[RFC PATCH v2 1/3] x86/sgx: Add SGX specific LSM hooks

[Andy Lutomirski]

On Thu, Jun 27, 2019 at 11:56 AM Cedric Xing <cedric.xing at intel.com> wrote:
>
> SGX enclaves are loaded from pages in regular memory. Given the ability to
> create executable pages, the newly added SGX subsystem may present a backdoor
> for adversaries to circumvent LSM policies, such as creating an executable
> enclave page from a modified regular page that would otherwise not be made
> executable as prohibited by LSM. Therefore arises the primary question of
> whether an enclave page should be allowed to be created from a given source
> page in regular memory.
>
> A related question is whether to grant/deny a mprotect() request on a given
> enclave page/range. mprotect() is traditionally covered by
> security_file_mprotect() hook, however, enclave pages have a different lifespan
> than either MAP_PRIVATE or MAP_SHARED. Particularly, MAP_PRIVATE pages have the
> same lifespan as the VMA while MAP_SHARED pages have the same lifespan as the
> backing file (on disk), but enclave pages have the lifespan of the enclave’s
> file descriptor. For example, enclave pages could be munmap()’ed then mmap()’ed
> again without losing contents (like MAP_SHARED), but all enclave pages will be
> lost once its file descriptor has been closed (like MAP_PRIVATE). That said,
> LSM modules need some new data structure for tracking protections of enclave
> pages/ranges so that they can make proper decisions at mmap()/mprotect()
> syscalls.
>
> The last question, which is orthogonal to the 2 above, is whether or not to
> allow a given enclave to launch/run. Enclave pages are not visible to the rest
> of the system, so to some extent offer a better place for malicious software to
> hide. Thus, it is sometimes desirable to whitelist/blacklist enclaves by their
> measurements, signing public keys, or image files.
>
> To address the questions above, 2 new LSM hooks are added for enclaves.
>   - security_enclave_load() – This hook allows LSM to decide whether or not to
>     allow instantiation of a range of enclave pages using the specified VMA. It
>     is invoked when a range of enclave pages is about to be loaded. It serves 3
>     purposes: 1) indicate to LSM that the file struct in subject is an enclave;
>     2) allow LSM to decide whether or not to instantiate those pages and 3)
>     allow LSM to initialize internal data structures for tracking
>     origins/protections of those pages.
>   - security_enclave_init() – This hook allows whitelisting/blacklisting or
>     performing whatever checks deemed appropriate before an enclave is allowed
>     to run. An LSM module may opt to use the file backing the SIGSTRUCT as a
>     proxy to dictate allowed protections for anonymous pages.
>
> mprotect() of enclave pages continue to be governed by
> security_file_mprotect(), with the expectation that LSM is able to distinguish
> between regular and enclave pages inside the hook. For mmap(), the SGX
> subsystem is expected to invoke security_file_mprotect() explicitly to check
> protections against the requested protections for existing enclave pages. As
> stated earlier, enclave pages have different lifespan than the existing
> MAP_PRIVATE and MAP_SHARED pages, so would require a new data structure outside
> of VMA to track their protections and/or origins. Enclave Memory Area (or EMA
> for short) has been introduced to address the need. EMAs are maintained by the
> LSM framework for all LSM modules to share. EMAs will be instantiated for
> enclaves only so will not impose memory/performance overheads for regular
> applications/files. Please see include/linux/lsm_ema.h and security/lsm_ema.c
> for details.
>
> A new setup parameter – lsm.ema.cache_decisions has been introduced to offer
> the choice between memory consumption and accuracy of audit logs. Enabling
> lsm.ema.cache_decisions causes LSM framework NOT to keep backing files open for
> EMAs. While that saves memory, it requires LSM modules to make and cache
> decisions ahead of time, and makes it difficult for LSM modules to generate
> accurate audit logs. System administrators are expected to run LSM in
> permissive mode with lsm.ema.cache_decisions off to determine the minimal
> permissions needed, and then turn it back on in enforcing mode for optimal
> performance and memory usage. lsm.ema.cache_decisions is on by default and
> could be turned off by appending “lsm.ema.cache_decisions=0” or
> “lsm.ema.cache_decisions=off” to the kernel command line.

Just on a very cursory review, this seems like it's creating a bunch
of complexity (a whole new library and data structure), and I'm not
convinced the result is any better than Sean's version.