[v1,0/6] Memory Mapping (VMA) protection using PKU - set 1

From: Jeff Xu <jeffxu@google.com>

This is the first set of Memory mapping (VMA) protection patches using PKU.

* * * 

Background:

As discussed previously in the kernel mailing list [1], V8 CFI [2] uses 
PKU to protect memory, and Stephen Röttger proposes to extend the PKU to 
memory mapping [3].

We're using PKU for in-process isolation to enforce control-flow integrity
for a JIT compiler. In our threat model, an attacker exploits a 
vulnerability and has arbitrary read/write access to the whole process
space concurrently to other threads being executed. This attacker can
manipulate some arguments to syscalls from some threads.

Under such a powerful attack, we want to create a “safe/isolated”
thread environment. We assign dedicated PKUs to this thread, 
and use those PKUs to protect the threads’ runtime environment.
The thread has exclusive access to its run-time memory. This
includes modifying the protection of the memory mapping, or
munmap the memory mapping after use. And the other threads
won’t be able to access the memory or modify the memory mapping
(VMA) belonging to the thread.

* * * 

Proposed changes:

This patch introduces a new flag, PKEY_ENFORCE_API, to the pkey_alloc()
function. When a PKEY is created with this flag, it is enforced that any
thread that wants to make changes to the memory mapping (such as mprotect)
of the memory must have write access to the PKEY. PKEYs created without
this flag will continue to work as they do now, for backwards 
compatibility.

Only PKEY created from user space can have the new flag set, the PKEY
allocated by the kernel internally will not have it. In other words,
ARCH_DEFAULT_PKEY(0) and execute_only_pkey won’t have this flag set,
and continue work as today.

This flag is checked only at syscall entry, such as mprotect/munmap in
this set of patches. It will not apply to other call paths. In other
words, if the kernel want to change attributes of VMA for some reasons,
the kernel is free to do that and not affected by this new flag.

This set of patch covers mprotect/munmap, I plan to work on other 
syscalls after this. 

* * * 

Testing:

I have tested this patch on a Linux kernel 5.15, 6,1, and 6.4-rc1,
new selftest is added in: pkey_enforce_api.c 

* * * 

Discussion:

We believe that this patch provides a valuable security feature. 
It allows us to create “safe/isolated” thread environments that are 
protected from attackers with arbitrary read/write access to 
the process space.

We believe that the interface change and the patch don't 
introduce backwards compatibility risk.

We would like to disucss this patch in Linux kernel community
for feedback and support. 

* * * 

Reference:

[1]https://lore.kernel.org/all/202208221331.71C50A6F@keescook/
[2]https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit?usp=sharing
[3]https://docs.google.com/document/d/1qqVoVfRiF2nRylL3yjZyCQvzQaej1HRPh3f5wj1AS9I/edit

* * * 
Current status:

There are on-going discussion related to threat model, io_uring, we will continue discuss using v0 thread.

* * * 
PATCH history:

v1: update code related review comments:
mprotect.c: 
	remove syscall from do_mprotect_pkey()
	remove pr_warn_ratelimited

munmap.c:
	change syscall to enum caller_origin
	remove pr_warn_ratelimited

v0:
https://lore.kernel.org/linux-mm/20230515130553.2311248-1-jeffxu@chromium.org/

Best Regards,
-Jeff Xu


Jeff Xu (6):
  PKEY: Introduce PKEY_ENFORCE_API flag
  PKEY: Add arch_check_pkey_enforce_api()
  PKEY: Apply PKEY_ENFORCE_API to mprotect
  PKEY:selftest pkey_enforce_api for mprotect
  PKEY: Apply PKEY_ENFORCE_API to munmap
  PKEY:selftest pkey_enforce_api for munmap

 arch/powerpc/include/asm/pkeys.h              |   19 +-
 arch/x86/include/asm/mmu.h                    |    7 +
 arch/x86/include/asm/pkeys.h                  |   92 +-
 arch/x86/mm/pkeys.c                           |    2 +-
 include/linux/mm.h                            |    8 +-
 include/linux/pkeys.h                         |   18 +-
 include/uapi/linux/mman.h                     |    5 +
 mm/mmap.c                                     |   31 +-
 mm/mprotect.c                                 |   17 +-
 mm/mremap.c                                   |    6 +-
 tools/testing/selftests/mm/Makefile           |    1 +
 tools/testing/selftests/mm/pkey_enforce_api.c | 1312 +++++++++++++++++
 12 files changed, 1499 insertions(+), 19 deletions(-)
 create mode 100644 tools/testing/selftests/mm/pkey_enforce_api.c


base-commit: ba0ad6ed89fd5dada3b7b65ef2b08e95d449d4ab