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Message-ID: <ca77e597-69de-9db4-f88a-26881ab53680@tycho.nsa.gov>
Date: Fri, 7 Jun 2019 17:16:01 -0400
From: Stephen Smalley <sds@...ho.nsa.gov>
To: Sean Christopherson <sean.j.christopherson@...el.com>,
Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>,
Cedric Xing <cedric.xing@...el.com>,
James Morris <jmorris@...ei.org>,
"Serge E . Hallyn" <serge@...lyn.com>,
LSM List <linux-security-module@...r.kernel.org>,
Paul Moore <paul@...l-moore.com>,
Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
Jethro Beekman <jethro@...tanix.com>,
Dave Hansen <dave.hansen@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
linux-sgx@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>, nhorman@...hat.com,
npmccallum@...hat.com, Serge Ayoun <serge.ayoun@...el.com>,
Shay Katz-zamir <shay.katz-zamir@...el.com>,
Haitao Huang <haitao.huang@...el.com>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Kai Svahn <kai.svahn@...el.com>,
Borislav Petkov <bp@...en8.de>,
Josh Triplett <josh@...htriplett.org>,
Kai Huang <kai.huang@...el.com>,
David Rientjes <rientjes@...gle.com>,
William Roberts <william.c.roberts@...el.com>,
Philip Tricca <philip.b.tricca@...el.com>
Subject: Re: [RFC PATCH v2 5/5] security/selinux: Add enclave_load()
implementation
On 6/5/19 10:11 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@...el.com>
> ---
> security/selinux/hooks.c | 69 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 69 insertions(+)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 3ec702cf46ca..3c5418edf51c 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6726,6 +6726,71 @@ 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)
> +{
> + const struct cred *cred = current_cred();
> + u32 sid = cred_sid(cred);
> + int ret;
> +
> + /* SGX is supported only in 64-bit kernels. */
> + WARN_ON_ONCE(!default_noexec);
> +
> + /* Only executable enclave pages are restricted in any way. */
> + if (!(prot & PROT_EXEC))
> + return 0;
prot/PROT_EXEC or vmflags/VM_EXEC
> +
> + /*
> + * The source page is exectuable, i.e. has already passed SELinux's
executable
> + * checks, and userspace is not requesting RW->RX capabilities.
Is it requesting W->X or WX?
> + */
> + if ((vma->vm_flags & VM_EXEC) && !(prot & PROT_WRITE))
> + return 0;
> +
> + /*
> + * The source page is not executable, or userspace is requesting the
> + * ability to do a RW->RX conversion. Permissions are required as
> + * follows, in order of increasing privelege:
> + *
> + * EXECUTE - Load an executable enclave page without RW->RX intent from
> + * a non-executable vma that is backed by a shared mapping to
> + * a regular file that has not undergone COW.
Shared mapping or unmodified private file mapping
> + *
> + * EXECMOD - Load an executable enclave page without RW->RX intent from
> + * a non-executable vma that is backed by a shared mapping to
> + * a regular file that *has* undergone COW.
modified private file mapping (write to shared mapping won't trigger
COW; it would have been checked by FILE__WRITE earlier)
> + *
> + * - Load an enclave page *with* RW->RX intent from a shared
> + * mapping to a regular file.
> + *
> + * EXECMEM - Load an exectuable enclave page from an anonymous mapping.
executable
> + *
> + * - Load an exectuable enclave page from a private file, e.g.
executable
> + * from a shared mapping to a hugetlbfs file.
> + *
> + * - Load an enclave page *with* RW->RX intent from a private
W->X or WX?
> + * mapping to a regular file.
> + *
> + * 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 (vma->vm_file && !IS_PRIVATE(file_inode(vma->vm_file) &&
> + ((vma->vm_flags & VM_SHARED) || !(prot & PROT_WRITE)))) {
> + if (!vma->anon_vma && !(prot & PROT_WRITE))
> + ret = file_has_perm(cred, vma->vm_file, FILE__EXECUTE);
> + else
> + ret = file_has_perm(cred, vma->vm_file, FILE__EXECMOD);
> + } else {
> + ret = avc_has_perm(&selinux_state,
> + sid, sid, SECCLASS_PROCESS,
> + PROCESS__EXECMEM, NULL);
> + }
> + return ret;
> +}
> +#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 +7033,10 @@ 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)
>
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