Re: [PATCH V3 14/30] x86/sgx: Support restricting of enclave page permissions
From: Jarkko Sakkinen
Date: Tue Apr 05 2022 - 01:03:36 EST
On Mon, 2022-04-04 at 09:49 -0700, Reinette Chatre wrote:
> In the initial (SGX1) version of SGX, pages in an enclave need to be
> created with permissions that support all usages of the pages, from the
> time the enclave is initialized until it is unloaded. For example,
> pages used by a JIT compiler or when code needs to otherwise be
> relocated need to always have RWX permissions.
>
> SGX2 includes a new function ENCLS[EMODPR] that is run from the kernel
> and can be used to restrict the EPCM permissions of regular enclave
> pages within an initialized enclave.
>
> Introduce ioctl() SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS to support
> restricting EPCM permissions. With this ioctl() the user specifies
> a page range and the EPCM permissions to be applied to all pages in
> the provided range. ENCLS[EMODPR] is run to restrict the EPCM
> permissions followed by the ENCLS[ETRACK] flow that will ensure
> no cached linear-to-physical address mappings to the changed
> pages remain.
>
> It is possible for the permission change request to fail on any
> page within the provided range, either with an error encountered
> by the kernel or by the SGX hardware while running
> ENCLS[EMODPR]. To support partial success the ioctl() returns an
> error code based on failures encountered by the kernel as well
> as two result output parameters: one for the number of pages
> that were successfully changed and one for the SGX return code.
>
> The page table entry permissions are not impacted by the EPCM
> permission changes. VMAs and PTEs will continue to allow the
> maximum vetted permissions determined at the time the pages
> are added to the enclave. The SGX error code in a page fault
> will indicate if it was an EPCM permission check that prevented
> an access attempt.
>
> No checking is done to ensure that the permissions are actually
> being restricted. This is because the enclave may have relaxed
> the EPCM permissions from within the enclave without letting the
> kernel know. An attempt to relax permissions using this call will
> be ignored by the hardware.
>
> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> ---
> Changes since V2:
> - Include the sgx_ioc_sgx2_ready() utility
> that previously was in "x86/sgx: Support relaxing of enclave page
> permissions" that is removed from the next version.
> - Few renames requested by Jarkko:
> struct sgx_enclave_restrict_perm ->
> struct sgx_enclave_restrict_permissions
> sgx_enclave_restrict_perm() ->
> sgx_enclave_restrict_permissions()
> sgx_ioc_enclave_restrict_perm() ->
> sgx_ioc_enclave_restrict_permissions()
> - Make EPCM permissions independent from kernel view of
> permissions. (Jarkko)
> - Remove attempt at runtime tracking of EPCM permissions
> (sgx_encl_page->vm_run_prot_bits).
> - Do not flush page table entries - they are no longer impacted by
> EPCM permission changes.
> - Modify changelog to reflect new architecture.
> - Ensure at least PROT_READ is requested - enclave requires read
> access to the page for commands like EMODPE and EACCEPT. (Jarkko)
>
> Changes since V1:
> - Change terminology to use "relax" instead of "extend" to refer to
> the case when enclave page permissions are added (Dave).
> - Use ioctl() in commit message (Dave).
> - Add examples on what permissions would be allowed (Dave).
> - Split enclave page permission changes into two ioctl()s, one for
> permission restricting (SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS)
> and one for permission relaxing (SGX_IOC_ENCLAVE_RELAX_PERMISSIONS)
> (Jarkko).
> - In support of the ioctl() name change the following names have been
> changed:
> struct sgx_page_modp -> struct sgx_enclave_restrict_perm
> sgx_ioc_page_modp() -> sgx_ioc_enclave_restrict_perm()
> sgx_page_modp() -> sgx_enclave_restrict_perm()
> - ioctl() takes entire secinfo as input instead of
> page permissions only (Jarkko).
> - Fix kernel-doc to include () in function name.
> - Create and use utility for the ETRACK flow.
> - Fixups in comments
> - Move kernel-doc to function that provides documentation for
> Documentation/x86/sgx.rst.
> - Remove redundant comment.
> - Make explicit which members of struct sgx_enclave_restrict_perm
> are for output (Dave).
