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Date: Wed, 16 Sep 2020 10:08:22 -0500
Subject: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

From: "Madhavan T. Venkataraman" <>


Dynamic code is used in many different user applications. Dynamic code is
often generated at runtime. Dynamic code can also just be a pre-defined
sequence of machine instructions in a data buffer. Examples of dynamic
code are trampolines, JIT code, DBT code, etc.

Dynamic code is placed either in a data page or in a stack page. In order
to execute dynamic code, the page it resides in needs to be mapped with
execute permissions. Writable pages with execute permissions provide an
attack surface for hackers. Attackers can use this to inject malicious
code, modify existing code or do other harm.

To mitigate this, LSMs such as SELinux implement W^X. That is, they may not
allow pages to have both write and execute permissions. This prevents
dynamic code from executing and blocks applications that use it. To allow
genuine applications to run, exceptions have to be made for them (by setting
execmem, etc) which opens the door to security issues.

The W^X implementation today is not complete. There exist many user level
tricks that can be used to load and execute dynamic code. E.g.,

- Load the code into a file and map the file with R-X.

- Load the code in an RW- page. Change the permissions to R--. Then,
  change the permissions to R-X.

- Load the code in an RW- page. Remap the page with R-X to get a separate
  mapping to the same underlying physical page.

IMO, these are all security holes as an attacker can exploit them to inject
his own code.

In the future, these holes will definitely be closed. For instance, LSMs
(such as the IPE proposal [1]) may only allow code in properly signed object
files to be mapped with execute permissions. This will do two things:

	- user level tricks using anonymous pages will fail as anonymous
	  pages have no file identity

	- loading the code in a temporary file and mapping it with R-X
	  will fail as the temporary file would not have a signature

We need a way to execute such code without making security exceptions.
Trampolines are a good example of dynamic code. A couple of examples
of trampolines are given below. My first use case for this RFC is

Examples of trampolines

libffi (A Portable Foreign Function Interface Library):

libffi allows a user to define functions with an arbitrary list of
arguments and return value through a feature called "Closures".
Closures use trampolines to jump to ABI handlers that handle calling
conventions and call a target function. libffi is used by a lot
of different applications. To name a few:

	- Python
	- Java
	- Javascript
	- Ruby FFI
	- Lisp
	- Objective C

GCC nested functions:

GCC has traditionally used trampolines for implementing nested
functions. The trampoline is placed on the user stack. So, the stack
needs to be executable.

Currently available solution

One solution that has been proposed to allow trampolines to be executed
without making security exceptions is Trampoline Emulation. See:

In this solution, the kernel recognizes certain sequences of instructions
as "well-known" trampolines. When such a trampoline is executed, a page
fault happens because the trampoline page does not have execute permission.
The kernel recognizes the trampoline and emulates it. Basically, the
kernel does the work of the trampoline on behalf of the application.

Currently, the emulated trampolines are the ones used in libffi and GCC
nested functions. To my knowledge, only X86 is supported at this time.

As noted in emutramp.txt, this is not a generic solution. For every new
trampoline that needs to be supported, new instruction sequences need to
be recognized by the kernel and emulated. And this has to be done for
every architecture that needs to be supported.

emutramp.txt notes the following:

"... the real solution is not in emulation but by designing a kernel API
for runtime code generation and modifying userland to make use of it."

Solution proposed in this RFC

>From this RFC's perspective, there are two scenarios for dynamic code:

Scenario 1

We know what code we need only at runtime. For instance, JIT code generated
for frequently executed Java methods. Only at runtime do we know what
methods need to be JIT compiled. Such code cannot be statically defined. It
has to be generated at runtime.

Scenario 2

We know what code we need in advance. User trampolines are a good example of
this. It is possible to define such code statically with some help from the

This RFC addresses (2). (1) needs a general purpose trusted code generator
and is out of scope for this RFC.

For (2), the solution is to convert dynamic code to static code and place it
in a source file. The binary generated from the source can be signed. The
kernel can use signature verification to authenticate the binary and
allow the code to be mapped and executed.

The problem is that the static code has to be able to find the data that it
needs when it executes. For functions, the ABI defines the way to pass
parameters. But, for arbitrary dynamic code, there isn't a standard ABI
compliant way to pass data to the code for most architectures. Each instance
of dynamic code defines its own way. For instance, co-location of code and
data and PC-relative data referencing are used in cases where the ISA
supports it.

We need one standard way that would work for all architectures and ABIs.

The solution proposed here is:

1. Write the static code assuming that the data needed by the code is already
   pointed to by a designated register.

2. Get the kernel to supply a small universal trampoline that does the

	- Load the address of the data in a designated register
	- Load the address of the static code in a designated register
	- Jump to the static code

User code would use a kernel supplied API to create and map the trampoline.
The address values would be baked into the code so that no special ISA
features are needed.

To conserve memory, the kernel will pack as many trampolines as possible in
a page and provide a trampoline table to user code. The table itself is
managed by the user.

Trampoline File Descriptor (trampfd)

I am proposing a kernel API using anonymous file descriptors that can be
used to create the trampolines. The API is described in patch 1/4 of this
patchset. I provide a summary here:

	- Create a trampoline file object

	- Write a code descriptor into the trampoline file and specify:

		- the number of trampolines desired
		- the name of the code register
		- user pointer to a table of code addresses, one address
		  per trampoline

	- Write a data descriptor into the trampoline file and specify:

		- the name of the data register
		- user pointer to a table of data addresses, one address
		  per trampoline

	- mmap() the trampoline file. The kernel generates a table of
	  trampolines in a page and returns the trampoline table address

	- munmap() a trampoline file mapping

	- Close the trampoline file

Each mmap() will only map a single base page. Large pages are not supported.

