Re: [PATCH V3 17/30] x86/sgx: Support modifying SGX page type

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Apr 04, 2022 at 09:49:25AM -0700, Reinette Chatre wrote:
> Every enclave contains one or more Thread Control Structures (TCS). The
> TCS contains meta-data used by the hardware to save and restore thread
> specific information when entering/exiting the enclave. With SGX1 an
> enclave needs to be created with enough TCSs to support the largest
> number of threads expecting to use the enclave and enough enclave pages
> to meet all its anticipated memory demands. In SGX1 all pages remain in
> the enclave until the enclave is unloaded.
> 
> SGX2 introduces a new function, ENCLS[EMODT], that is used to change
> the type of an enclave page from a regular (SGX_PAGE_TYPE_REG) enclave
> page to a TCS (SGX_PAGE_TYPE_TCS) page or change the type from a
> regular (SGX_PAGE_TYPE_REG) or TCS (SGX_PAGE_TYPE_TCS)
> page to a trimmed (SGX_PAGE_TYPE_TRIM) page (setting it up for later
> removal).
> 
> With the existing support of dynamically adding regular enclave pages
> to an initialized enclave and changing the page type to TCS it is
> possible to dynamically increase the number of threads supported by an
> enclave.
> 
> Changing the enclave page type to SGX_PAGE_TYPE_TRIM is the first step
> of dynamically removing pages from an initialized enclave. The complete
> page removal flow is:
> 1) Change the type of the pages to be removed to SGX_PAGE_TYPE_TRIM
>    using the SGX_IOC_ENCLAVE_MODIFY_TYPE ioctl() introduced here.
> 2) Approve the page removal by running ENCLU[EACCEPT] from within
>    the enclave.
> 3) Initiate actual page removal using the ioctl() introduced in the
>    following patch.
> 
> Add ioctl() SGX_IOC_ENCLAVE_MODIFY_TYPE to support changing SGX
> enclave page types within an initialized enclave. With
> SGX_IOC_ENCLAVE_MODIFY_TYPE the user specifies a page range and the
> enclave page type to be applied to all pages in the provided range.
> The ioctl() itself can return an error code based on failures
> encountered by the kernel. It is also possible for SGX specific
> failures to be encountered.  Add a result output parameter to
> communicate the SGX return code. It is possible for the enclave page
> type change request to fail on any page within the provided range.
> Support partial success by returning the number of pages that were
> successfully changed.
> 
> After the page type is changed the page continues to be accessible
> from the kernel perspective with page table entries and internal
> state. The page may be moved to swap. Any access until ENCLU[EACCEPT]
> will encounter a page fault with SGX flag set in error code.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Changes since V2:
> - Adjust ioctl number after removal of SGX_IOC_ENCLAVE_RELAX_PERMISSIONS.
> - Remove attempt at runtime tracking of EPCM permissions
>   (sgx_encl_page->vm_run_prot_bits). (Jarkko)
> - Change names to follow guidance of using detailed names (Jarkko):
>   struct sgx_enclave_modt -> struct sgx_enclave_modify_type
>   sgx_enclave_modt() -> sgx_enclave_modify_type()
>   sgx_ioc_enclave_modt() -> sgx_ioc_enclave_modify_type()
> 
> Changes since V1:
> - Remove the "Earlier changes ..." paragraph (Jarkko).
> - Change "new ioctl" text to "Add SGX_IOC_ENCLAVE_MOD_TYPE" (Jarkko).
> - Discussion about EPCM interaction and the EPCM MODIFIED bit is moved
>   to new patch that introduces the ENCLS[EMODT] wrapper while keeping
>   the higher level discussion on page accessibility in
>   this commit log (Jarkko).
> - Rename SGX_IOC_PAGE_MODT ioctl() to SGX_IOC_ENCLAVE_MODIFY_TYPE
>   (Jarkko).
