Re: [PATCH v4 00/10] Function Granular KASLR
From: Joe Lawrence
Date: Tue Aug 04 2020 - 14:24:20 EST
On Fri, Jul 17, 2020 at 09:59:57AM -0700, Kristen Carlson Accardi wrote:
> Function Granular Kernel Address Space Layout Randomization (fgkaslr)
> ---------------------------------------------------------------------
>
> 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.
>
> Changes in v4:
> -------------
> * dropped the patch to split out change to STATIC definition in
> x86/boot/compressed/misc.c and replaced with a patch authored
> by Kees Cook to avoid the duplicate malloc definitions
> * Added a section to Documentation/admin-guide/kernel-parameters.txt
> to document the fgkaslr boot option.
> * redesigned the patch to hide the new layout when reading
> /proc/kallsyms. The previous implementation utilized a dynamically
> allocated linked list to display the kernel and module symbols
> in alphabetical order. The new implementation uses a randomly
> shuffled index array to display the kernel and module symbols
> in a random order.
>
> Changes in v3:
> -------------
> * Makefile changes to accommodate CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
> * removal of extraneous ALIGN_PAGE from _etext changes
> * changed variable names in x86/tools/relocs to be less confusing
> * split out change to STATIC definition in x86/boot/compressed/misc.c
> * Updates to Documentation to make it more clear what is preserved in .text
> * much more detailed commit message for function granular KASLR patch
> * minor tweaks and changes that make for more readable code
> * this cover letter updated slightly to add additional details
>
> Changes in v2:
> --------------
> * Fix to address i386 build failure
> * Allow module reordering patch to be configured separately so that
> arm (or other non-x86_64 arches) can take advantage of module function
> reordering. This support has not be tested by me, but smoke tested by
> Ard Biesheuvel <ardb@xxxxxxxxxx> on arm.
> * Fix build issue when building on arm as reported by
> Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> Patches to objtool are included because they are dependencies for this
> patchset, however they have been submitted by their maintainer separately.
>
> 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. It is designed to work in combination
> with the existing solution. The implementation is really pretty simple, and
> there are 2 main area where changes occur:
>
> * Build time
>
> GCC has had an option to place functions into individual .text sections for
> many years now. This option can be used to implement function reordering at
> load time. The final compiled vmlinux retains all the section headers, which
> can be used to help find the address ranges of each function. Using this
> information and an expanded table of relocation addresses, individual text
> sections can be suffled immediately after decompression. Some data tables
> inside the kernel that have assumptions about order require re-sorting
> after being updated when applying relocations. In order to modify these tables,
> a few key symbols are excluded 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. Anything that is written in C and not present in a
> special input section is randomized. The final binary segment 0 retains a
> consolidated .text section, as well as all the individual .text.* sections.
> 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. The consolidated .text section
> is skipped and not moved. The sections are shuffled randomly, and copied
> into memory following the .text section in a new random order. 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. Any tables that need to be modified with new
> addresses or resorted are updated using the symbol addresses parsed from the
> elf symbol table.
>
> In order to hide our new layout, symbols reported through /proc/kallsyms
> will be displayed in a random order.
>
> Security Considerations
> -----------------------
> The objective of this patch set is to improve a technology that is already
> merged into the kernel (KASLR). This code will not prevent all 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.
>
> 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
> that are randomized matters. This implementation keeps the existing .text
> section for code that cannot be randomized - for example, because it was
> assembly code. 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. Because the layout
> changes per boot, each time a system is rebooted the performance of a workload
> may change.
>
> 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%.
>
> Memory Usage
> ------------
> fgkaslr increases the amount of heap that is required at boot time,
> although this extra memory is released when the kernel has finished
> decompression. As a result, it may not be appropriate to use this feature on
> systems without much memory.
>
> 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.
>
> Although fgkaslr for the kernel is only supported for the X86_64 architecture,
> it is possible to use fgkaslr with modules on other architectures. To enable
> this feature, select
>
> CONFIG_MODULE_FG_KASLR=y
>
> This option is selected automatically for X86_64 when CONFIG_FG_KASLR is set.
>
> Disabling
> ---------
> Disabling normal KASLR using the nokaslr command line option also disables
> fgkaslr. It is also possible to disable fgkaslr separately by booting with
> fgkaslr=off on the commandline.
