Re: [PATCH] x86/sgx: Suppress WARN on inability to sanitize EPC if ksgxd is stopped

Borislav Petkov
2021-06-17T16:45:59Z

On Tue, Jun 15, 2021 at 05:44:58PM -0700, Sean Christopherson wrote:
> Don't WARN on having unsanitized EPC pages if ksgxd is stopped early,
> e.g. if sgx_init() realizes there will be no downstream consumers of EPC.
> If ksgxd is stopped early, EPC pages may be left on the dirty list, but
> that's ok because ksgxd is only stopped if SGX initialization failed or
> if the kernel is going down. In either case, the EPC won't be used.
>
> This bug was exposed by the addition of KVM support, but has existed and
> was hittable since the original sanitization code was added. Prior to
> adding KVM support, if Launch Control was not fully enabled, e.g. when
> running on older hardware, sgx_init() bailed immediately before spawning
> ksgxd because X86_FEATURE_SGX was cleared if X86_FEATURE_SGX_LC was
> unsupported.
>
> With KVM support, sgx_drv_init() handles the X86_FEATURE_SGX_LC check
> manually, so now there's any easy-to-hit case where sgx_init() will spawn
> ksgxd and _then_ fail to initialize, which results in sgx_init() stopping
> ksgxd before it finishes sanitizing the EPC.
>
> Prior to KVM support, the bug was much harder to hit because it basically
> required char device registration to fail.
>
> Reported-by: Du Cheng <ducheng2@gmail.com>
> Fixes: e7e0545299d8 ("x86/sgx: Initialize metadata for Enclave Page Cache (EPC) sections")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>
> Lightly tested due to lack of hardware. I hacked the flow to verify that
> stopping early will leave work pending, and that rechecking should_stop()
> suppress the resulting WARN.
>
> arch/x86/kernel/cpu/sgx/main.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index ad904747419e..fbad2b9625a5 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -425,7 +425,7 @@ static int ksgxd(void *p)
> __sgx_sanitize_pages(&sgx_dirty_page_list);
>
> /* sanity check: */
> - WARN_ON(!list_empty(&sgx_dirty_page_list));
> + WARN_ON(!list_empty(&sgx_dirty_page_list) && !kthread_should_stop());
>
> while (!kthread_should_stop()) {
> if (try_to_freeze())
> --

Hmm, this looks weird. Why aren't we starting ksgxd only after
*everything* has initialized successfully? I.e., after both kvm and
native drivers' init functions have succeeded?

Then you won't have to do this kthread_should_stop() thing after the
fact.

Btw, you have the same thing in the while loop's termination condition
two lines down which, if I have to look at it later, would make me
scratch head as to what TH is going on here.

Thx.

--
Regards/Gruss,
Boris.