> - Rename struct sgx_page_modt to struct sgx_enclave_modt in support
>   of ioctl() rename.
> - Rename sgx_page_modt() to sgx_enclave_modt() and sgx_ioc_page_modt()
>   to sgx_ioc_enclave_modt() in support of ioctl() rename.
> - Provide secinfo as parameter to ioctl() instead of just
>   page type (Jarkko).
> - Update comments to refer to new ioctl() names.
> - Use new SGX2 checking helper().
> - Use ETRACK flow utility.
> - Move kernel-doc to function that provides documentation for
>   Documentation/x86/sgx.rst.
> - Remove redundant comment.
> - Use offset/length validation utility.
> - Make explicit which members of struct sgx_enclave_modt are for
>   output (Dave).
> 
>  arch/x86/include/uapi/asm/sgx.h |  20 +++
>  arch/x86/kernel/cpu/sgx/ioctl.c | 209 ++++++++++++++++++++++++++++++++
>  2 files changed, 229 insertions(+)
> 
> diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
> index a0a24e94fb27..529f4ab28410 100644
> --- a/arch/x86/include/uapi/asm/sgx.h
> +++ b/arch/x86/include/uapi/asm/sgx.h
> @@ -31,6 +31,8 @@ enum sgx_page_flags {
>  	_IO(SGX_MAGIC, 0x04)
>  #define SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS \
>  	_IOWR(SGX_MAGIC, 0x05, struct sgx_enclave_restrict_permissions)
> +#define SGX_IOC_ENCLAVE_MODIFY_TYPE \
> +	_IOWR(SGX_MAGIC, 0x06, struct sgx_enclave_modify_type)
>  
>  /**
>   * struct sgx_enclave_create - parameter structure for the
> @@ -97,6 +99,24 @@ struct sgx_enclave_restrict_permissions {
>  	__u64 count;
>  };
>  
> +/**
> + * struct sgx_enclave_modify_type - parameters for %SGX_IOC_ENCLAVE_MODIFY_TYPE
> + * @offset:	starting page offset (page aligned relative to enclave base
> + *		address defined in SECS)
> + * @length:	length of memory (multiple of the page size)
> + * @secinfo:	address for the SECINFO data containing the new type
> + *		for pages in range described by @offset and @length
> + * @result:	(output) SGX result code of ENCLS[EMODT] function
> + * @count:	(output) bytes successfully changed (multiple of page size)
> + */
> +struct sgx_enclave_modify_type {
> +	__u64 offset;
> +	__u64 length;
> +	__u64 secinfo;
> +	__u64 result;
> +	__u64 count;
> +};
> +
>  struct sgx_enclave_run;
>  
>  /**
> diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> index 4d88bfd163e7..6f769e67ec2d 100644
> --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> @@ -898,6 +898,212 @@ static long sgx_ioc_enclave_restrict_permissions(struct sgx_encl *encl,
>  	return ret;
>  }
>  
> +/**
> + * sgx_enclave_modify_type() - Modify type of SGX enclave pages
> + * @encl:	Enclave to which the pages belong.
> + * @modt:	Checked parameters from user about which pages need modifying.
> + * @page_type:	New page type.
> + *
> + * Return:
> + * - 0:		Success
> + * - -errno:	Otherwise
> + */
> +static long sgx_enclave_modify_type(struct sgx_encl *encl,
> +				    struct sgx_enclave_modify_type *modt,
> +				    enum sgx_page_type page_type)
> +{
> +	unsigned long max_prot_restore;
> +	struct sgx_encl_page *entry;
> +	struct sgx_secinfo secinfo;
> +	unsigned long prot;
> +	unsigned long addr;
> +	unsigned long c;
> +	void *epc_virt;
> +	int ret;
> +
> +	/*
> +	 * The only new page types allowed by hardware are PT_TCS and PT_TRIM.
