Re: [RFC PATCH 05/23] x86/cpu/intel: Allow SGX virtualization without Launch Control support

Dave Hansen
2021-01-06T19:56:00Z

On 1/5/21 5:55 PM, Kai Huang wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Allow SGX virtualization on systems without Launch Control support, i.e.
> allow KVM to expose SGX to guests that support non-LC configurations.

Context, please.

The kernel will currently disable all SGX support if the hardware does
not support launch control. Make it more permissive to allow SGX
virtualization on systems without Launch Control support. This will
allow KVM to expose SGX to guests that have less-strict requirements on
the availability of flexible launch control.

> Introduce clear_sgx_lc() to clear SGX_LC feature bit only if SGX Launch
> Control is locked by BIOS when SGX virtualization is enabled, to prevent
> SGX driver being enabled.

This is another run-on, and it makes it really hard to figure out what
it is trying to say.

> Improve error message to distinguish three cases: 1) SGX disabled
> completely by BIOS; 2) SGX disabled completely due to SGX LC is locked
> by BIOS, and SGX virtualization is also disabled; 3) Only SGX driver is
> disabled due to SGX LC is locked by BIOS, but SGX virtualization is
> enabled.

Editing for grammar and clarity again...

Improve error message to distinguish between three cases. There are two
cases where SGX support is completely disabled:
1) SGX has been disabled completely by the BIOS
2) SGX LC is locked by the BIOS. Bare-metal support is disabled because
of LC unavailability. SGX virtualization is unavailable (because of
Kconfig).
One where it is partially available:
3) SGX LC is locked by the BIOS. Bare-metal support is disabled because
of LC unavailability. SGX virtualization is supported.

> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Co-developed-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
> arch/x86/kernel/cpu/feat_ctl.c | 48 +++++++++++++++++++++++++---------
> 1 file changed, 36 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/feat_ctl.c b/arch/x86/kernel/cpu/feat_ctl.c
> index 4fcd57fdc682..b07452b68538 100644
> --- a/arch/x86/kernel/cpu/feat_ctl.c
> +++ b/arch/x86/kernel/cpu/feat_ctl.c
> @@ -101,6 +101,11 @@ static void clear_sgx_caps(void)
> setup_clear_cpu_cap(X86_FEATURE_SGX2);
> }
>
> +static void clear_sgx_lc(void)
> +{
> + setup_clear_cpu_cap(X86_FEATURE_SGX_LC);
> +}
> +
> static int __init nosgx(char *str)
> {
> clear_sgx_caps();
> @@ -113,7 +118,7 @@ early_param("nosgx", nosgx);
> void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> {
> bool tboot = tboot_enabled();
> - bool enable_sgx;
> + bool enable_sgx_virt, enable_sgx_driver;
> u64 msr;
>
> if (rdmsrl_safe(MSR_IA32_FEAT_CTL, &msr)) {
> @@ -123,12 +128,19 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> }
>
> /*
> - * Enable SGX if and only if the kernel supports SGX and Launch Control
> - * is supported, i.e. disable SGX if the LE hash MSRs can't be written.
> + * Enable SGX if and only if the kernel supports SGX. Require Launch
> + * Control support if SGX virtualization is *not* supported, i.e.
> + * disable SGX if the LE hash MSRs can't be written and SGX can't be
> + * exposed to a KVM guest (which might support non-LC configurations).
> */
> - enable_sgx = cpu_has(c, X86_FEATURE_SGX) &&
> - cpu_has(c, X86_FEATURE_SGX_LC) &&
> - IS_ENABLED(CONFIG_X86_SGX);
> + enable_sgx_driver = cpu_has(c, X86_FEATURE_SGX) &&
> + cpu_has(c, X86_FEATURE_SGX1) &&
> + IS_ENABLED(CONFIG_X86_SGX) &&
> + cpu_has(c, X86_FEATURE_SGX_LC);
> + enable_sgx_virt = cpu_has(c, X86_FEATURE_SGX) &&
> + cpu_has(c, X86_FEATURE_SGX1) &&
> + IS_ENABLED(CONFIG_X86_SGX) &&
> + IS_ENABLED(CONFIG_X86_SGX_VIRTUALIZATION);

Don't we also need some runtime checks here? What if we boot on
hardware that doesn't support KVM?

> if (msr & FEAT_CTL_LOCKED)
> goto update_caps;
> @@ -151,8 +163,11 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> msr |= FEAT_CTL_VMX_ENABLED_INSIDE_SMX;
> }
>
> - if (enable_sgx)
> - msr |= FEAT_CTL_SGX_ENABLED | FEAT_CTL_SGX_LC_ENABLED;
> + if (enable_sgx_driver || enable_sgx_virt) {
> + msr |= FEAT_CTL_SGX_ENABLED;
> + if (enable_sgx_driver)
> + msr |= FEAT_CTL_SGX_LC_ENABLED;
> + }
>
> wrmsrl(MSR_IA32_FEAT_CTL, msr);
>
> @@ -175,10 +190,19 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> }
>
> update_sgx:
> - if (!(msr & FEAT_CTL_SGX_ENABLED) ||
> - !(msr & FEAT_CTL_SGX_LC_ENABLED) || !enable_sgx) {
> - if (enable_sgx)
> - pr_err_once("SGX disabled by BIOS\n");
> + if (!(msr & FEAT_CTL_SGX_ENABLED)) {
> + if (enable_sgx_driver || enable_sgx_virt)
> + pr_err_once("SGX disabled by BIOS.\n");
> clear_sgx_caps();
> }
> + if (!(msr & FEAT_CTL_SGX_LC_ENABLED) &&
> + (enable_sgx_driver || enable_sgx_virt)) {
> + if (!enable_sgx_virt) {
> + pr_err_once("SGX Launch Control is locked. Disable SGX.\n");
> + clear_sgx_caps();
> + } else if (enable_sgx_driver) {
> + pr_err_once("SGX Launch Control is locked. Disable SGX driver.\n");

Should we have an explicit message for enabling virtualization? I'm not
sure how many people will understand that "SGX driver" actually doesn't
mean /dev/sgx_epc_virt.

> + clear_sgx_lc();
> + }
> + }
> }
>