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

[Sean Christopherson]

On Fri, May 24, 2019 at 10:42:43AM -0700, Sean Christopherson wrote:
> Hmm, I've been thinking more about pulling permissions from the source
> page.  Conceptually I'm not sure we need to meet the same requirements as
> non-enclave DSOs while the enclave is being built, i.e. do we really need
> to force userspace to fully map the enclave in normal memory?
>  
> Consider the Graphene scenario where it's building an enclave on the fly.
> Pulling permissions from the source VMAs means Graphene has to map the
> code pages of the enclave with X.  This means Graphene will need EXEDMOD
> (or EXECMEM if Graphene isn't careful).  In a non-SGX scenario this makes
> perfect sense since there is no way to verify the end result of RW->RX.
> 
> But for SGX, assuming enclaves are whitelisted by their sigstruct (checked
> at EINIT) and because page permissions affect sigstruct.MRENCLAVE, it *is*
> possible to verify the resulting RX contents.  E.g. for the purposes of
> LSMs, can't we use the .sigstruct file as a proxy for the enclave and
> require FILE__EXECUTE on the .sigstruct inode to map/run the enclave?
> 
> Stephen, is my logic sound?
> 
> 
> If so...
> 
>   - Require FILE__READ+FILE__EXECUTE on .sigstruct to mmap() the enclave.
> 
>   - Prevent userspace from mapping the enclave with permissions beyond the
>     original permissions of the enclave.  This can be done by populating
>     VM_MAY{READ,WRITE,EXEC} from the SECINFO (same basic concept as Andy's
>     proposals).  E.g. pre-EINIT, mmap() and mprotect() can only succeed
>     with PROT_NONE.
> 
>   - Require FILE__{READ,WRITE,EXECUTE} on /dev/sgx/enclave for simplicity,
>     or provide an alternate SGX_IOC_MPROTECT if we want to sidestep the
>     FILE__WRITE requirement.

One more thought.  EADD (and the equivalent SGX2 flow) could do
security_mmap_file() with a NULL file on the SECINFO permissions, which
would trigger PROCESS_EXECMEM if an enclave attempts to map a page RWX.

> No changes are required to LSMs, SGX1 has a single LSM touchpoint in its
> mmap(), and I *think* the only required userspace change is to mmap()
> PROT_NONE when allocating the enclave's virtual address range.
> 
> As for Graphene, it doesn't need extra permissions to run its enclaves,
> it just needs a way to install .sigstruct, which is a generic permissions
> problem and not SGX specific.
> 
> 
> For SGX2 maybe:
> 
>   - No additional requirements to map an EAUG'd page as RW page.  Not
>     aligned with standard MAP_SHARED behavior, but we really don't want
>     to require FILE__WRITE, and thus allow writes to .sigstruct. 
>  
>   - Require FILE__EXECMOD on the .sigstruct to map previously writable
>     page as executable (which indirectly includes all EAUG'd pages).
>     Wiring this up will be a little funky, but we again we don't want
>     to require FILE__WRITE on .sigstruct.
>