Re: [PATCH v4 1/9] x86/sgx: Introduce mechanism to prevent new initializations of EPC pages

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Apr 21, 2022 at 07:03:18PM +0800, Cathy Zhang wrote:
> == Background ==
> 
> EUPDATESVN is a new SGX instruction which allows enclave attestation
> to include information about updated microcode without a reboot.
> 
> The SGX hardware maintains metadata for each enclave page to help
> enforce its security guarantees. This includes things like a record
> of the enclave to which the page belongs and the type of the page:
> SGX metadata like "VA" or "SECS" pages, or regular enclave pages
> like those that store user data.
> 
> Before an EUPDATESVN operation can be successful, all SGX memory (aka.
> EPC) must be marked as "unused" in the SGX hardware metadata (aka,
> EPCM). The SGX microcode now maintains a reference count of pages
> which are unused to aid in determining when all pages reach the
> "unused" state.
> 
> Both bare-metal and KVM guest EPC must be made unused. To increase
> the chance of a successful EUPDATESVN, the kernel prevents existing
> enclaves from creating new, valid pages and prevents new enclave
> creation (creating an enclave involves initializing a "SECS" page).
> 
> The entire EUPDATESVN process is very slow since it potentially
> affects gigabytes of enclave memory. It can potentially take seconds
> or minutes to complete. Userspace may encounter -EBUSY errors during
> the update and is expected to retry.
> 
> == Patch contents ==
> 
> Introduce mechanism to prevent new initializations of EPC pages.
> 
> Use a flag to indicate when SGX EPC pages are "locked", which means
> it's not allowed to allocate new EPC page for use. Check it in all
> paths that can initialize an EPC page. Use SRCU to ensure that the
> flag is visible across the system before proceeding with an update.
> 
> Add checks to all sites that call SGX instructions that can transition
> pages from unused to initialized to ensure that the SRCU lock is held.
> 
> Signed-off-by: Cathy Zhang <cathy.zhang@intel.com>
> 
> ---
> Changes since v3:
>  - Rename lable "out" as "err" in sgx_ioc_enclave_create, update error
>    branch accordingly. (Jarrko Sakkinen)
> 
> Changes since v2:
>  - Move out the SGX2 related change to remove the dependency.
>   (Jarkko Sakkinen, Reinette Chatre).
> ---
>  arch/x86/kernel/cpu/sgx/encls.h |  8 ++++++
>  arch/x86/kernel/cpu/sgx/sgx.h   |  3 +++
>  arch/x86/kernel/cpu/sgx/encl.c  | 19 +++++++++++++++
>  arch/x86/kernel/cpu/sgx/ioctl.c | 43 +++++++++++++++++++++++++++------
>  arch/x86/kernel/cpu/sgx/main.c  | 37 ++++++++++++++++++++++++++++
>  arch/x86/kernel/cpu/sgx/virt.c  | 20 +++++++++++++++
>  6 files changed, 123 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/encls.h b/arch/x86/kernel/cpu/sgx/encls.h
> index fa04a73daf9c..60321c5f5718 100644
> --- a/arch/x86/kernel/cpu/sgx/encls.h
> +++ b/arch/x86/kernel/cpu/sgx/encls.h
> @@ -138,6 +138,8 @@ static inline bool encls_failed(int ret)
>  
>  static inline int __ecreate(struct sgx_pageinfo *pginfo, void *secs)
>  {
> +	lockdep_assert_held(&sgx_lock_epc_srcu);
> +
>  	return __encls_2(ECREATE, pginfo, secs);
>  }
>  
> @@ -148,6 +150,8 @@ static inline int __eextend(void *secs, void *addr)
>  
>  static inline int __eadd(struct sgx_pageinfo *pginfo, void *addr)
>  {
> +	lockdep_assert_held(&sgx_lock_epc_srcu);
> +
>  	return __encls_2(EADD, pginfo, addr);
>  }
>  
> @@ -179,6 +183,8 @@ static inline int __etrack(void *addr)
>  static inline int __eldu(struct sgx_pageinfo *pginfo, void *addr,
>  			 void *va)
>  {
> +	lockdep_assert_held(&sgx_lock_epc_srcu);
> +
>  	return __encls_ret_3(ELDU, pginfo, addr, va);
>  }
>  
> @@ -191,6 +197,8 @@ static inline int __epa(void *addr)
>  {
>  	unsigned long rbx = SGX_PAGE_TYPE_VA;
>  
> +	lockdep_assert_held(&sgx_lock_epc_srcu);
> +
