Re: [RFC PATCH 03/23] x86/sgx: Introduce virtual EPC for use by KVM guests

[Kai Huang <kai.huang@intel.com>]

On Tue, 12 Jan 2021 01:38:23 +0200 Jarkko Sakkinen wrote:
> On Wed, Jan 06, 2021 at 02:55:20PM +1300, Kai Huang wrote:
> > From: Sean Christopherson <sean.j.christopherson@intel.com>
> > 
> > Add a misc device /dev/sgx_virt_epc to allow userspace to allocate "raw"
> > EPC without an associated enclave.  The intended and only known use case
> > for raw EPC allocation is to expose EPC to a KVM guest, hence the
> > virt_epc moniker, virt.{c,h} files and X86_SGX_VIRTUALIZATION Kconfig.
> > 
> > Modify sgx_init() to always try to initialize virtual EPC driver, even
> > when SGX driver is disabled due to SGX Launch Control is in locked mode,
> > or not present at all, since SGX virtualization allows to expose SGX to
> > guests that support non-LC configurations.
> > 
> > Implement the "raw" EPC allocation in the x86 core-SGX subsystem via
> > /dev/sgx_virt_epc rather than in KVM. Doing so has two major advantages:
> > 
> >   - Does not require changes to KVM's uAPI, e.g. EPC gets handled as
> >     just another memory backend for guests.
> > 
> >   - EPC management is wholly contained in the SGX subsystem, e.g. SGX
> >     does not have to export any symbols, changes to reclaim flows don't
> >     need to be routed through KVM, SGX's dirty laundry doesn't have to
> >     get aired out for the world to see, and so on and so forth.
> > 
> > The virtual EPC allocated to guests is currently not reclaimable, due to
> > oversubscription of EPC for KVM guests is not currently supported. Due
> > to the complications of handling reclaim conflicts between guest and
> > host, KVM EPC oversubscription is significantly more complex than basic
> > support for SGX virtualization.
> > 
> > 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>
> 
> The commit message does not describe the code changes. It should
> have an understandable explanation of fops. There is nothing about
> the implementation right now.

Thanks for feedback. Does "understabdable explanation of fops" mean I
should add one sentence to say, for instance: "userspace hypervisor should open
the /dev/sgx_virt_epc, use mmap() to get a valid address range, and then use
that address range to create KVM memory region"?

Or should I include an example of how to use /dev/sgx_virt_epc in userspace, for
instance, below?

	fd = open("/dev/sgx_virt_epc", O_RDWR);
	void *addr = mmap(NULL, size, ..., fd);
	/* userspace hypervisor uses addr, size to create KVM memory slot */
	...

I dug the SGX driver side to understand what should I add, but in below commit I
don't see description of fops either:

	commit 3fe0778edac8628637e2fd23835996523b1a3372
	Author: Jarkko Sakkinen <jarkko@kernel.org>
	Date:   Fri Nov 13 00:01:22 2020 +0200

