Re: [RFC PATCH 11/23] x86/sgx: Add helpers to expose ECREATE and EINIT to KVM

Dave Hansen
2021-01-06T21:24:37Z

On 1/6/21 1:04 PM, Sean Christopherson wrote:
> On Wed, Jan 06, 2021, Dave Hansen wrote:
>> On 1/5/21 5:56 PM, Kai Huang wrote:
>>> From: Sean Christopherson <sean.j.christopherson@intel.com>
>>>
>>> Provide wrappers around __ecreate() and __einit() to hide the ugliness
>>> of overloading the ENCLS return value to encode multiple error formats
>>> in a single int. KVM will trap-and-execute ECREATE and EINIT as part
>>> of SGX virtualization, and on an exception, KVM needs the trapnr so that
>>> it can inject the correct fault into the guest.
>>
>> This is missing a bit of a step about how and why ECREATE needs to be
>> run in the host in the first place.
>
> There's (hopefully) good info in the KVM usage patch that can be borrowed:
>
> Add an ECREATE handler that will be used to intercept ECREATE for the
> purpose of enforcing and enclave's MISCSELECT, ATTRIBUTES and XFRM, i.e.
> to allow userspace to restrict SGX features via CPUID. ECREATE will be
> intercepted when any of the aforementioned masks diverges from hardware
> in order to enforce the desired CPUID model, i.e. inject #GP if the
> guest attempts to set a bit that hasn't been enumerated as allowed-1 in
> CPUID.

OK, so in plain language: the bare-metal kernel must intercept ECREATE
to be able to impose policies on guests. When it does this, the
bare-metal kernel runs ECREATE against the userspace mapping of the
virtualized EPC.

>>> diff --git a/arch/x86/include/asm/sgx.h b/arch/x86/include/asm/sgx.h
>>> new file mode 100644
>>> index 000000000000..0d643b985085
>>> --- /dev/null
>>> +++ b/arch/x86/include/asm/sgx.h
>>> @@ -0,0 +1,16 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +#ifndef _ASM_X86_SGX_H
>>> +#define _ASM_X86_SGX_H
>>> +
>>> +#include <linux/types.h>
>>> +
>>> +#ifdef CONFIG_X86_SGX_VIRTUALIZATION
>>> +struct sgx_pageinfo;
>>> +
>>> +int sgx_virt_ecreate(struct sgx_pageinfo *pageinfo, void __user *secs,
>>> + int *trapnr);
>>> +int sgx_virt_einit(void __user *sigstruct, void __user *token,
>>> + void __user *secs, u64 *lepubkeyhash, int *trapnr);
>>> +#endif
>>> +
>>> +#endif /* _ASM_X86_SGX_H */
>>> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
>>> index d625551ccf25..4e9810ba9259 100644
>>> --- a/arch/x86/kernel/cpu/sgx/virt.c
>>> +++ b/arch/x86/kernel/cpu/sgx/virt.c
>>> @@ -261,3 +261,58 @@ int __init sgx_virt_epc_init(void)
>>>
>>> return misc_register(&sgx_virt_epc_dev);
>>> }
>>> +
>>> +int sgx_virt_ecreate(struct sgx_pageinfo *pageinfo, void __user *secs,
>>> + int *trapnr)
>>> +{
>>> + int ret;
>>> +
>>> + __uaccess_begin();
>>> + ret = __ecreate(pageinfo, (void *)secs);
>>> + __uaccess_end();
>>
>> The __uaccess_begin/end() worries me. There are *very* few of these in
>> the kernel and it seems like something we want to use as sparingly as
>> possible.
>>
>> Why don't we just use the kernel mapping for 'secs' and not have to deal
>> with stac/clac?
>
> The kernel mapping isn't readily available.

Oh, duh. There's no kernel mapping for EPC... it's not RAM in the first
place.

> At this point, it's not even
> guaranteed that @secs points at an EPC page. Unlike the driver code, where the
> EPC page is allocated on-demand by the kernel, the pointer here is userspace
> (technically guest) controlled. The caller (KVM) is responsible for ensuring
> it's a valid userspace address, but the SGX/EPC specific checks are mostly
> deferred to hardware.

Ahh, got it. Kai, could we get some of this into comments or the changelog?

>> I'm also just generally worried about casting away an __user without
>> doing any checking. How is that OK?
>
> Short answer, KVM validates the virtual addresses.
>
> KVM validates the host virtual addresses (HVA) when creating a memslot (maps
> GPA->HVA). The HVAs that are passed to these helpers are generated/retrieved
> by KVM translating GVA->GPA->HVA; the GPA->HVA stage ensures the address is in a
> valid memslot, and thus a valid user address.

There is something a *bit* unpalatable about having KVM fill an
'unsigned long' only to cast it to a (void __user *), the to cast it
back to a (void *) to pass it to the SGX inlines.

I guess sparse would catch us in the window that it is __user if someone
tried to dereference it.

Adding access_ok()'s sounds like a good idea to me. Or, at *least*
commenting why they're not necessary.