[v9,0/4] KASLR feature to randomize each loadable module

Message

Rick Edgecombe Nov. 10, 2018, 1:38 a.m. UTC

This is V9 of the "KASLR feature to randomize each loadable module" patchset.
The purpose is to increase the randomization for the module space from 10 to 17
bits, and also to make the modules randomized in relation to each other instead
of just the address where the allocations begin, so that if one module leaks the
location of the others can't be inferred.

Why its useful
==============
Randomizing the location of executable code is a defense against control flow
attacks, where the kernel is tricked into jumping, or speculatively executing
code other than what is intended. By randomizing the location of the code, the
attacker doesn't know where to redirect the control flow.

Today the RANDOMIZE_BASE feature randomizes the base address where the module
allocations begin with 10 bits of entropy for this purpose. From here, a highly
deterministic algorithm allocates space for the modules as they are loaded and
unloaded. If an attacker can predict the order and identities for modules that
will be loaded (either by the system, or controlled by the user with
request_module or BPF), then a single text address leak can give the attacker
access to the locations of other modules. So in this case this new algorithm can
take the entropy of the other modules from ~0 to 17, making it much more robust.

Another problem today is that the low 10 bits of entropy makes brute force
attacks feasible, especially in the case of speculative execution where a wrong
guess won't necessarily cause a crash. In this case, increasing the
randomization will force attacks to take longer, and so increase the time an
attacker may be detected on a system.

In the past KASLR has been considered mostly a remote defense, due to available
methods of de-randomizing the kernel text locally, but previous easier local
de-randomizing methods have been blocked by KPTI.

There are multiple efforts to apply more randomization to the core kernel text
as well, and so this module space piece can be a first step to increasing
randomization for all kernel space executable code.

Userspace ASLR can get 28 bits of entropy or more, so at least increasing this
to 17 for now improves what is currently a pretty low amount of randomization
for the higher privileged kernel space.

How it works
============
The algorithm is pretty simple. It just breaks the module space in two, a random
area (2/3 of module space) and a backup area (1/3 of module space). It first
tries to allocate up to 10000 randomly located starting pages inside the random
section. If this fails, it will allocate in the backup area. The backup area
base will be offset in the same way as current algorithm does for the base area,
which has 10 bits of entropy.

The vmalloc allocator can be used to try an allocation at a specific address,
however it is usually used to try an allocation over a large address range, and
so some behaviors which are non-issues in normal usage can be be sub-optimal
when trying the an allocation at 10000 small ranges. So this patch also includes
a new vmalloc function __vmalloc_node_try_addr and some other vmalloc tweaks
that allow for more efficient trying of addresses.

This algorithm targets maintaining high entropy for many 1000's of module
allocations. This is because there are other users of the module space besides
kernel modules, like eBPF JIT, classic BPF socket filter JIT and kprobes.

Performance
===========
Simulations were run using module sizes derived from the x86_64 modules to
measure the allocation performance at various levels of fragmentation and
whether the backup area was used.

Capacity
--------
There is a slight reduction in the capacity of modules as simulated by the
x86_64 module sizes of <1000. Note this is a worst case, since in practice
module allocations in the 1000's will consist of smaller BPF JIT allocations or
kprobes which would fit better in the random area.

Allocation time
---------------
Below are three sets of measurements in ns of the allocation time as measured by
the included kselftests. The first two columns are this new algorithm with and
with out the vmalloc optimizations for trying random addresses quickly. They are
included for consideration of whether the changes are worth it. The last column
is the performance of the original algorithm.

Modules Vmalloc optimization    No Vmalloc Optimization Existing Module KASLR
1000    1433                    1993                    3821
2000    2295                    3681                    7830
3000    4424                    7450                    13012
4000    7746                    13824                   18106
5000    12721                   21852                   22572
6000    19724                   33926                   26443
7000    27638                   47427                   30473
8000    37745                   64443                   34200

These allocations are not taking very long, but it may show up on systems with
very high usage of the module space (BPF JITs). If the trade-off of touching
vmalloc doesn't seem worth it to people, I can remove the optimizations.

Randomness
----------
Unlike the existing algorithm, the amount of randomness provided has a
dependency on the number of modules allocated and the sizes of the modules text
sections. The entropy provided for the Nth allocation will come from three
sources of randomness, the range of addresses for the random area, the
probability the section will be allocated in the backup area and randomness from
the number of modules already allocated in the backup area. For computing a
lower bound entropy in the following calculations, the randomness of the modules
already in the backup area, or overlapping from the random area, is ignored
since it is usually small and will only increase the entropy. Below is an
attempt to compute a worst case value for entropy to compare to the existing
algorithm.

