Re: [RFC PATCH 9/9] security/selinux: Add enclave_load() implementation
From: Stephen Smalley
Date: Mon Jun 03 2019 - 11:06:02 EST
On 5/31/19 7:31 PM, Sean Christopherson wrote:
The goal of selinux_enclave_load() is to provide a facsimile of the
existing selinux_file_mprotect() and file_map_prot_check() policies,
but tailored to the unique properties of SGX.
For example, an enclave page is technically backed by a MAP_SHARED file,
but the "file" is essentially shared memory that is never persisted
anywhere and also requires execute permissions (for some pages).
The basic concept is to require appropriate execute permissions on the
source of the enclave for pages that are requesting PROT_EXEC, e.g. if
an enclave page is being loaded from a regular file, require
FILE__EXECUTE and/or FILE__EXECMOND, and if it's coming from an
anonymous/private mapping, require PROCESS__EXECMEM since the process
is essentially executing from the mapping, albeit in a roundabout way.
Note, FILE__READ and FILE__WRITE are intentionally not required even if
the source page is backed by a regular file. Writes to the enclave page
are contained to the EPC, i.e. never hit the original file, and read
permissions have already been vetted (or the VMA doesn't have PROT_READ,
in which case loading the page into the enclave will fail).
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
security/selinux/hooks.c | 85 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 85 insertions(+)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 3ec702cf46ca..f436a055dda7 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6726,6 +6726,87 @@ static void selinux_bpf_prog_free(struct bpf_prog_aux *aux)
}
#endif
+#ifdef CONFIG_INTEL_SGX
+int selinux_enclave_load(struct vm_area_struct *vma, unsigned long prot,
+ unsigned long *allowed_prot)
+{
+ const struct cred *cred = current_cred();
+ u32 sid = cred_sid(cred);
+ int rc;
+
+ /* SGX is supported only in 64-bit kernels. */
+ WARN_ON_ONCE(!default_noexec);
+
+ /*
+ * SGX is responsible for checking @prot vs @allowed_prot, and SELinux
+ * only cares about execute related permissions for enclaves.
+ */
+ if (!(*allowed_prot & PROT_EXEC))
+ return 0;
+
+ /*
+ * Loading an executable enclave page from a VMA that is not executable
+ * itself requires EXECUTE permissions on the source file, or if there
+ * is no regular source file, EXECMEM since the page is being loaded
+ * from a non-executable anonymous mapping.
+ */
+ if (!(vma->vm_flags & VM_EXEC)) {
+ if (vma->vm_file && !IS_PRIVATE(file_inode(vma->vm_file)))
+ rc = file_has_perm(cred, vma->vm_file, FILE__EXECUTE);
We might need an EXECMOD check here as well if (vma->vm_file &&
vma->anon_vma). The scenario would be that the host application mapped
the file with PROT_WRITE, modified it, but haven't mapped it PROT_EXEC.
Now the enclave loader requests PROT_EXEC without PROT_WRITE or allows
it. FILE__EXECUTE is insufficient for this case.
+ else
+ rc = avc_has_perm(&selinux_state,
+ sid, sid, SECCLASS_PROCESS,
+ PROCESS__EXECMEM, NULL);
These calls will audit FILE__EXECUTE or PROCESS__EXECMEM denials even
when userspace never asked for PROT_EXEC. Possibly we should use
avc_has_perm_noaudit() and only call avc_audit() if (prot & PROT_EXEC)?
And similarly introduce file_has_perm_noaudit() ->
inode_has_perm_noaudit() -> avc_has_perm_noaudit() or inline here and
switch to avc_has_perm_noaudit() throughout?
+
+ /*
+ * Reject the load if the enclave *needs* the page to be
+ * executable, otherwise prevent it from becoming executable.
+ */
+ if (rc) {
+ if (prot & PROT_EXEC)
+ return rc;
+
+ *allowed_prot &= ~PROT_EXEC;
+ }
+ }
+
+ /*
+ * An enclave page that may do RW->RX or W+X requires EXECMOD (backed
+ * by a regular file) or EXECMEM (loaded from an anonymous mapping).
At present EXECMEM is also triggered for W+X private file mappings, to
allow denying W+X while permitting exceptions for W->X for text relocations.
+ * Note, this hybrid EXECMOD and EXECMEM behavior is intentional and
+ * reflects the nature of enclaves and the EPC, e.g. EPC is effectively
+ * a non-persistent shared file, but each enclave is a private domain
+ * within that shared file, so delegate to the source of the enclave.
+ */
+ if ((*allowed_prot & PROT_EXEC) && (*allowed_prot & PROT_WRITE)) {
+ if (vma->vm_file && !IS_PRIVATE(file_inode(vma->vm_file)))
+ rc = file_has_perm(cred, vma->vm_file, FILE__EXECMOD);
+ else
+ rc = avc_has_perm(&selinux_state,
+ sid, sid, SECCLASS_PROCESS,
+ PROCESS__EXECMEM, NULL);
Same issue wrt auditing here. Could also potentially skip the EXECMEM
check this time if we performed it above (if so, then we must have
passed it because *allowed_prot still had PROT_EXEC set).
+ /*
+ * Clear ALLOW_EXEC instead of ALLOWED_WRITE if permissions are
+ * lacking and @prot has neither PROT_WRITE or PROT_EXEC. If
+ * userspace wanted RX they would have requested RX, and due to
+ * lack of permissions they can never get RW->RX, i.e. the only
+ * useful transition is R->RW.
+ */
+ if (rc) {
+ if ((prot & PROT_EXEC) && (prot & PROT_WRITE))
+ return rc;
+
+ if (prot & PROT_EXEC)
+ *allowed_prot &= ~PROT_WRITE;
+ else
+ *allowed_prot &= ~PROT_EXEC;
+ }
+ }
+
+ return 0;
+}
+#endif
+
struct lsm_blob_sizes selinux_blob_sizes __lsm_ro_after_init = {
.lbs_cred = sizeof(struct task_security_struct),
.lbs_file = sizeof(struct file_security_struct),
@@ -6968,6 +7049,10 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(bpf_map_free_security, selinux_bpf_map_free),
LSM_HOOK_INIT(bpf_prog_free_security, selinux_bpf_prog_free),
#endif
+
+#ifdef CONFIG_INTEL_SGX
+ LSM_HOOK_INIT(enclave_load, selinux_enclave_load),
+#endif
};
static __init int selinux_init(void)