linux-kernel - Re: [PATCH v2 2/2] x86: sgx_vepc: implement SGX_IOC_VEPC

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Date:   Fri, 15 Oct 2021 22:29:46 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        dave.hansen@...ux.intel.com, x86@...nel.org, yang.zhong@...el.com,
        jarkko@...nel.org
Subject: Re: [PATCH v2 2/2] x86: sgx_vepc: implement SGX_IOC_VEPC_REMOVE ioctl

On Tue, Oct 12, 2021, Paolo Bonzini wrote:
> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
> index 59cdf3f742ac..81a0a0f22007 100644
> --- a/arch/x86/kernel/cpu/sgx/virt.c
> +++ b/arch/x86/kernel/cpu/sgx/virt.c
> @@ -150,6 +150,46 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
>  	return 0;
>  }
>  
> +static long sgx_vepc_remove_all(struct sgx_vepc *vepc)
> +{
> +	struct sgx_epc_page *entry;
> +	unsigned long index;
> +	long failures = 0;
> +
> +	xa_for_each(&vepc->page_array, index, entry) {

Might be worth a comment that xa_for_each() is safe to use concurrently with
xa_load/xa_store, i.e. this doesn't need to take vepc->lock.  It does raise the
question of whether or not the kernel is responsible for providing deterministic
results if userspace/guest is accessing previously-unallocated pages.

> +		int ret = sgx_vepc_remove_page(entry);

I don't see anything that prevents userspace from doing SGX_IOC_VEPC_REMOVE_ALL
on multiple threads with the same vEPC.  That means userspace can induce a #GP
due to concurrent access.  Taking vepc->lock would solve that particular problem,
but I think that's a moot point because the EREMOVE locking rules are relative to
the SECS, not the individual page (because of refcounting).  SGX_IOC_VEPC_REMOVE_ALL
on any two arbitrary vEPCs could induce a fault if they have children belonging to
the same enclave, i.e. share an SECS.

Sadly, I think this needs to be:

		if (ret == SGX_CHILD_PRESENT)
			failures++;
		else if (ret)
			return -EBUSY;

> +		switch (ret) {
> +		case 0:
> +			break;
> +
> +		case SGX_CHILD_PRESENT:
> +			failures++;
> +			break;
> +
> +		case SGX_ENCLAVE_ACT:
> +			/*
> +			 * Unlike in sgx_vepc_free_page, userspace could be calling
> +			 * the ioctl while logical processors are running in the
> +			 * enclave; do not warn.
> +			 */
> +			return -EBUSY;
> +
> +		default:
> +			WARN_ONCE(1, EREMOVE_ERROR_MESSAGE, ret, ret);
> +			failures++;
> +			break;
> +		}
> +		cond_resched();
> +	}
> +
> +	/*
> +	 * Return the number of pages that failed to be removed, so
> +	 * userspace knows that there are still SECS pages lying
> +	 * around.
> +	 */
> +	return failures;
> +}