    	    x86/sgx: Add an SGX misc driver interface


> 
> /Jarkko
> 
> > ---
> >  arch/x86/Kconfig                 |  12 ++
> >  arch/x86/kernel/cpu/sgx/Makefile |   1 +
> >  arch/x86/kernel/cpu/sgx/main.c   |   5 +-
> >  arch/x86/kernel/cpu/sgx/virt.c   | 263 +++++++++++++++++++++++++++++++
> >  arch/x86/kernel/cpu/sgx/virt.h   |  14 ++
> >  5 files changed, 294 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/x86/kernel/cpu/sgx/virt.c
> >  create mode 100644 arch/x86/kernel/cpu/sgx/virt.h
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 618d1aabccb8..a7318175509b 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -1947,6 +1947,18 @@ config X86_SGX
> >  
> >  	  If unsure, say N.
> >  
> > +config X86_SGX_VIRTUALIZATION
> > +	bool "Software Guard eXtensions (SGX) Virtualization"
> > +	depends on X86_SGX && KVM_INTEL
> > +	help
> > +
> > +	  Enables KVM guests to create SGX enclaves.
> > +
> > +	  This includes support to expose "raw" unreclaimable enclave memory to
> > +	  guests via a device node, e.g. /dev/sgx_virt_epc.
> > +
> > +	  If unsure, say N.
> > +
> >  config EFI
> >  	bool "EFI runtime service support"
> >  	depends on ACPI
> > diff --git a/arch/x86/kernel/cpu/sgx/Makefile b/arch/x86/kernel/cpu/sgx/Makefile
> > index 91d3dc784a29..7a25bf63adfb 100644
> > --- a/arch/x86/kernel/cpu/sgx/Makefile
> > +++ b/arch/x86/kernel/cpu/sgx/Makefile
> > @@ -3,3 +3,4 @@ obj-y += \
> >  	encl.o \
> >  	ioctl.o \
> >  	main.o
> > +obj-$(CONFIG_X86_SGX_VIRTUALIZATION)	+= virt.o
> > diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> > index 95aad183bb65..02993a327a1f 100644
> > --- a/arch/x86/kernel/cpu/sgx/main.c
> > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > @@ -9,9 +9,11 @@
> >  #include <linux/sched/mm.h>
> >  #include <linux/sched/signal.h>
> >  #include <linux/slab.h>
> > +#include "arch.h"
> >  #include "driver.h"
> >  #include "encl.h"
> >  #include "encls.h"
> > +#include "virt.h"
> >  
> >  struct sgx_epc_section sgx_epc_sections[SGX_MAX_EPC_SECTIONS];
> >  static int sgx_nr_epc_sections;
> > @@ -726,7 +728,8 @@ static void __init sgx_init(void)
> >  	if (!sgx_page_reclaimer_init())
> >  		goto err_page_cache;
> >  
> > -	ret = sgx_drv_init();
> > +	/* Success if the native *or* virtual EPC driver initialized cleanly. */
> > +	ret = !!sgx_drv_init() & !!sgx_virt_epc_init();
> >  	if (ret)
> >  		goto err_kthread;
> >  
> > diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
> > new file mode 100644
> > index 000000000000..d625551ccf25
> > --- /dev/null
> > +++ b/arch/x86/kernel/cpu/sgx/virt.c
> > @@ -0,0 +1,263 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*  Copyright(c) 2016-20 Intel Corporation. */
> > +
> > +#include <linux/miscdevice.h>
> > +#include <linux/mm.h>
> > +#include <linux/mman.h>
> > +#include <linux/sched/mm.h>
> > +#include <linux/sched/signal.h>
> > +#include <linux/slab.h>
> > +#include <linux/xarray.h>
> > +#include <asm/sgx.h>
> > +#include <uapi/asm/sgx.h>
> > +
> > +#include "encls.h"
> > +#include "sgx.h"
> > +#include "virt.h"
> > +
> > +struct sgx_virt_epc {
> > +	struct xarray page_array;
> > +	struct mutex lock;
> > +	struct mm_struct *mm;
> > +};
> > +
> > +static struct mutex virt_epc_lock;
> > +static struct list_head virt_epc_zombie_pages;
> > +
> > +static int __sgx_virt_epc_fault(struct sgx_virt_epc *epc,
> > +				struct vm_area_struct *vma, unsigned long addr)
> > +{
> > +	struct sgx_epc_page *epc_page;
> > +	unsigned long index, pfn;
> > +	int ret;
> > +
> > +	/* epc->lock must already have been hold */
> > +
> > +	/* Calculate index of EPC page in virtual EPC's page_array */
> > +	index = vma->vm_pgoff + PFN_DOWN(addr - vma->vm_start);
> > +
> > +	epc_page = xa_load(&epc->page_array, index);
> > +	if (epc_page)
> > +		return 0;
> > +
