[llvm-dev] [RFC] Pagerando: Page-granularity code randomization

Davide Italiano via llvm-dev llvm-dev at lists.llvm.org
Sat Jun 10 16:09:33 PDT 2017

On Tue, Jun 6, 2017 at 10:55 AM, Stephen Crane via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> This RFC describes pagerando, an improvement upon ASLR for shared
> libraries. We're planning to submit this work for upstreaming and
> would appreciate feedback before we get to the patch submission stage.
> Pagerando randomizes the location of individual memory pages (ASLR
> only randomizes the library base address). This increases security
> against code-reuse attacks (such as ROP) by tolerating pointer leaks.
> Pagerando splits libraries into page-aligned bins at compile time. At
> load time, each bin is mapped to a random address. The code in each
> bin is immutable and thus shared between processes.
> To implement pagerando, the compiler and linker need to build shared
> libraries with text segments split into page-aligned (and ideally
> page-sized) bins. All inter-bin references are indirected through a
> table initialized by the dynamic loader that holds the absolute
> address of each bin. At load time the loader randomly chooses an
> address for each bin and maps the bin pages from disk into memory.
> We're focusing on ARM and AArch64 initially, although there is nothing
> particularly target specific that precludes support for other LLVM
> backends.
> ## Design Goals
> 1. Improve security over ASLR. The randomization granularity
> determines how much information a single code pointer leaks. A pointer
> to a page reveals less about the location of other code than a pointer
> into a contiguous library would.
> 2. Avoid randomizing files on disk. Modern operating systems provide
> verified boot techniques to detect tampering with files. Randomizing
> the on-disk layout of system libraries would interfere with the
> trusted boot process. Randomizing libraries at compile or link time
> would also needlessly complicate deployment and provisioning.
> 3. Preserve code page sharing. The OS reduces memory usage by mapping
> shared file pages to the same physical memory in each process and
> locates these pages at different virtual addresses with ASLR. To
> preserve sharing of code pages, we cannot modify the contents of
> file-mapped pages at load time and are restricted to changing their
> ordering and placement in the virtual address space.
> 4. Backwards compatibility. Randomized code must interoperate
> transparently with existing, unmodified executables and shared
> libraries. Calls into randomized code must work as-is according to the
> normal ABI.
> 5. Compatibility with other mitigations. Enabling randomization must
> not preclude deploying other mitigations such as control-flow
> integrity as well.
> ## Pagerando Design
> Pagerando requires a platform-specific extension to the dynamic
> loading ABI for compatible libraries to opt-in to. In order to
> decouple the address of each code bin (segment) from that of other
> bins and global data, we must disallow relative addressing between
> different bin segments as well as between legacy segments and bin
> segments.
> To prepare a library for pagerando, the compiler must first allocate
> functions into page-aligned bins corresponding to segments in the
> final ELF file. Since these bins will be independently positioned, the
> compiler must redirect all inter-bin references through an indirection
> table – the Page Offset Table (POT) – which stores the virtual address
> of each bin in the library. Indices of POT entries and bin offsets are
> statically determined at link time so code will not require any
> dynamic relocations to reference functions in another bin or globals
> outside of bins. We reserve a register in pagerando-compatible code to
> hold the address of the POT. This register is initialized on entry to
> the shared library. At load time the dynamic loader maps code bins at
> independent, random addresses and updates the dynamic relocations in
> the POT.
> Reserving a register to hold the POT address changes the internal ABI
> calling convention and requires that the POT register be correctly
> initialized when entering a library from external code. To initialize
> the register, the compiler emits entry wrappers which save the old
> contents of the POT register if necessary, initialize the POT
> register, and call the target function. Each externally visible
> function (conservatively including all address taken functions) needs
> an entry wrapper which replaces the function for all external uses.
> To optimally pack functions into bins and avoid new static
> relocations, we propose using (traditional) LTO. With new static
> relocations (i.e. linker cooperation), LTO would not be necessary, but
> it is still desirable for more efficient bin packing.
> The design of pagerando is based on the mitigations proposed by Backes
> and Nürnberger [1], with improvements for compatibility and
> deployability. The present design is a refinement of our first
> pagerando prototype [2].
> ## LLVM Changes
> To implement pagerando, we propose the following LLVM changes:
> New module pass to create entry wrapper functions. This pass will
> create entry wrappers as described above and replace exported function
> names and all address taken uses with the wrapper. This pass will only
> be run when pagerando is enabled.
> Instruction Lowering. Pagerando-compatible code must access all global