> +	 */
> +	if (page_type != SGX_PAGE_TYPE_TCS && page_type != SGX_PAGE_TYPE_TRIM)
> +		return -EINVAL;
> +
> +	memset(&secinfo, 0, sizeof(secinfo));
> +
> +	secinfo.flags = page_type << 8;
> +
> +	for (c = 0 ; c < modt->length; c += PAGE_SIZE) {
> +		addr = encl->base + modt->offset + c;
> +
> +		mutex_lock(&encl->lock);
> +
> +		entry = sgx_encl_load_page(encl, addr);
> +		if (IS_ERR(entry)) {
> +			ret = PTR_ERR(entry) == -EBUSY ? -EAGAIN : -EFAULT;
> +			goto out_unlock;
> +		}
> +
> +		/*
> +		 * Borrow the logic from the Intel SDM. Regular pages
> +		 * (SGX_PAGE_TYPE_REG) can change type to SGX_PAGE_TYPE_TCS
> +		 * or SGX_PAGE_TYPE_TRIM but TCS pages can only be trimmed.
> +		 * CET pages not supported yet.
> +		 */
> +		if (!(entry->type == SGX_PAGE_TYPE_REG ||
> +		      (entry->type == SGX_PAGE_TYPE_TCS &&
> +		       page_type == SGX_PAGE_TYPE_TRIM))) {
> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +
> +		max_prot_restore = entry->vm_max_prot_bits;
> +
> +		/*
> +		 * Once a regular page becomes a TCS page it cannot be
> +		 * changed back. So the maximum allowed protection reflects
> +		 * the TCS page that is always RW from kernel perspective but
> +		 * will be inaccessible from within enclave. Before doing
> +		 * so, do make sure that the new page type continues to
> +		 * respect the originally vetted page permissions.
> +		 */
> +		if (entry->type == SGX_PAGE_TYPE_REG &&
> +		    page_type == SGX_PAGE_TYPE_TCS) {
> +			if (~entry->vm_max_prot_bits & (VM_READ | VM_WRITE)) {
> +				ret = -EPERM;
> +				goto out_unlock;
> +			}
> +			prot = PROT_READ | PROT_WRITE;
> +			entry->vm_max_prot_bits = calc_vm_prot_bits(prot, 0);
> +
> +			/*
> +			 * Prevent page from being reclaimed while mutex
> +			 * is released.
> +			 */
> +			if (sgx_unmark_page_reclaimable(entry->epc_page)) {
> +				ret = -EAGAIN;
> +				goto out_entry_changed;
> +			}
> +
> +			/*
> +			 * Do not keep encl->lock because of dependency on
> +			 * mmap_lock acquired in sgx_zap_enclave_ptes().
> +			 */
> +			mutex_unlock(&encl->lock);
> +
> +			sgx_zap_enclave_ptes(encl, addr);
> +
> +			mutex_lock(&encl->lock);
> +
> +			sgx_mark_page_reclaimable(entry->epc_page);
> +		}
> +
> +		/* Change EPC type */
> +		epc_virt = sgx_get_epc_virt_addr(entry->epc_page);
> +		ret = __emodt(&secinfo, epc_virt);
> +		if (encls_faulted(ret)) {
> +			/*
> +			 * All possible faults should be avoidable:
> +			 * parameters have been checked, will only change
> +			 * valid page types, and no concurrent
> +			 * SGX1/SGX2 ENCLS instructions since these are
> +			 * protected with mutex.