> > +	epc_page = sgx_alloc_epc_page(epc, false);
> > +	if (IS_ERR(epc_page))
> > +		return PTR_ERR(epc_page);
> > +
> > +	ret = xa_err(xa_store(&epc->page_array, index, epc_page, GFP_KERNEL));
> > +	if (ret)
> > +		goto err_free;
> > +
> > +	pfn = PFN_DOWN(sgx_get_epc_phys_addr(epc_page));
> > +
> > +	ret = vmf_insert_pfn(vma, addr, pfn);
> > +	if (ret != VM_FAULT_NOPAGE) {
> > +		ret = -EFAULT;
> > +		goto err_delete;
> > +	}
> > +
> > +	return 0;
> > +
> > +err_delete:
> > +	xa_erase(&epc->page_array, index);
> > +err_free:
> > +	sgx_free_epc_page(epc_page);
> > +	return ret;
> > +}
> > +
> > +static vm_fault_t sgx_virt_epc_fault(struct vm_fault *vmf)
> > +{
> > +	struct vm_area_struct *vma = vmf->vma;
> > +	struct sgx_virt_epc *epc = vma->vm_private_data;
> > +	int ret;
> > +
> > +	mutex_lock(&epc->lock);
> > +	ret = __sgx_virt_epc_fault(epc, vma, vmf->address);
> > +	mutex_unlock(&epc->lock);
> > +
> > +	if (!ret)
> > +		return VM_FAULT_NOPAGE;
> > +
> > +	if (ret == -EBUSY && (vmf->flags & FAULT_FLAG_ALLOW_RETRY)) {
> > +		mmap_read_unlock(vma->vm_mm);
> > +		return VM_FAULT_RETRY;
> > +	}
> > +
> > +	return VM_FAULT_SIGBUS;
> > +}
> > +
> > +const struct vm_operations_struct sgx_virt_epc_vm_ops = {
> > +	.fault = sgx_virt_epc_fault,
> > +};
> > +
> > +static int sgx_virt_epc_mmap(struct file *file, struct vm_area_struct *vma)
> > +{
> > +	struct sgx_virt_epc *epc = file->private_data;
> > +
> > +	if (!(vma->vm_flags & VM_SHARED))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * Don't allow mmap() from child after fork(), since child and parent
> > +	 * cannot map to the same EPC.
> > +	 */
> > +	if (vma->vm_mm != epc->mm)
> > +		return -EINVAL;
> > +
> > +	vma->vm_ops = &sgx_virt_epc_vm_ops;
> > +	/* Don't copy VMA in fork() */
> > +	vma->vm_flags |= VM_PFNMAP | VM_IO | VM_DONTDUMP | VM_DONTCOPY;
> > +	vma->vm_private_data = file->private_data;
> > +
> > +	return 0;
> > +}
> > +
> > +static int sgx_virt_epc_free_page(struct sgx_epc_page *epc_page)
> > +{
> > +	int ret;
> > +
> > +	if (!epc_page)
> > +		return 0;
> > +
> > +	/*
> > +	 * Explicitly EREMOVE virtual EPC page. Virtual EPC is only used by
> > +	 * guest, and in normal condition guest should have done EREMOVE for
> > +	 * all EPC pages before they are freed here. But it's possible guest
> > +	 * is killed or crashed unnormally in which case EREMOVE has not been
> > +	 * done. Do EREMOVE unconditionally here to cover both cases, because
> > +	 * it's not possible to tell whether guest has done EREMOVE, since
> > +	 * virtual EPC page status is not tracked. And it is fine to EREMOVE
> > +	 * EPC page multiple times.
> > +	 */
> > +	ret = __eremove(sgx_get_epc_virt_addr(epc_page));
> > +	if (ret) {
> > +		/*
> > +		 * Only SGX_CHILD_PRESENT is expected, which is because of
> > +		 * EREMOVE-ing an SECS still with child, in which case it can
> > +		 * be handled by EREMOVE-ing the SECS again after all pages in
> > +		 * virtual EPC have been EREMOVE-ed. See comments in below in
> > +		 * sgx_virt_epc_release().
> > +		 */
> > +		WARN_ON_ONCE(ret != SGX_CHILD_PRESENT);
> > +		return ret;
> > +	}
> > +
> > +	__sgx_free_epc_page(epc_page);
> > +	return 0;
> > +}
> > +
> > +static int sgx_virt_epc_release(struct inode *inode, struct file *file)
> > +{
> > +	struct sgx_virt_epc *epc = file->private_data;
> > +	struct sgx_epc_page *epc_page, *tmp, *entry;
> > +	unsigned long index;
> > +
> > +	LIST_HEAD(secs_pages);
> > +
> > +	mmdrop(epc->mm);
> > +
> > +	xa_for_each(&epc->page_array, index, entry) {
> > +		/*
> > +		 * Virtual EPC pages are not tracked, so it's possible for
> > +		 * EREMOVE to fail due to, e.g. a SECS page still has children
> > +		 * if guest was shutdown unexpectedly. If it is the case, leave
> > +		 * it in the xarray and retry EREMOVE below later.
> > +		 */