>  	return __encls_2(EPA, rbx, addr);
>  }
>  
> diff --git a/arch/x86/kernel/cpu/sgx/sgx.h b/arch/x86/kernel/cpu/sgx/sgx.h
> index b30cee4de903..d7a1490d90bb 100644
> --- a/arch/x86/kernel/cpu/sgx/sgx.h
> +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> @@ -103,4 +103,7 @@ static inline int __init sgx_vepc_init(void)
>  
>  void sgx_update_lepubkeyhash(u64 *lepubkeyhash);
>  
> +extern struct srcu_struct sgx_lock_epc_srcu;
> +bool sgx_epc_is_locked(void);
> +
>  #endif /* _X86_SGX_H */
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 02ff9ac83985..68c8d65a8dee 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -142,6 +142,7 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
>  	unsigned long phys_addr;
>  	struct sgx_encl *encl;
>  	vm_fault_t ret;
> +	int srcu_idx;
>  
>  	encl = vma->vm_private_data;
>  
> @@ -153,11 +154,18 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
>  	if (unlikely(!encl))
>  		return VM_FAULT_SIGBUS;
>  
> +	srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +	if (sgx_epc_is_locked()) {
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +		return VM_FAULT_SIGBUS;
> +	}
> +
>  	mutex_lock(&encl->lock);
>  
>  	entry = sgx_encl_load_page(encl, addr, vma->vm_flags);
>  	if (IS_ERR(entry)) {
>  		mutex_unlock(&encl->lock);
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
>  
>  		if (PTR_ERR(entry) == -EBUSY)
>  			return VM_FAULT_NOPAGE;
> @@ -170,12 +178,14 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
>  	ret = vmf_insert_pfn(vma, addr, PFN_DOWN(phys_addr));
>  	if (ret != VM_FAULT_NOPAGE) {
>  		mutex_unlock(&encl->lock);
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
>  
>  		return VM_FAULT_SIGBUS;
>  	}
>  
>  	sgx_encl_test_and_clear_young(vma->vm_mm, entry);
>  	mutex_unlock(&encl->lock);
> +	srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
>  
>  	return VM_FAULT_NOPAGE;
>  }
> @@ -323,6 +333,7 @@ static int sgx_vma_access(struct vm_area_struct *vma, unsigned long addr,
>  	struct sgx_encl_page *entry = NULL;
>  	char data[sizeof(unsigned long)];
>  	unsigned long align;
> +	int srcu_idx;
>  	int offset;
>  	int cnt;
>  	int ret = 0;
> @@ -339,8 +350,15 @@ static int sgx_vma_access(struct vm_area_struct *vma, unsigned long addr,
>  		return -EFAULT;
>  
>  	for (i = 0; i < len; i += cnt) {
> +		srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +		if (sgx_epc_is_locked()) {
> +			ret = -EBUSY;
> +			goto out;
> +		}
> +
>  		entry = sgx_encl_reserve_page(encl, (addr + i) & PAGE_MASK,
>  					      vma->vm_flags);
> +
>  		if (IS_ERR(entry)) {
>  			ret = PTR_ERR(entry);
>  			break;
> @@ -366,6 +384,7 @@ static int sgx_vma_access(struct vm_area_struct *vma, unsigned long addr,
>  
>  out:
>  		mutex_unlock(&encl->lock);
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
>  
>  		if (ret)
>  			break;
> diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> index b3c2e8d58142..a4df72f715d7 100644
> --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> @@ -147,24 +147,42 @@ static int sgx_encl_create(struct sgx_encl *encl, struct sgx_secs *secs)
>  static long sgx_ioc_enclave_create(struct sgx_encl *encl, void __user *arg)
>  {
>  	struct sgx_enclave_create create_arg;
> +	int srcu_idx;
>  	void *secs;
>  	int ret;
>  
> -	if (test_bit(SGX_ENCL_CREATED, &encl->flags))
> -		return -EINVAL;
> +	if (test_bit(SGX_ENCL_CREATED, &encl->flags)) {
> +		ret = -EINVAL;
> +		goto err;
> +	}
>  
> -	if (copy_from_user(&create_arg, arg, sizeof(create_arg)))
> -		return -EFAULT;
> +	if (copy_from_user(&create_arg, arg, sizeof(create_arg))) {
> +		ret = -EFAULT;
> +		goto err;
> +	}
>  
>  	secs = kmalloc(PAGE_SIZE, GFP_KERNEL);
> -	if (!secs)
> -		return -ENOMEM;
> +	if (!secs) {
> +		ret = -ENOMEM;
> +		goto err;
> +	}
>  
>  	if (copy_from_user(secs, (void __user *)create_arg.src, PAGE_SIZE))
>  		ret = -EFAULT;

