Re: SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)

From: Stephen Smalley
Date: Fri May 17 2019 - 15:23:02 EST

On 5/17/19 2:05 PM, Stephen Smalley wrote:
On 5/17/19 1:12 PM, Andy Lutomirski wrote:


On May 17, 2019, at 9:37 AM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:

On 5/17/19 12:20 PM, Stephen Smalley wrote:
On 5/17/19 11:09 AM, Sean Christopherson wrote:
On Fri, May 17, 2019 at 09:53:06AM -0400, Stephen Smalley wrote:
On 5/16/19 6:23 PM, Xing, Cedric wrote:
I thought EXECMOD applied to files (and memory mappings backed by them) but
I was probably wrong. It sounds like EXECMOD applies to the whole process so
would allow all pages within a process's address space to be modified then
executed, regardless the backing files. Am I correct this time?

No, you were correct the first time I think; EXECMOD is used to control
whether a process can make executable a private file mapping that has
previously been modified (e.g. text relocation); it is a special case to
support text relocations without having to allow full EXECMEM (i.e. execute
arbitrary memory).

SELinux checks relevant to W^X include:

- EXECMEM: mmap/mprotect PROT_EXEC an anonymous mapping (regardless of
PROT_WRITE, since we know the content has to have been written at some
point) or a private file mapping that is also PROT_WRITE.
- EXECMOD: mprotect PROT_EXEC a private file mapping that has been
previously modified, typically for text relocations,
- FILE__WRITE: mmap/mprotect PROT_WRITE a shared file mapping,
- FILE__EXECUTE: mmap/mprotect PROT_EXEC a file mapping.

(ignoring EXECSTACK and EXECHEAP here since they aren't really relevant to
this discussion)

So if you want to ensure W^X, then you wouldn't allow EXECMEM for the
process, EXECMOD by the process to any file, and the combination of both
FILE__WRITE and FILE__EXECUTE by the process to any file.

If the /dev/sgx/enclave mappings are MAP_SHARED and you aren't using an
anonymous inode, then I would expect that only the FILE__WRITE and
FILE__EXECUTE checks are relevant.

Yep, I was just typing this up in a different thread:

I think we may want to change the SGX API to alloc an anon inode for each
enclave instead of hanging every enclave off of the /dev/sgx/enclave inode.
Because /dev/sgx/enclave is NOT private, SELinux's file_map_prot_check()
will only require FILE__WRITE and FILE__EXECUTE to mprotect() enclave VMAs
to RWX. Backing each enclave with an anon inode will make SELinux treat
EPC memory like anonymous mappings, which is what we want (I think), e.g.
making *any* EPC page executable will require PROCESS__EXECMEM (SGX is
64-bit only at this point, so SELinux will always have default_noexec).
I don't think we want to require EXECMEM (or equivalently both FILE__WRITE and FILE__EXECUTE to /dev/sgx/enclave) for making any EPC page executable, only if the page is also writable or previously modified. The intent is to prevent arbitrary code execution without EXECMEM (or FILE__WRITE|FILE__EXECUTE), while still allowing enclaves to be created without EXECMEM as long as the EPC page mapping is only ever mapped RX and its initial contents came from an unmodified file mapping that was PROT_EXEC (and hence already checked via FILE__EXECUTE).

Also, just to be clear, there is nothing inherently better about checking EXECMEM instead of checking both FILE__WRITE and FILE__EXECUTE to the /dev/sgx/enclave inode, so I wouldn't switch to using anon inodes for that reason. Using anon inodes also unfortunately disables SELinux inode-based checking since we no longer have any useful inode information, so you'd lose out on SELinux ioctl whitelisting on those enclave inodes if that matters.

How can that work? Unless the API changes fairly radically, users fundamentally need to both write and execute the enclave. Some of it will be written only from already executable pages, and some privilege should be needed to execute any enclave page that was not loaded like this.

I'm not sure what the API is. Let's say they do something like this:

fd = open("/dev/sgx/enclave", O_RDONLY);
addr = mmap(NULL, size, PROT_READ | PROT_EXEC, MAP_SHARED, fd, 0);
stuff addr into ioctl args
ioctl(fd, ENCLAVE_CREATE, &ioctlargs);
ioctl(fd, ENCLAVE_ADD_PAGE, &ioctlargs);
ioctl(fd, ENCLAVE_INIT, &ioctlargs);

The important points are that they do not open /dev/sgx/enclave with write access (otherwise they will trigger FILE__WRITE at open time, and later encounter FILE__EXECUTE as well during mmap, thereby requiring both to be allowed to /dev/sgx/enclave), and that they do not request PROT_WRITE to the resulting mapping (otherwise they will trigger FILE__WRITE at mmap time). Then only FILE__READ and FILE__EXECUTE are required to /dev/sgx/enclave in policy.

If they switch to an anon inode, then any mmap PROT_EXEC of the opened file will trigger an EXECMEM check, at least as currently implemented, as we have no useful backing inode information.

FWIW, looking at the selftest for SGX in the patch series, they open /dev/sgx/enclave O_RDWR (probably not necessary?) and mmap the open file RWX. If that is necessary then I'd rather it show up as FILE__WRITE and FILE__EXECUTE to /dev/sgx/enclave instead of EXECMEM, so that we can allow the process the ability to perform that mmap without allowing it to make other mappings WX. So staying with the single /dev/sgx/enclave inode is better in that regard.