> values (including functions) through the POT since PC-relative memory
> addressing is not allowed between a bin and another segment. We
> propose that when pagerando is enabled, all global variable accesses
> from functions marked as pagerando-compatible must be lowered into
> GOT-relative accesses and added to the GOT address loaded from the POT
> (currently stored in the first POT entry). Lowering of direct function
> calls targeting pagerando-compatible code is slightly more complicated
> because we need to determine the POT index of the bin containing the
> target function if the target is not in the same bin. However, we
> can't properly allocate functions to bins before they are lowered and
> an approximate size is available. Therefore, during lowering we should
> assume that all function calls must be made indirectly through the POT
> with the computation of the POT index and bin offset of the target
> function postponed until assembly printing.
> New machine module LTO pass to allocate functions into bins. This pass
> relies on targets implementing TargetInstrInfo::getInstSizeInBytes
> (MachineInstr) so that it knows (approximately) how large the final
> function code will be. Functions can also be packed in such a way that
> the number of inter-bin calls are minimized by taking the function
> call graph and/or execution profiles into account while packing. This
> pass only needs to run when pagerando is enabled.
> Code Emission. After functions are assigned to bins, we create an
> individual MCSection for each bin. These MCSections will map to
> independent segments during linking. The AsmPrinter is responsible for
> emitting the POT entries during code emission. We cannot easily
> represent the POT as a standard IR object because it needs to contain
> bin (MCSection) addresses. The AsmPrinter instead can query the
> MCContext for the list of bin symbols and emit these symbols directly
> into a global POT array.
> Gold Plugin Interface. If using LTO to build the module, LLVM can
> generate the complete POT for the module and instrument all references
> that need to use the POT. However, we must still ensure that bin
> sections are each placed into an independent segment so that the
> dynamic loader can map each bin separately. The gold plugin interface
> currently provides support to assign sections to unique output
> segments. However, it does not yet provide plugins an opportunity to
> call this interface for new, plugin-created input files. Gold requires
> that the plugin provide the file handle of the input section to assign
> a section to a unique segment. We will need to upstream a small patch
> for gold that provides a new callback to the LTO plugin when gold
> receives a new, plugin-generated input file. This would allow the
> plugin to obtain the new file’s handle and map its sections to unique
> segments. The linker must mark pagerando bin segments in such a way
> that the dynamic loader knows that it can randomize each bin segment
> independently. We propose a new ELF segment flag PF_RAND_ADDR that can
> communicate this for each compatible segment. The compiler and/or
> linker must add this flag to compatible segments for the loader to
> recognize and randomize the relevant segments.
> ## Target-Specific Details
> We will initially support pagerando for ARM and AArch64, so several
> details are worth considering on those targets. For ARM/AArch64, the
> r9 register is a platform-specific register that can be used as the
> static base register, which is similar in many ways to pagerando. When
> not specified by the platform, r9 is a callee-saved general-purpose
> register. Thus, using r9 as the POT register will be backwards
> compatible when calling out of pagerando code into either legacy code
> or a different module; the callee will preserve r9 for use after
> returning to pagerando code. In AArch64, r18 is designated as a
> platform-specific register, however, it is not specified as
> callee-saved when not reserved by the target platform. Thus, to
> interoperate with unmodified legacy AArch64 software, we would need to
> save r18 in pagerando code before calling into any external code. When
> using LTO, the compiler will see the entire module and therefore be
> able to identify calls into external vs internal code. Without LTO, it
> will likely be more efficient to use a callee-saved register to avoid
> the need to save the POT register before each call. We will experiment
> with both caller- and callee-saved registers to determine which is
> most efficient.
> [1] M. Backes and S. Nürnberger. Oxymoron - making fine-grained memory
> randomization practical by allowing code sharing. In USENIX Security
> Symposium, 2014. https://www.usenix.org/node/184466
> [2] S. Crane, A. Homescu, and P. Larsen. Code randomization: Haven’t
> we solved this problem yet? In IEEE Cybersecurity Development
> Conference (SecDev), 2016.
> http://www.ics.uci.edu/~perl/sd16_pagerando.pdf
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Out of curiosity, Did you measure what's the impact on performances
of the generated executable? We tried something akin to your proposal
in the past (i.e. randomizing ELF sections layout) and it turned out to be a
sledgehammer for performances (in some cases, i.e. when
-ffunction-sections/-fdata-sections was specified the performances of
the runtime executable dropped by > 10% [cc:ing Michael as he did the


"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare

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