  	if (copy_from_user(secs, (void __user *)create_arg.src, PAGE_SIZE)) {
  		ret = -EFAULT;
                goto err;
        }

Right?


> -	else
> +	else {
> +		srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +		if (sgx_epc_is_locked()) {
> +			srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +			ret = -EBUSY;
> +			goto err;
> +		}
> +
>  		ret = sgx_encl_create(encl, secs);
>  
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +	}

I mean less branching for the common (success) case.

> +
> +err:
>  	kfree(secs);
>  	return ret;
>  }
> @@ -418,6 +436,7 @@ static long sgx_ioc_enclave_add_pages(struct sgx_encl *encl, void __user *arg)
>  	struct sgx_enclave_add_pages add_arg;
>  	struct sgx_secinfo secinfo;
>  	unsigned long c;
> +	int srcu_idx;
>  	int ret;
>  
>  	if (!test_bit(SGX_ENCL_CREATED, &encl->flags) ||
> @@ -455,8 +474,18 @@ static long sgx_ioc_enclave_add_pages(struct sgx_encl *encl, void __user *arg)
>  		if (need_resched())
>  			cond_resched();
>  
> +		srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +		if (sgx_epc_is_locked()) {
> +			ret = -EBUSY;
> +			srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +			break;
> +		}
> +
>  		ret = sgx_encl_add_page(encl, add_arg.src + c, add_arg.offset + c,
>  					&secinfo, add_arg.flags);
> +
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +
>  		if (ret)
>  			break;
>  	}
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index b3aba1e1274c..10360f06c0df 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -23,6 +23,17 @@ static int sgx_nr_epc_sections;
>  static struct task_struct *ksgxd_tsk;
>  static DECLARE_WAIT_QUEUE_HEAD(ksgxd_waitq);
>  static DEFINE_XARRAY(sgx_epc_address_space);
> +/*
> + * The flag sgx_epc_locked prevents any new SGX flows that
> + * may attempt to allocate a new EPC page.
> + */
> +static bool __rcu sgx_epc_locked;
> +/*
> + * By synchronizing around sgx_epc_locked SRCU ensures that any executing
> + * SGX flows have completed before proceeding with an SVN update. New SGX flows
> + * will be prevented from starting during an SVN update.
> + */
> +DEFINE_SRCU(sgx_lock_epc_srcu);
>  
>  /*
>   * These variables are part of the state of the reclaimer, and must be accessed
> @@ -407,6 +418,8 @@ static bool sgx_should_reclaim(unsigned long watermark)
>  
>  static int ksgxd(void *p)
>  {
> +	int srcu_idx;
> +
>  	set_freezable();
>  
>  	/*
> @@ -427,9 +440,15 @@ static int ksgxd(void *p)
>  				     kthread_should_stop() ||
>  				     sgx_should_reclaim(SGX_NR_HIGH_PAGES));
>  
> +		srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +		if (sgx_epc_is_locked())
> +			goto maybe_resched;
> +
>  		if (sgx_should_reclaim(SGX_NR_HIGH_PAGES))
>  			sgx_reclaim_pages();
>  
> +maybe_resched:
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
>  		cond_resched();
>  	}
>  
> @@ -972,3 +991,21 @@ static int __init sgx_init(void)
>  }
>  
>  device_initcall(sgx_init);
> +
> +static void sgx_lock_epc(void)
> +{
> +	sgx_epc_locked = true;
> +	synchronize_srcu(&sgx_lock_epc_srcu);
> +}
> +
> +static void sgx_unlock_epc(void)
> +{
> +	sgx_epc_locked = false;
> +	synchronize_srcu(&sgx_lock_epc_srcu);
> +}
> +
> +bool sgx_epc_is_locked(void)
> +{
> +	lockdep_assert_held(&sgx_lock_epc_srcu);
> +	return sgx_epc_locked;
> +}
> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
> index 6a77a14eee38..e953816d7c8b 100644
> --- a/arch/x86/kernel/cpu/sgx/virt.c
> +++ b/arch/x86/kernel/cpu/sgx/virt.c
> @@ -75,10 +75,21 @@ static vm_fault_t sgx_vepc_fault(struct vm_fault *vmf)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct sgx_vepc *vepc = vma->vm_private_data;
> +	int srcu_idx;
>  	int ret;
>  
>  	mutex_lock(&vepc->lock);
> +	srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +
> +	if (sgx_epc_is_locked()) {
> +		ret = -EBUSY;
> +		goto out_unlock;
> +	}
> +
>  	ret = __sgx_vepc_fault(vepc, vma, vmf->address);
> +
> +out_unlock:
> +	srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
>  	mutex_unlock(&vepc->lock);
>  
>  	if (!ret)
> @@ -331,6 +342,7 @@ int __init sgx_vepc_init(void)
>  int sgx_virt_ecreate(struct sgx_pageinfo *pageinfo, void __user *secs,
>  		     int *trapnr)
>  {
> +	int srcu_idx;
>  	int ret;
>  
>  	/*
> @@ -347,6 +359,12 @@ int sgx_virt_ecreate(struct sgx_pageinfo *pageinfo, void __user *secs,
>  	if (WARN_ON_ONCE(!access_ok(secs, PAGE_SIZE)))
>  		return -EINVAL;
>  
> +	srcu_idx = srcu_read_lock(&sgx_lock_epc_srcu);
> +	if (sgx_epc_is_locked()) {
> +		srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +		return -EBUSY;
> +	}
> +
>  	__uaccess_begin();
>  	ret = __ecreate(pageinfo, (void *)secs);
>  	__uaccess_end();
> @@ -356,6 +374,8 @@ int sgx_virt_ecreate(struct sgx_pageinfo *pageinfo, void __user *secs,
>  		return -EFAULT;
>  	}
>  
> +	srcu_read_unlock(&sgx_lock_epc_srcu, srcu_idx);
> +
>  	/* ECREATE doesn't return an error code, it faults or succeeds. */
>  	WARN_ON_ONCE(ret);
>  	return 0;
> -- 
> 2.17.1
> 

BR, Jarkko