> +			 */
> +			pr_err_once("EMODT encountered exception %d\n",
> +				    ENCLS_TRAPNR(ret));
> +			ret = -EFAULT;
> +			goto out_entry_changed;
> +		}
> +		if (encls_failed(ret)) {
> +			modt->result = ret;
> +			ret = -EFAULT;
> +			goto out_entry_changed;
> +		}
> +
> +		ret = sgx_enclave_etrack(encl);
> +		if (ret) {
> +			ret = -EFAULT;
> +			goto out_unlock;
> +		}
> +
> +		entry->type = page_type;
> +
> +		mutex_unlock(&encl->lock);
> +	}
> +
> +	ret = 0;
> +	goto out;
> +
> +out_entry_changed:
> +	entry->vm_max_prot_bits = max_prot_restore;
> +out_unlock:
> +	mutex_unlock(&encl->lock);
> +out:
> +	modt->count = c;
> +
> +	return ret;
> +}
> +
> +/**
> + * sgx_ioc_enclave_modify_type() - handler for %SGX_IOC_ENCLAVE_MODIFY_TYPE
> + * @encl:	an enclave pointer
> + * @arg:	userspace pointer to a &struct sgx_enclave_modify_type instance
> + *
> + * Ability to change the enclave page type supports the following use cases:
> + *
> + * * It is possible to add TCS pages to an enclave by changing the type of
> + *   regular pages (%SGX_PAGE_TYPE_REG) to TCS (%SGX_PAGE_TYPE_TCS) pages.
> + *   With this support the number of threads supported by an initialized
> + *   enclave can be increased dynamically.
> + *
> + * * Regular or TCS pages can dynamically be removed from an initialized
> + *   enclave by changing the page type to %SGX_PAGE_TYPE_TRIM. Changing the
> + *   page type to %SGX_PAGE_TYPE_TRIM marks the page for removal with actual
> + *   removal done by handler of %SGX_IOC_ENCLAVE_REMOVE_PAGES ioctl() called
> + *   after ENCLU[EACCEPT] is run on %SGX_PAGE_TYPE_TRIM page from within the
> + *   enclave.
> + *
> + * Return:
> + * - 0:		Success
> + * - -errno:	Otherwise
> + */
> +static long sgx_ioc_enclave_modify_type(struct sgx_encl *encl, void __user *arg)
> +{
> +	struct sgx_enclave_modify_type params;
> +	enum sgx_page_type page_type;
> +	struct sgx_secinfo secinfo;
> +	long ret;
> +
> +	ret = sgx_ioc_sgx2_ready(encl);
> +	if (ret)
> +		return ret;
> +
> +	if (copy_from_user(&params, arg, sizeof(params)))
> +		return -EFAULT;
> +
> +	if (sgx_validate_offset_length(encl, params.offset, params.length))
> +		return -EINVAL;
> +
> +	if (copy_from_user(&secinfo, (void __user *)params.secinfo,
> +			   sizeof(secinfo)))
> +		return -EFAULT;
> +
> +	if (secinfo.flags & ~SGX_SECINFO_PAGE_TYPE_MASK)
> +		return -EINVAL;
> +
> +	if (memchr_inv(secinfo.reserved, 0, sizeof(secinfo.reserved)))
> +		return -EINVAL;
> +
> +	if (params.result || params.count)
> +		return -EINVAL;
> +
> +	page_type = (secinfo.flags & SGX_SECINFO_PAGE_TYPE_MASK) >> 8;
> +	ret = sgx_enclave_modify_type(encl, &params, page_type);
> +
> +	if (copy_to_user(arg, &params, sizeof(params)))
> +		return -EFAULT;
> +
> +	return ret;
> +}
> +
>  long sgx_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>  {
>  	struct sgx_encl *encl = filep->private_data;
> @@ -923,6 +1129,9 @@ long sgx_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>  		ret = sgx_ioc_enclave_restrict_permissions(encl,
>  							   (void __user *)arg);
>  		break;
> +	case SGX_IOC_ENCLAVE_MODIFY_TYPE:
> +		ret = sgx_ioc_enclave_modify_type(encl, (void __user *)arg);
> +		break;
>  	default:
>  		ret = -ENOIOCTLCMD;
>  		break;
> -- 
> 2.25.1
> 

To be coherent with other names, this should be
SGX_IOC_ENCLAVE_MODIFY_TYPES.

BR, Jarkko