>
> 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 (2):
> x86/boot: Allow a "silent" kaslr random byte fetch
> x86/boot/compressed: Avoid duplicate malloc() implementations
>
> 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: Add relative relocs for randomized functions
> x86: Add support for function granular KASLR
> kallsyms: Hide layout
> module: Reorder functions
>
> .../admin-guide/kernel-parameters.txt | 7 +
> Documentation/security/fgkaslr.rst | 172 ++++
> Documentation/security/index.rst | 1 +
> Makefile | 6 +-
> arch/x86/Kconfig | 4 +
> arch/x86/Makefile | 5 +
> arch/x86/boot/compressed/Makefile | 9 +-
> arch/x86/boot/compressed/fgkaslr.c | 811 ++++++++++++++++++
> arch/x86/boot/compressed/kaslr.c | 4 -
> arch/x86/boot/compressed/misc.c | 157 +++-
> arch/x86/boot/compressed/misc.h | 30 +
> arch/x86/boot/compressed/utils.c | 11 +
> arch/x86/boot/compressed/vmlinux.symbols | 17 +
> arch/x86/include/asm/boot.h | 15 +-
> arch/x86/kernel/vmlinux.lds.S | 17 +-
> arch/x86/lib/kaslr.c | 18 +-
> arch/x86/tools/relocs.c | 143 ++-
> arch/x86/tools/relocs.h | 4 +-
> arch/x86/tools/relocs_common.c | 15 +-
> include/asm-generic/vmlinux.lds.h | 18 +-
> include/linux/decompress/mm.h | 12 +-
> include/uapi/linux/elf.h | 1 +
> init/Kconfig | 26 +
> kernel/kallsyms.c | 163 +++-
> kernel/module.c | 81 ++
> tools/objtool/elf.c | 8 +-
> 26 files changed, 1670 insertions(+), 85 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
>
>
> base-commit: 11ba468877bb23f28956a35e896356252d63c983
> --
> 2.20.1
>
Apologies in advance if this has already been discussed elsewhere, but I
did finally get around to testing the patchset against the livepatching
kselftests.
The livepatching kselftests fail as all livepatches stall their
transitions. It appears that reliable (ORC) stack unwinding is broken
when fgkaslr is enabled.
Relevant config options:
CONFIG_ARCH_HAS_FG_KASLR=y
CONFIG_ARCH_STACKWALK=y
CONFIG_FG_KASLR=y
CONFIG_HAVE_LIVEPATCH=y
CONFIG_HAVE_RELIABLE_STACKTRACE=y
CONFIG_LIVEPATCH=y
CONFIG_MODULE_FG_KASLR=y
CONFIG_TEST_LIVEPATCH=m
CONFIG_UNWINDER_ORC=y
The livepatch transitions are stuck along this call path:
klp_check_stack
stack_trace_save_tsk_reliable
arch_stack_walk_reliable
/* Check for stack corruption */
if (unwind_error(&state))
return -EINVAL;
where the unwinder error is set by unwind_next_frame():
arch/x86/kernel/unwind_orc.c
bool unwind_next_frame(struct unwind_state *state)
sometimes here:
/* End-of-stack check for kernel threads: */
if (orc->sp_reg == ORC_REG_UNDEFINED) {
if (!orc->end)
goto err;
goto the_end;
}
or here:
/* Prevent a recursive loop due to bad ORC data: */
if (state->stack_info.type == prev_type &&
on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
state->sp <= prev_sp) {
orc_warn_current("stack going in the wrong direction? at %pB\n",
(void *)orig_ip);
goto err;
}
(and probably other places the ORC unwinder gets confused.)
It also manifests itself in other, more visible ways. For example, a
kernel module that calls dump_stack() in its init function or even
/proc/<pid>/stack:
(fgkaslr on)
------------
Call Trace:
? dump_stack+0x57/0x73
? 0xffffffffc0850000
? mymodule_init+0xa/0x1000 [dumpstack]
? do_one_initcall+0x46/0x1f0
? free_unref_page_commit+0x91/0x100
? _cond_resched+0x15/0x30
? kmem_cache_alloc_trace+0x14b/0x210
? do_init_module+0x5a/0x220
? load_module+0x1912/0x1b20
? __do_sys_finit_module+0xa8/0x110
? __do_sys_finit_module+0xa8/0x110
? do_syscall_64+0x47/0x80
? entry_SYSCALL_64_after_hwframe+0x44/0xa9
% sudo cat /proc/$$/stack
[<0>] do_wait+0x1c3/0x230
[<0>] kernel_wait4+0xa6/0x140
fgkaslr=off
-----------
Call Trace:
dump_stack+0x57/0x73
? 0xffffffffc04f2000
mymodule_init+0xa/0x1000 [readonly]
do_one_initcall+0x46/0x1f0
? free_unref_page_commit+0x91/0x100
? _cond_resched+0x15/0x30
? kmem_cache_alloc_trace+0x14b/0x210
do_init_module+0x5a/0x220
load_module+0x1912/0x1b20
? __do_sys_finit_module+0xa8/0x110
__do_sys_finit_module+0xa8/0x110
do_syscall_64+0x47/0x80
entry_SYSCALL_64_after_hwframe+0x44/0xa9
% sudo cat /proc/$$/stack
[<0>] do_wait+0x1c3/0x230
[<0>] kernel_wait4+0xa6/0x140
[<0>] __do_sys_wait4+0x83/0x90
[<0>] do_syscall_64+0x47/0x80
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
I would think fixing and verifying these latter cases would be easier than
chasing livepatch transitions (but would still probably fix klp case, too).
Perhaps Josh or someone has other ORC unwinder tests that could be used?
-- Joe