A trampoline file can only be mapped once in an address space.

Trampoline file mappings cannot be shared across address spaces. So,
sending the trampoline file descriptor over a unix domain socket and
mapping it in another process will not work.

It is recommended that the code descriptor and the code table be placed
in the .rodata section so an attacker cannot modify them.

Trampoline use and reuse

The code for trampoline X in the trampoline table is:

	load	&code_table[X], code_reg
	load	(code_reg), code_reg
	load	&data_table[X], data_reg
	load	(data_reg), data_reg
	jump	code_reg

The addresses &code_table[X] and &data_table[X] are baked into the
trampoline code. So, PC-relative data references are not needed. The user
can modify code_table[X] and data_table[X] dynamically.

For instance, within libffi, the same trampoline X can be used for different
closures at different times by setting:

	data_table[X] = closure;
	code_table[X] = ABI handling code;

Advantages of the Trampoline File Descriptor approach

- Using this support from the kernel, dynamic code can be converted to
  static code with a little effort so applications and libraries can move to
  a more secure model. In the simplest cases such as libffi, dynamic code can
  even be eliminated.

- This initial work is targeted towards X86 and ARM. But it can be supported
  easily on all architectures. We don't need any special ISA features such
  as PC-relative data referencing.

- The only code generation needed is for this small, universal trampoline.

- The kernel does not have to deal with any ABI issues in the generation of
  this trampoline.

- The kernel provides a trampoline table to conserve memory.

- An SELinux setting called "exectramp" can be implemented along the
  lines of "execmem", "execstack" and "execheap" to selectively allow the
  use of trampolines on a per application basis.

- In version 1, a trip to the kernel was required to execute the trampoline.
  In version 2, that is not required. So, there are no performance
  concerns in this approach.


I have implemented my solution for libffi and provided the changes for
X86 and ARM, 32-bit and 64-bit. Here is the reference patch:

If the trampfd patchset gets accepted, I will send the libffi changes
to the maintainers for a review. BTW, I have also successfully executed
the libffi self tests.

Work that is pending

- I am working on implementing the SELinux setting - "exectramp".

- I have a test program to test the kernel API. I am working on adding it
  to selftests.




	Introduced the Trampfd feature.

	- Changed the system call. Version 2 does not support different
	  trampoline types and their associated type structures. It only
	  supports a kernel generated trampoline.

	  The system call now returns information to the user that is
	  used to define trampoline descriptors. E.g., the maximum
	  number of trampolines that can be packed in a single page.

	- Removed all the trampoline contexts such as register contexts
	  and stack contexts. This is based on the feedback that the kernel
	  should not have to worry about ABI issues and H/W features that
	  may deal with the context of a process.

	- Removed the need to make a trip into the kernel on trampoline
	  invocation. This is based on the feedback about performance.

	- Removed the ability to share trampolines across address spaces.
	  This would have made sense to different trampoline types based
	  on their semantics. But since I support only one specific
	  trampoline, sharing does not make sense.

	- Added calls to specify trampoline descriptors that the kernel
	  uses to generate trampolines.

	- Added architecture-specific code to generate the small, universal
	  trampoline for X86 32 and 64-bit, ARM 32 and 64-bit.

	- Implemented the trampoline table in a page.
Madhavan T. Venkataraman (4):
  Implement the kernel API for the trampoline file descriptor.
  Implement i386 and X86 support for the trampoline file descriptor.
  Implement ARM64 support for the trampoline file descriptor.
  Implement ARM support for the trampoline file descriptor.

 arch/arm/include/uapi/asm/ptrace.h     |  21 +++
 arch/arm/kernel/Makefile               |   1 +
 arch/arm/kernel/trampfd.c              | 124 +++++++++++++
 arch/arm/tools/syscall.tbl             |   1 +
 arch/arm64/include/asm/unistd.h        |   2 +-
 arch/arm64/include/asm/unistd32.h      |   2 +
 arch/arm64/include/uapi/asm/ptrace.h   |  59 ++++++
 arch/arm64/kernel/Makefile             |   2 +
 arch/arm64/kernel/trampfd.c            | 244 +++++++++++++++++++++++++
 arch/x86/entry/syscalls/syscall_32.tbl |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl |   1 +
 arch/x86/include/uapi/asm/ptrace.h     |  38 ++++
 arch/x86/kernel/Makefile               |   1 +
 arch/x86/kernel/trampfd.c              | 238 ++++++++++++++++++++++++
 fs/Makefile                            |   1 +
 fs/trampfd/Makefile                    |   5 +
 fs/trampfd/trampfd_fops.c              | 241 ++++++++++++++++++++++++
 fs/trampfd/trampfd_map.c               | 142 ++++++++++++++
 include/linux/syscalls.h               |   2 +
 include/linux/trampfd.h                |  49 +++++
 include/uapi/asm-generic/unistd.h      |   4 +-
 include/uapi/linux/trampfd.h           | 184 +++++++++++++++++++
 init/Kconfig                           |   7 +
 kernel/sys_ni.c                        |   3 +
 24 files changed, 1371 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm/kernel/trampfd.c
 create mode 100644 arch/arm64/kernel/trampfd.c
 create mode 100644 arch/x86/kernel/trampfd.c
 create mode 100644 fs/trampfd/Makefile
 create mode 100644 fs/trampfd/trampfd_fops.c
 create mode 100644 fs/trampfd/trampfd_map.c
 create mode 100644 include/linux/trampfd.h
 create mode 100644 include/uapi/linux/trampfd.h


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