> > +		if (sgx_virt_epc_free_page(entry))
> > +			continue;
> > +
> > +		xa_erase(&epc->page_array, index);
> > +	}
> > +
> > +	/*
> > +	 * Retry all failed pages after iterating through the entire tree, at
> > +	 * which point all children should be removed and the SECS pages can be
> > +	 * nuked as well...unless userspace has exposed multiple instance of
> > +	 * virtual EPC to a single VM.
> > +	 */
> > +	xa_for_each(&epc->page_array, index, entry) {
> > +		epc_page = entry;
> > +		/*
> > +		 * Error here means that EREMOVE failed due to a SECS page
> > +		 * still has child on *another* EPC instance.  Put it to a
> > +		 * temporary SECS list which will be spliced to 'zombie page
> > +		 * list' and will be EREMOVE-ed again when freeing another
> > +		 * virtual EPC instance.
> > +		 */
> > +		if (sgx_virt_epc_free_page(epc_page))
> > +			list_add_tail(&epc_page->list, &secs_pages);
> > +
> > +		xa_erase(&epc->page_array, index);
> > +	}
> > +
> > +	/*
> > +	 * Third time's a charm.  Try to EREMOVE zombie SECS pages from virtual
> > +	 * EPC instances that were previously released, i.e. free SECS pages
> > +	 * that were in limbo due to having children in *this* EPC instance.
> > +	 */
> > +	mutex_lock(&virt_epc_lock);
> > +	list_for_each_entry_safe(epc_page, tmp, &virt_epc_zombie_pages, list) {
> > +		/*
> > +		 * Speculatively remove the page from the list of zombies, if
> > +		 * the page is successfully EREMOVE it will be added to the
> > +		 * list of free pages.  If EREMOVE fails, throw the page on the
> > +		 * local list, which will be spliced on at the end.
> > +		 */
> > +		list_del(&epc_page->list);
> > +
> > +		if (sgx_virt_epc_free_page(epc_page))
> > +			list_add_tail(&epc_page->list, &secs_pages);
> > +	}
> > +
> > +	if (!list_empty(&secs_pages))
> > +		list_splice_tail(&secs_pages, &virt_epc_zombie_pages);
> > +	mutex_unlock(&virt_epc_lock);
> > +
> > +	kfree(epc);
> > +
> > +	return 0;
> > +}
> > +
> > +static int sgx_virt_epc_open(struct inode *inode, struct file *file)
> > +{
> > +	struct sgx_virt_epc *epc;
> > +
> > +	epc = kzalloc(sizeof(struct sgx_virt_epc), GFP_KERNEL);
> > +	if (!epc)
> > +		return -ENOMEM;
> > +	/*
> > +	 * Keep the current->mm to virtual EPC. It will be checked in
> > +	 * sgx_virt_epc_mmap() to prevent, in case of fork, child being
> > +	 * able to mmap() to the same virtual EPC pages.
> > +	 */
> > +	mmgrab(current->mm);
> > +	epc->mm = current->mm;
> > +	mutex_init(&epc->lock);
> > +	xa_init(&epc->page_array);
> > +
> > +	file->private_data = epc;
> > +
> > +	return 0;
> > +}
> > +
> > +static const struct file_operations sgx_virt_epc_fops = {
> > +	.owner			= THIS_MODULE,
> > +	.open			= sgx_virt_epc_open,
> > +	.release		= sgx_virt_epc_release,
> > +	.mmap			= sgx_virt_epc_mmap,
> > +};
> > +
> > +static struct miscdevice sgx_virt_epc_dev = {
> > +	.minor = MISC_DYNAMIC_MINOR,
> > +	.name = "sgx_virt_epc",
> > +	.nodename = "sgx_virt_epc",
> > +	.fops = &sgx_virt_epc_fops,
> > +};
> > +
> > +int __init sgx_virt_epc_init(void)
> > +{
> > +	INIT_LIST_HEAD(&virt_epc_zombie_pages);
> > +	mutex_init(&virt_epc_lock);
> > +
> > +	return misc_register(&sgx_virt_epc_dev);
> > +}
> > diff --git a/arch/x86/kernel/cpu/sgx/virt.h b/arch/x86/kernel/cpu/sgx/virt.h
> > new file mode 100644
> > index 000000000000..e5434541a122
> > --- /dev/null
> > +++ b/arch/x86/kernel/cpu/sgx/virt.h
> > @@ -0,0 +1,14 @@
> > +/* SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause) */
> > +#ifndef _ASM_X86_SGX_VIRT_H
> > +#define _ASM_X86_SGX_VIRT_H
> > +
> > +#ifdef CONFIG_X86_SGX_VIRTUALIZATION
> > +int __init sgx_virt_epc_init(void);
> > +#else
> > +static inline int __init sgx_virt_epc_init(void)
> > +{
> > +	return -ENODEV;
> > +}
> > +#endif
> > +
> > +#endif /* _ASM_X86_SGX_VIRT_H */
> > -- 
> > 2.29.2
> > 
> >