[RFC,v1,0/3] security/x86/sgx: SGX specific LSM hooks

Message

Xing, Cedric June 10, 2019, 7:03 a.m. UTC

This series intends to make the new SGX subsystem and the existing LSM
architecture work together smoothly so that, say, SGX cannot be abused to work
around restrictions set forth by LSM. This series applies on top of Jarkko
Sakkinen's SGX series v20 (https://lkml.org/lkml/2019/4/17/344), where abundant
details of this SGX/LSM problem could be found.

This series is an alternative to Sean Christopherson's recent RFC series
(https://lkml.org/lkml/2019/6/5/1070) that was trying to solve the same
problem. The key problem is for LSM to determine the "maximal (most permissive)
protection" allowed for individual enclave pages. Sean's approach is to take
that from user mode code as a parameter of the EADD ioctl, validate it with LSM
ahead of time, and then enforce it inside the SGX subsystem. The major
disadvantage IMHO is that a priori knowledge of "maximal protection" is needed,
but it isn't always available in certain use cases. In fact, it is an unusual
approach to take "maximal protection" from user code, as what SELinux is doing
today is to determine "maximal protection" of a vma using attributes associated
with vma->vm_file instead. When it comes to enclaves, vma->vm_file always
points /dev/sgx/enclave, so what's missing is a new way for LSM modules to
remember origins of enclave pages so that they don't solely depend on
vma->vm_file to determine "maximal protection".

This series takes advantage of the fact that enclave pages cannot be remapped
(to different linear address), therefore the pair of { vma->vm_file,
linear_address } can be used to uniquely identify an enclave page. Then by
notifying LSM on creation of every enclave page (via a new LSM hook -
security_enclave_load), LSM modules would be able to track origin and
protection changes of every page, hence be able to judge correctly upon
mmap/mprotect requests.

Cedric Xing (3):
  LSM/x86/sgx: Add SGX specific LSM hooks
  LSM/x86/sgx: Implement SGX specific hooks in SELinux
  LSM/x86/sgx: Call new LSM hooks from SGX subsystem

 arch/x86/kernel/cpu/sgx/driver/ioctl.c |  72 +++++-
 arch/x86/kernel/cpu/sgx/driver/main.c  |  12 +-
 include/linux/lsm_hooks.h              |  33 +++
 include/linux/security.h               |  26 +++
 security/security.c                    |  21 ++
 security/selinux/Makefile              |   2 +
 security/selinux/hooks.c               |  77 ++++++-
 security/selinux/include/intel_sgx.h   |  18 ++
 security/selinux/include/objsec.h      |   3 +
 security/selinux/intel_sgx.c           | 292 +++++++++++++++++++++++++
 10 files changed, 545 insertions(+), 11 deletions(-)
 create mode 100644 security/selinux/include/intel_sgx.h
 create mode 100644 security/selinux/intel_sgx.c

Comments

Jarkko Sakkinen June 10, 2019, 5:36 p.m. UTC | #1

On Mon, Jun 10, 2019 at 12:03:03AM -0700, Cedric Xing wrote:
> This series intends to make the new SGX subsystem and the existing LSM
> architecture work together smoothly so that, say, SGX cannot be abused to work
> around restrictions set forth by LSM. This series applies on top of Jarkko
> Sakkinen's SGX series v20 (https://lkml.org/lkml/2019/4/17/344), where abundant
> details of this SGX/LSM problem could be found.
> 
> This series is an alternative to Sean Christopherson's recent RFC series
> (https://lkml.org/lkml/2019/6/5/1070) that was trying to solve the same
> problem. The key problem is for LSM to determine the "maximal (most permissive)
> protection" allowed for individual enclave pages. Sean's approach is to take
> that from user mode code as a parameter of the EADD ioctl, validate it with LSM
> ahead of time, and then enforce it inside the SGX subsystem. The major
> disadvantage IMHO is that a priori knowledge of "maximal protection" is needed,
> but it isn't always available in certain use cases. In fact, it is an unusual
> approach to take "maximal protection" from user code, as what SELinux is doing
> today is to determine "maximal protection" of a vma using attributes associated
> with vma->vm_file instead. When it comes to enclaves, vma->vm_file always
> points /dev/sgx/enclave, so what's missing is a new way for LSM modules to
> remember origins of enclave pages so that they don't solely depend on
> vma->vm_file to determine "maximal protection".
> 
> This series takes advantage of the fact that enclave pages cannot be remapped
> (to different linear address), therefore the pair of { vma->vm_file,
> linear_address } can be used to uniquely identify an enclave page. Then by
> notifying LSM on creation of every enclave page (via a new LSM hook -
> security_enclave_load), LSM modules would be able to track origin and
> protection changes of every page, hence be able to judge correctly upon
> mmap/mprotect requests.
> 
> Cedric Xing (3):
>   LSM/x86/sgx: Add SGX specific LSM hooks
>   LSM/x86/sgx: Implement SGX specific hooks in SELinux
>   LSM/x86/sgx: Call new LSM hooks from SGX subsystem

A patch set containing direct LSM changes should consider all LSMs.
This will allow all the LSM maintainers to consider the changes. Now we
have a limited audience and we are favoring one LSM.

There is no good reason why direct LSM changes cannot be done
post-upstreaming like we do for virtualization.

Looking at Sean's patches, overally 1/5-3/5 make perfect sense.

/Jarkko