>
> arch/x86/include/uapi/asm/sgx.h | 21 +++
> arch/x86/kernel/cpu/sgx/ioctl.c | 242 ++++++++++++++++++++++++++++++++
> 2 files changed, 263 insertions(+)
>
> diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
> index f4b81587e90b..a0a24e94fb27 100644
> --- a/arch/x86/include/uapi/asm/sgx.h
> +++ b/arch/x86/include/uapi/asm/sgx.h
> @@ -29,6 +29,8 @@ enum sgx_page_flags {
> _IOW(SGX_MAGIC, 0x03, struct sgx_enclave_provision)
> #define SGX_IOC_VEPC_REMOVE_ALL \
> _IO(SGX_MAGIC, 0x04)
> +#define SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS \
> + _IOWR(SGX_MAGIC, 0x05, struct sgx_enclave_restrict_permissions)
>
> /**
> * struct sgx_enclave_create - parameter structure for the
> @@ -76,6 +78,25 @@ struct sgx_enclave_provision {
> __u64 fd;
> };
>
> +/**
> + * struct sgx_enclave_restrict_permissions - parameters for ioctl
> + * %SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS
> + * @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 permission bits
> + * for pages in range described by @offset and @length
> + * @result: (output) SGX result code of ENCLS[EMODPR] function
> + * @count: (output) bytes successfully changed (multiple of page size)
> + */
> +struct sgx_enclave_restrict_permissions {
> + __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 0460fd224a05..4d88bfd163e7 100644
> --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> @@ -660,6 +660,244 @@ static long sgx_ioc_enclave_provision(struct sgx_encl *encl, void __user *arg)
> return sgx_set_attribute(&encl->attributes_mask, params.fd);
> }
>
> +/*
> + * Ensure enclave is ready for SGX2 functions. Readiness is checked
> + * by ensuring the hardware supports SGX2 and the enclave is initialized
> + * and thus able to handle requests to modify pages within it.
> + */
> +static int sgx_ioc_sgx2_ready(struct sgx_encl *encl)
> +{
> + if (!(cpu_feature_enabled(X86_FEATURE_SGX2)))
> + return -ENODEV;
> +
> + if (!test_bit(SGX_ENCL_INITIALIZED, &encl->flags))
> + return -EINVAL;
> +
> + return 0;
> +}
> +
> +/*
> + * Return valid permission fields from a secinfo structure provided by
> + * user space. The secinfo structure is required to only have bits in
> + * the permission fields set.
> + */
> +static int sgx_perm_from_user_secinfo(void __user *_secinfo, u64 *secinfo_perm)
> +{
> + struct sgx_secinfo secinfo;
> + u64 perm;
> +
> + if (copy_from_user(&secinfo, (void __user *)_secinfo,
> + sizeof(secinfo)))
> + return -EFAULT;
> +
> + if (secinfo.flags & ~SGX_SECINFO_PERMISSION_MASK)
> + return -EINVAL;
> +
> + if (memchr_inv(secinfo.reserved, 0, sizeof(secinfo.reserved)))
> + return -EINVAL;
> +
> + perm = secinfo.flags & SGX_SECINFO_PERMISSION_MASK;
> +
> + /*
> + * Read access is required for the enclave to be able to use the page.
> + * SGX instructions like ENCLU[EMODPE] and ENCLU[EACCEPT] require
> + * read access.
> + */
> + if (!(perm & SGX_SECINFO_R))
> + return -EINVAL;
> +
> + *secinfo_perm = perm;
> +
> + return 0;
> +}
> +
> +/*
> + * Some SGX functions require that no cached linear-to-physical address
> + * mappings are present before they can succeed. Collaborate with
> + * hardware via ENCLS[ETRACK] to ensure that all cached
> + * linear-to-physical address mappings belonging to all threads of
> + * the enclave are cleared. See sgx_encl_cpumask() for details.
> + */
> +static int sgx_enclave_etrack(struct sgx_encl *encl)
> +{
> + void *epc_virt;
> + int ret;
> +
> + epc_virt = sgx_get_epc_virt_addr(encl->secs.epc_page);
> + ret = __etrack(epc_virt);
> + if (ret) {
> + /*
> + * ETRACK only fails when there is an OS issue. For
> + * example, two consecutive ETRACK was sent without
> + * completed IPI between.
> + */
> + pr_err_once("ETRACK returned %d (0x%x)", ret, ret);
> + /*
> + * Send IPIs to kick CPUs out of the enclave and
> + * try ETRACK again.
> + */
> + on_each_cpu_mask(sgx_encl_cpumask(encl), sgx_ipi_cb, NULL, 1);
> + ret = __etrack(epc_virt);
> + if (ret) {
> + pr_err_once("ETRACK repeat returned %d (0x%x)",
> + ret, ret);
> + return -EFAULT;
> + }
> + }
> + on_each_cpu_mask(sgx_encl_cpumask(encl), sgx_ipi_cb, NULL, 1);
> +
> + return 0;
> +}
> +
> +/**
> + * sgx_enclave_restrict_permissions() - Restrict EPCM permissions
> + * @encl: Enclave to which the pages belong.
> + * @modp: Checked parameters from user on which pages need modifying.
> + * @secinfo_perm: New (validated) permission bits.
> + *
> + * Return:
> + * - 0: Success.
> + * - -errno: Otherwise.
