[PATCH V2 15/32] x86/sgx: Support relaxing of enclave page permissions

Reinette Chatre
2022-02-08T01:13:14Z

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.

With the SGX2 function ENCLU[EMODPE] an enclave is able to relax
the EPCM permissions of its pages after the enclave is initialized.
Relaxing EPCM permissions is not possible from outside the enclave,
including from the kernel. The kernel does control the PTEs though
and the enclave still depends on the kernel to install PTEs with the
new relaxed permissions before it (the enclave) can access the pages
using the new permissions.

Introduce ioctl() SGX_IOC_ENCLAVE_RELAX_PERMISSIONS to support
relaxing of EPCM permissions done from within the enclave. With
this ioctl() the user specifies a page range and the permissions to
be applied to all pages in the provided range. After checking
the new permissions (more detail below) the PTEs are reset and
it is ensured that any new PTEs will contain the new, relaxed,
permissions.

The permission change request could fail on any page within the
provided range. To support partial success the ioctl() returns
an error code based on failures encountered by the kernel and
the number of pages that were successfully changed.

Checking user provided new permissions
======================================

Enclave page permission changes need to be approached with care and
for this reason permission changes are only allowed if
the new permissions are the same or more restrictive that the
vetted permissions. Thus, even though an enclave is able to relax
the EPCM permissions of its pages beyond what was originally vetted,
the kernel will not. The kernel will only install PTEs that respect
the vetted enclave page permissions.

For example, enclave pages with vetted EPCM permissions in brackets
below are allowed to have PTE permissions as follows:
* (RWX) R => RW => RX => RWX
* (RW) R => RW
* (RX) R => RX

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
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_relax_perm
sgx_ioc_page_modp() -> sgx_ioc_enclave_relax_perm()
sgx_page_modp() -> sgx_enclave_relax_perm()
- ioctl() takes entire secinfo as input instead of
page permissions only (Jarkko).
- Fix kernel-doc to include () in function name.
- Introduce small helper to check for SGX2 readiness instead of
duplicating the same two checks in every SGX2 supporting ioctl().
- Fixups in comments
- Move kernel-doc to function that provides documentation for
Documentation/x86/sgx.rst.
- Remove redundant comment.
- Make explicit which member of struct sgx_enclave_relax_perm is
for output (Dave).

arch/x86/include/uapi/asm/sgx.h | 19 +++
arch/x86/kernel/cpu/sgx/ioctl.c | 199 ++++++++++++++++++++++++++++++++
2 files changed, 218 insertions(+)

diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
index f4b81587e90b..5c678b27bb72 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_RELAX_PERMISSIONS \
+ _IOWR(SGX_MAGIC, 0x05, struct sgx_enclave_relax_perm)

/**
* struct sgx_enclave_create - parameter structure for the
@@ -76,6 +78,23 @@ struct sgx_enclave_provision {
__u64 fd;
};

+/**
+ * struct sgx_enclave_relax_perm - parameters for ioctl
+ * %SGX_IOC_ENCLAVE_RELAX_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
+ * @count: (output) bytes successfully changed (multiple of page size)
+ */
+struct sgx_enclave_relax_perm {
+ __u64 offset;
+ __u64 length;
+ __u64 secinfo;
+ __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 b8336d5d9029..9cc6af404bf6 100644
--- a/arch/x86/kernel/cpu/sgx/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/ioctl.c
@@ -698,6 +698,202 @@ static long sgx_ioc_enclave_provision(struct sgx_encl *encl, void __user *arg)
return sgx_set_attribute(&encl->attributes_mask, params.fd);
}