For probability of the Nth allocation being in the backup area, p, a lower bound
entropy estimate is calculated here as:

Random Area Slots = ((2/3)*1073741824)/4096 = 174762

Entropy = -( (1-p)*log2((1-p)/174762) + p*log2(p/1024) )

For >8000 modules the entropy remains above 17.3. For non-speculative control
flow attacks, an attack might crash the system. So the probability of the
first guess being right can be more important than the Nth guess. KASLR schemes
usually have equal probability for each possible position, but in this scheme
that is not the case. So a more conservative comparison to existing schemes is
the amount of information that would have to be guessed correctly for the
position that has the highest probability for having the Nth module allocated
(as that would be the attackers best guess):

Min Info = MIN(-log2(p/1024), -log2((1-p)/174762))

Allocations	Entropy
1000		17.4
2000		17.4
3000		17.4
4000		16.8
5000		15.8
6000		14.9
7000		14.8
8000		14.2

If anyone is keeping track, these numbers are different than as reported in V2,
because they are generated using the more compact allocation size heuristic that
is included in the kselftest rather than the real much larger dataset. The
heuristic generates randomization benchmarks that are slightly slower than the
real dataset. The real dataset also isn't representative of the case of mostly
smaller BPF filters, so it represents a worst case lower bound for entropy and
in practice 17+ bits should be maintained to much higher number of modules.

PTE usage
---------
Since the allocations are spread out over a wider address space, there is
increased PTE usage which should not exceed 1.3MB more than the old algorithm.


Changes for V9:
 - Better explanations in commit messages, instructions in kselftests (Andrew
   Morton)

Changes for V8:
 - Simplify code by removing logic for optimum handling of lazy free areas

Changes for V7:
 - More 0-day build fixes, readability improvements (Kees Cook)

Changes for V6:
 - 0-day build fixes by removing un-needed functional testing, more error
   handling

Changes for V5:
 - Add module_alloc test module

Changes for V4:
 - Fix issue caused by KASAN, kmemleak being provided different allocation
   lengths (padding).
 - Avoid kmalloc until sure its needed in __vmalloc_node_try_addr.
 - Fixed issues reported by 0-day.

Changes for V3:
 - Code cleanup based on internal feedback. (thanks to Dave Hansen and Andriy
   Shevchenko)
 - Slight refactor of existing algorithm to more cleanly live along side new
   one.
 - BPF synthetic benchmark

Changes for V2:
 - New implementation of __vmalloc_node_try_addr based on the
   __vmalloc_node_range implementation, that only flushes TLB when needed.
 - Modified module loading algorithm to try to reduce the TLB flushes further.
 - Increase "random area" tries in order to increase the number of modules that
   can get high randomness.
 - Increase "random area" size to 2/3 of module area in order to increase the
   number of modules that can get high randomness.
 - Fix for 0day failures on other architectures.
 - Fix for wrong debugfs permissions. (thanks to Jann Horn)
 - Spelling fix. (thanks to Jann Horn)
 - Data on module_alloc performance and TLB flushes. (brought up by Kees Cook
   and Jann Horn)
 - Data on memory usage. (suggested by Jann)


Rick Edgecombe (4):
  vmalloc: Add __vmalloc_node_try_addr function
  x86/modules: Increase randomization for modules
  vmalloc: Add debugfs modfraginfo
  Kselftest for module text allocation benchmarking

 arch/x86/Kconfig                              |   3 +
 arch/x86/include/asm/kaslr_modules.h          |  38 ++
 arch/x86/include/asm/pgtable_64_types.h       |   7 +
 arch/x86/kernel/module.c                      | 111 ++++--
 include/linux/vmalloc.h                       |   3 +
 lib/Kconfig.debug                             |   9 +
 lib/Makefile                                  |   1 +
 lib/test_mod_alloc.c                          | 375 ++++++++++++++++++
 mm/vmalloc.c                                  | 228 +++++++++--
 tools/testing/selftests/bpf/test_mod_alloc.sh |  29 ++
 10 files changed, 743 insertions(+), 61 deletions(-)
 create mode 100644 arch/x86/include/asm/kaslr_modules.h
 create mode 100644 lib/test_mod_alloc.c
 create mode 100755 tools/testing/selftests/bpf/test_mod_alloc.sh