> + */
> +static long
> +sgx_enclave_restrict_permissions(struct sgx_encl *encl,
> + struct sgx_enclave_restrict_permissions *modp,
> + u64 secinfo_perm)
> +{
> + struct sgx_encl_page *entry;
> + struct sgx_secinfo secinfo;
> + unsigned long addr;
> + unsigned long c;
> + void *epc_virt;
> + int ret;
> +
> + memset(&secinfo, 0, sizeof(secinfo));
> + secinfo.flags = secinfo_perm;
> +
> + for (c = 0 ; c < modp->length; c += PAGE_SIZE) {
> + addr = encl->base + modp->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;
> + }
> +
> + /*
> + * Changing EPCM permissions is only supported on regular
> + * SGX pages. Attempting this change on other pages will
> + * result in #PF.
> + */
> + if (entry->type != SGX_PAGE_TYPE_REG) {
> + ret = -EINVAL;
> + goto out_unlock;
> + }
> +
> + /*
> + * Do not verify the permission bits requested. Kernel
> + * has no control over how EPCM permissions can be relaxed
> + * from within the enclave. ENCLS[EMODPR] can only
> + * remove existing EPCM permissions, attempting to set
> + * new permissions will be ignored by the hardware.
> + */
> +
> + /* Change EPCM permissions. */
> + epc_virt = sgx_get_epc_virt_addr(entry->epc_page);
> + ret = __emodpr(&secinfo, epc_virt);
> + if (encls_faulted(ret)) {
> + /*
> + * All possible faults should be avoidable:
> + * parameters have been checked, will only change
> + * permissions of a regular page, and no concurrent
> + * SGX1/SGX2 ENCLS instructions since these
> + * are protected with mutex.
> + */
> + pr_err_once("EMODPR encountered exception %d\n",
> + ENCLS_TRAPNR(ret));
> + ret = -EFAULT;
> + goto out_unlock;
> + }
> + if (encls_failed(ret)) {
> + modp->result = ret;
> + ret = -EFAULT;
> + goto out_unlock;
> + }
> +
> + ret = sgx_enclave_etrack(encl);
> + if (ret) {
> + ret = -EFAULT;
> + goto out_unlock;
> + }
> +
> + mutex_unlock(&encl->lock);
> + }
> +
> + ret = 0;
> + goto out;
> +
> +out_unlock:
> + mutex_unlock(&encl->lock);
> +out:
> + modp->count = c;
> +
> + return ret;
> +}
> +
> +/**
> + * sgx_ioc_enclave_restrict_permissions() - handler for
> + * %SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS
> + * @encl: an enclave pointer
> + * @arg: userspace pointer to a &struct sgx_enclave_restrict_permissions
> + * instance
> + *
> + * SGX2 distinguishes between relaxing and restricting the enclave page
> + * permissions maintained by the hardware (EPCM permissions) of pages
> + * belonging to an initialized enclave (after SGX_IOC_ENCLAVE_INIT).
> + *
> + * EPCM permissions cannot be restricted from within the enclave, the enclave
> + * requires the kernel to run the privileged level 0 instructions ENCLS[EMODPR]
> + * and ENCLS[ETRACK]. An attempt to relax EPCM permissions with this call
> + * will be ignored by the hardware.
> + *
> + * Return:
> + * - 0: Success
> + * - -errno: Otherwise
> + */
> +static long sgx_ioc_enclave_restrict_permissions(struct sgx_encl *encl,
> + void __user *arg)
> +{
> + struct sgx_enclave_restrict_permissions params;
> + u64 secinfo_perm;
> + long ret;
> +
> + ret = sgx_ioc_sgx2_ready(encl);
> + if (ret)
> + return ret;
> +
> + if (copy_from_user(¶ms, arg, sizeof(params)))
> + return -EFAULT;
> +
> + if (sgx_validate_offset_length(encl, params.offset, params.length))
> + return -EINVAL;
> +
> + ret = sgx_perm_from_user_secinfo((void __user *)params.secinfo,
> + &secinfo_perm);
> + if (ret)
> + return ret;
> +
> + if (params.result || params.count)
> + return -EINVAL;
> +
> + ret = sgx_enclave_restrict_permissions(encl, ¶ms, secinfo_perm);
> +
> + if (copy_to_user(arg, ¶ms, 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;
> @@ -681,6 +919,10 @@ long sgx_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> case SGX_IOC_ENCLAVE_PROVISION:
> ret = sgx_ioc_enclave_provision(encl, (void __user *)arg);
> break;
> + case SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS:
> + ret = sgx_ioc_enclave_restrict_permissions(encl,
> + (void __user *)arg);
> + break;
> default:
> ret = -ENOIOCTLCMD;
> break;
I think this a big improvement all things considered. I just put
a kernel building and see if I get this wired to our code:
https://github.com/jarkkojs/aur-linux-sgx/actions/runs/2094084943
I'll report my findings later on.
BR, Jarkko