+static unsigned long vm_prot_from_secinfo(u64 secinfo_perm)
+{
+ unsigned long vm_prot;
+
+ vm_prot = _calc_vm_trans(secinfo_perm, SGX_SECINFO_R, PROT_READ) |
+ _calc_vm_trans(secinfo_perm, SGX_SECINFO_W, PROT_WRITE) |
+ _calc_vm_trans(secinfo_perm, SGX_SECINFO_X, PROT_EXEC);
+ vm_prot = calc_vm_prot_bits(vm_prot, 0);
+
+ return vm_prot;
+}
+
+/**
+ * sgx_enclave_relax_perm() - Update OS after permissions relaxed by enclave
+ * @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_relax_perm(struct sgx_encl *encl,
+ struct sgx_enclave_relax_perm *modp,
+ u64 secinfo_perm)
+{
+ struct sgx_encl_page *entry;
+ unsigned long vm_prot;
+ unsigned long addr;
+ unsigned long c;
+ int ret;
+
+ vm_prot = vm_prot_from_secinfo(secinfo_perm);
+
+ for (c = 0 ; c < modp->length; c += PAGE_SIZE) {
+ addr = encl->base + modp->offset + c;
+
+ mutex_lock(&encl->lock);
+
+ entry = xa_load(&encl->page_array, PFN_DOWN(addr));
+ if (!entry) {
+ ret = -EFAULT;
+ goto out_unlock;
+ }
+
+ /*
+ * Changing EPCM permissions is only supported on regular
+ * SGX pages.
+ */
+ if (entry->type != SGX_PAGE_TYPE_REG) {
+ ret = -EINVAL;
+ goto out_unlock;
+ }
+
+ /*
+ * Do not accept permissions that are more relaxed
+ * than vetted permissions.
+ * If this check fails then EPCM permissions may be more
+ * relaxed that what would be allowed by the kernel via
+ * PTEs.
+ */
+ if ((entry->vm_max_prot_bits & vm_prot) != vm_prot) {
+ ret = -EPERM;
+ goto out_unlock;
+ }
+
+ /*
+ * Change runtime protection before zapping PTEs to ensure
+ * any new #PF uses new permissions.
+ */
+ entry->vm_run_prot_bits = vm_prot;
+
+ mutex_unlock(&encl->lock);
+ /*
+ * Do not keep encl->lock because of dependency on
+ * mmap_lock acquired in sgx_zap_enclave_ptes().
+ */
+ sgx_zap_enclave_ptes(encl, addr);
+ }
+
+ ret = 0;
+ goto out;
+
+out_unlock:
+ mutex_unlock(&encl->lock);
+out:
+ modp->count = c;
+
+ return ret;
+}
+
+/*
+ * 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;
+
+ if ((perm & SGX_SECINFO_W) && !(perm & SGX_SECINFO_R))
+ return -EINVAL;
+
+ *secinfo_perm = perm;
+
+ return 0;
+}
+
+/**
+ * sgx_ioc_enclave_relax_perm() - handler for
+ * %SGX_IOC_ENCLAVE_RELAX_PERMISSIONS
+ * @encl: an enclave pointer
+ * @arg: userspace pointer to a &struct sgx_enclave_relax_perm 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 can be relaxed anytime directly from within the enclave
+ * with no visibility from the kernel. This is accomplished with
+ * ENCLU[EMODPE] run from within the enclave. Accessing pages with
+ * the new, relaxed permissions requires the kernel to update the PTE
+ * to handle the subsequent #PF correctly.
+ *
+ * Enclave page permissions are not allowed to exceed the
+ * maximum vetted permissions maintained in
+ * &struct sgx_encl_page->vm_max_prot_bits. If the enclave
+ * exceeds these permissions by running ENCLU[EMODPE] from within the enclave
+ * the kernel will prevent access to the pages via PTE and
+ * VMA permissions.
+ *
+ * Return:
+ * - 0: Success
+ * - -errno: Otherwise
+ */
+static long sgx_ioc_enclave_relax_perm(struct sgx_encl *encl, void __user *arg)
+{
+ struct sgx_enclave_relax_perm params;
+ u64 secinfo_perm;
+ 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;
+
+ ret = sgx_perm_from_user_secinfo((void __user *)params.secinfo,
+ &secinfo_perm);
+ if (ret)
+ return ret;
+
+ if (params.count)
+ return -EINVAL;
+
+ ret = sgx_enclave_relax_perm(encl, &params, secinfo_perm);
+
+ 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;
@@ -719,6 +915,9 @@ 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_RELAX_PERMISSIONS:
+ ret = sgx_ioc_enclave_relax_perm(encl, (void __user *)arg);
+ break;
default:
ret = -ENOIOCTLCMD;
break;
--
2.25.1