[RFC PATCH v4 08/12] security/selinux: Require SGX_MAPWX to map enclave page WX

[Stephen Smalley]

On 6/19/19 6:23 PM, Sean Christopherson wrote:
> Hook enclave_map() to require a new per-process capability, SGX_MAPWX,
> when mapping an enclave as simultaneously writable and executable.
> Note, @prot contains the actual protection bits that will be set by the
> kernel, not the maximal protection bits specified by userspace when the
> page was first loaded into the enclave.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson at intel.com>
> ---
>   security/selinux/hooks.c            | 21 +++++++++++++++++++++
>   security/selinux/include/classmap.h |  3 ++-
>   2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 3ec702cf46ca..fc239e541b62 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6726,6 +6726,23 @@ static void selinux_bpf_prog_free(struct bpf_prog_aux *aux)
>   }
>   #endif
>   
> +#ifdef CONFIG_INTEL_SGX
> +static int selinux_enclave_map(unsigned long prot)
> +{
> +	const struct cred *cred = current_cred();
> +	u32 sid = cred_sid(cred);
> +
> +	/* SGX is supported only in 64-bit kernels. */
> +	WARN_ON_ONCE(!default_noexec);
> +
> +	if ((prot & PROT_EXEC) && (prot & PROT_WRITE))
> +		return avc_has_perm(&selinux_state, sid, sid,
> +				    SECCLASS_PROCESS2, PROCESS2__SGX_MAPWX,
> +				    NULL);

Possibly we should use a slightly more general name for the permission 
to allow reusing it in the future if/when another architecture 
introduces a similar construct under a different branding?  ENCLAVE_* 
seems slightly more generic than SGX_*.

I was interested in testing this code but sadly the driver reports the 
following on my development workstation:

[    1.644191] sgx: The launch control MSRs are not writable
[    1.695477] sgx: EPC section 0x70200000-0x75f7ffff
[    1.771760] sgx: The public key MSRs are not writable

I guess I'm out of luck until/unless I get a NUC or server class 
hardware that supports flexible launch control?  Seems developer unfriendly.

> +	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 +6985,10 @@ 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_map, selinux_enclave_map),
> +#endif
>   };
>   
>   static __init int selinux_init(void)
> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> index 201f7e588a29..cfd91e879bdf 100644
> --- a/security/selinux/include/classmap.h
> +++ b/security/selinux/include/classmap.h
> @@ -51,7 +51,8 @@ struct security_class_mapping secclass_map[] = {
>   	    "execmem", "execstack", "execheap", "setkeycreate",
>   	    "setsockcreate", "getrlimit", NULL } },
>   	{ "process2",
> -	  { "nnp_transition", "nosuid_transition", NULL } },
> +	  { "nnp_transition", "nosuid_transition",
> +	    "sgx_mapwx", NULL } },
>   	{ "system",
>   	  { "ipc_info", "syslog_read", "syslog_mod",
>   	    "syslog_console", "module_request", "module_load", NULL } },
>