the kernel has to perform at
boot time vs. standard KASLR, and it also requires a lookup on each address
that needs to be relocated to see if it was in a randomized section and needs
to be adjusted by a new offset. Finally, it re-sorts a few data tables that
are required to be sorted by address.

Booting a test VM on a modern, well appointed system showed an increase in
latency of approximately 1 second.

* Run time
The performance impact at run-time of function reordering varies by workload.
Using kcbench, a kernel compilation benchmark, the performance of a kernel
build with finer grained KASLR was about 1% slower than a kernel with standard
KASLR. Analysis with perf showed a slightly higher percentage of 
L1-icache-load-misses. Other workloads were examined as well, with varied
results. Some workloads performed significantly worse under FGKASLR, while
others stayed the same or were mysteriously better. In general, it will
depend on the code flow whether or not finer grained KASLR will impact
your workload, and how the underlying code was designed. Future work could
identify hot areas that may not be randomized and either leave them in the
.text section or group them together into a single section that may be
randomized. If grouping things together helps, one other thing to consider
is that if we could identify text blobs that should be grouped together
to benefit a particular code flow, it could be interesting to explore
whether this security feature could be also be used as a performance
feature if you are interested in optimizing your kernel layout for a
particular workload at boot time. Optimizing function layout for a particular
workload has been researched and proven effective - for more information
read the Facebook paper "Optimizing Function Placement for Large-Scale
Data-Center Applications" (see references section below).

Image Size
----------
Adding additional section headers as a result of compiling with
-ffunction-sections will increase the size of the vmlinux ELF file.
With a standard distro config, the resulting vmlinux was increased by
about 3%. The compressed image is also increased due to the header files,
as well as the extra relocations that must be added. You can expect fgkaslr
to increase the size of the compressed image by about 15%.

Building
--------
To enable fine grained KASLR, you need to have the following config options
set (including all the ones you would use to build normal KASLR)

CONFIG_FG_KASLR=y

In addition, fgkaslr is only supported for the X86_64 architecture.

Modules
-------
Modules are randomized similarly to the rest of the kernel by shuffling
the sections at load time prior to moving them into memory. The module must
also have been build with the -ffunction-sections compiler option.

References
----------
There are a lot of academic papers which explore finer grained ASLR.
This paper in particular contributed the most to my implementation design
as well as my overall understanding of the problem space:

Selfrando: Securing the Tor Browser against De-anonymization Exploits,
M. Conti, S. Crane, T. Frassetto, et al.

For more information on how function layout impacts performance, see:

Optimizing Function Placement for Large-Scale Data-Center Applications,
G. Ottoni, B. Maher

Kees Cook (1):
  x86/boot: Allow a "silent" kaslr random byte fetch

Kristen Carlson Accardi (8):
  objtool: do not assume order of parent/child functions
  x86: tools/relocs: Support >64K section headers
  x86: Makefile: Add build and config option for CONFIG_FG_KASLR
  x86: make sure _etext includes function sections
  x86/tools: Adding relative relocs for randomized functions
  x86: Add support for function granular KASLR
  kallsyms: hide layout
  module: Reorder functions

 Documentation/security/fgkaslr.rst       | 151 +++++
 Documentation/security/index.rst         |   1 +
 Makefile                                 |   4 +
 arch/x86/Kconfig                         |  13 +
 arch/x86/boot/compressed/Makefile        |  10 +-
 arch/x86/boot/compressed/fgkaslr.c       | 809 +++++++++++++++++++++++
 arch/x86/boot/compressed/kaslr.c         |   7 +-
 arch/x86/boot/compressed/misc.c          | 107 ++-
 arch/x86/boot/compressed/misc.h          |  31 +
 arch/x86/boot/compressed/utils.c         |  12 +
 arch/x86/boot/compressed/vmlinux.symbols |  18 +
 arch/x86/include/asm/boot.h              |  15 +-
 arch/x86/kernel/vmlinux.lds.S            |  18 +-
 arch/x86/lib/kaslr.c                     |  18 +-
 arch/x86/tools/relocs.c                  | 143 +++-
 arch/x86/tools/relocs.h                  |   4 +-
 arch/x86/tools/relocs_common.c           |  14 +-
 include/asm-generic/vmlinux.lds.h        |   2 +-
 include/uapi/linux/elf.h                 |   1 +
 kernel/kallsyms.c                        | 138 +++-
 kernel/module.c                          |  82 +++
 tools/objtool/elf.c                      |   8 +-
 22 files changed, 1543 insertions(+), 63 deletions(-)
 create mode 100644 Documentation/security/fgkaslr.rst
 create mode 100644 arch/x86/boot/compressed/fgkaslr.c
 create mode 100644 arch/x86/boot/compressed/utils.c
 create mode 100644 arch/x86/boot/compressed/vmlinux.symbols

Comments

Kees Cook April 20, 2020, 5:48 p.m. UTC | #1

On Wed, Apr 15, 2020 at 02:04:42PM -0700, Kristen Carlson Accardi wrote:
> Kristen Carlson Accardi (8):
>   objtool: do not assume order of parent/child functions
>   x86: tools/relocs: Support >64K section headers

Can these two patches land separately (i.e. "now")? CFI will need the
relocs fix too, so better to have this landed upstream for both
features. I see Josh's Ack in the first...