Re: [RFC PATCH 9/9] security/selinux: Add enclave_load() implementation

[Stephen Smalley <sds@tycho.nsa.gov>]

On 5/31/19 7:31 PM, Sean Christopherson wrote:
> The goal of selinux_enclave_load() is to provide a facsimile of the
> existing selinux_file_mprotect() and file_map_prot_check() policies,
> but tailored to the unique properties of SGX.
> 
> For example, an enclave page is technically backed by a MAP_SHARED file,
> but the "file" is essentially shared memory that is never persisted
> anywhere and also requires execute permissions (for some pages).
> 
> The basic concept is to require appropriate execute permissions on the
> source of the enclave for pages that are requesting PROT_EXEC, e.g. if
> an enclave page is being loaded from a regular file, require
> FILE__EXECUTE and/or FILE__EXECMOND, and if it's coming from an
> anonymous/private mapping, require PROCESS__EXECMEM since the process
> is essentially executing from the mapping, albeit in a roundabout way.
> 
> Note, FILE__READ and FILE__WRITE are intentionally not required even if
> the source page is backed by a regular file.  Writes to the enclave page
> are contained to the EPC, i.e. never hit the original file, and read
> permissions have already been vetted (or the VMA doesn't have PROT_READ,
> in which case loading the page into the enclave will fail).
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>   security/selinux/hooks.c | 85 ++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 85 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 3ec702cf46ca..f436a055dda7 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6726,6 +6726,87 @@ static void selinux_bpf_prog_free(struct bpf_prog_aux *aux)
>   }
>   #endif
>   
> +#ifdef CONFIG_INTEL_SGX
> +int selinux_enclave_load(struct vm_area_struct *vma, unsigned long prot,
> +			 unsigned long *allowed_prot)
> +{
> +	const struct cred *cred = current_cred();
> +	u32 sid = cred_sid(cred);
> +	int rc;
> +
> +	/* SGX is supported only in 64-bit kernels. */
> +	WARN_ON_ONCE(!default_noexec);
> +
> +	/*
> +	 * SGX is responsible for checking @prot vs @allowed_prot, and SELinux
> +	 * only cares about execute related permissions for enclaves.
> +	 */
> +	if (!(*allowed_prot & PROT_EXEC))
> +		return 0;
> +
> +	/*
> +	 * Loading an executable enclave page from a VMA that is not executable
> +	 * itself requires EXECUTE permissions on the source file, or if there
> +	 * is no regular source file, EXECMEM since the page is being loaded
> +	 * from a non-executable anonymous mapping.
> +	 */
> +	if (!(vma->vm_flags & VM_EXEC)) {
> +		if (vma->vm_file && !IS_PRIVATE(file_inode(vma->vm_file)))
> +			rc = file_has_perm(cred, vma->vm_file, FILE__EXECUTE);

We might need an EXECMOD check here as well if (vma->vm_file && 
vma->anon_vma).  The scenario would be that the host application mapped 
the file with PROT_WRITE, modified it, but haven't mapped it PROT_EXEC. 
Now the enclave loader requests PROT_EXEC without PROT_WRITE or allows 
it.  FILE__EXECUTE is insufficient for this case.

> +		else
> +			rc = avc_has_perm(&selinux_state,
> +					  sid, sid, SECCLASS_PROCESS,
> +					  PROCESS__EXECMEM, NULL);

These calls will audit FILE__EXECUTE or PROCESS__EXECMEM denials even 
when userspace never asked for PROT_EXEC. Possibly we should use 
avc_has_perm_noaudit() and only call avc_audit() if (prot & PROT_EXEC)? 
And similarly introduce file_has_perm_noaudit() -> 
inode_has_perm_noaudit() -> avc_has_perm_noaudit() or inline here and 
switch to avc_has_perm_noaudit() throughout?

> +
> +		/*
> +		 * Reject the load if the enclave *needs* the page to be
> +		 * executable, otherwise prevent it from becoming executable.
> +		 */
> +		if (rc) {
> +			if (prot & PROT_EXEC)
> +				return rc;
> +
> +			*allowed_prot &= ~PROT_EXEC;
> +		}
> +	}
> +
> +	/*
> +	 * An enclave page that may do RW->RX or W+X requires EXECMOD (backed
> +	 * by a regular file) or EXECMEM (loaded from an anonymous mapping).

At present EXECMEM is also triggered for W+X private file mappings, to 
allow denying W+X while permitting exceptions for W->X for text relocations.

> +	 * Note, this hybrid EXECMOD and EXECMEM behavior is intentional and
> +	 * reflects the nature of enclaves and the EPC, e.g. EPC is effectively
> +	 * a non-persistent shared file, but each enclave is a private domain
> +	 * within that shared file, so delegate to the source of the enclave.
> +	 */
> +	if ((*allowed_prot & PROT_EXEC) && (*allowed_prot & PROT_WRITE)) {
> +		if (vma->vm_file && !IS_PRIVATE(file_inode(vma->vm_file)))
> +			rc = file_has_perm(cred, vma->vm_file, FILE__EXECMOD);
> +		else
> +			rc = avc_has_perm(&selinux_state,
> +					  sid, sid, SECCLASS_PROCESS,
> +					  PROCESS__EXECMEM, NULL);

Same issue wrt auditing here.  Could also potentially skip the EXECMEM 
check this time if we performed it above (if so, then we must have 
passed it because *allowed_prot still had PROT_EXEC set).

> +		/*
> +		 * Clear ALLOW_EXEC instead of ALLOWED_WRITE if permissions are
> +		 * lacking and @prot has neither PROT_WRITE or PROT_EXEC.  If
> +		 * userspace wanted RX they would have requested RX, and due to
> +		 * lack of permissions they can never get RW->RX, i.e. the only
> +		 * useful transition is R->RW.
> +		 */
> +		if (rc) {
> +			if ((prot & PROT_EXEC) && (prot & PROT_WRITE))
> +				return rc;
> +
> +			if (prot & PROT_EXEC)
> +				*allowed_prot &= ~PROT_WRITE;
> +			else
> +				*allowed_prot &= ~PROT_EXEC;
> +		}
> +	}
> +
> +	return 0;
> +}
> +#endif
> +
>   struct lsm_blob_sizes selinux_blob_sizes __lsm_ro_after_init = {
>   	.lbs_cred = sizeof(struct task_security_struct),
>   	.lbs_file = sizeof(struct file_security_struct),
> @@ -6968,6 +7049,10 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
>   	LSM_HOOK_INIT(bpf_map_free_security, selinux_bpf_map_free),
>   	LSM_HOOK_INIT(bpf_prog_free_security, selinux_bpf_prog_free),
>   #endif
> +
> +#ifdef CONFIG_INTEL_SGX
> +	LSM_HOOK_INIT(enclave_load, selinux_enclave_load),
> +#endif
>   };
>   
>   static __init int selinux_init(void)
>