Re: [RFC PATCH 8/9] LSM: x86/sgx: Introduce ->enclave_load() hook for Intel SGX

[Stephen Smalley <sds@tycho.nsa.gov>]

On 6/3/19 10:42 AM, Sean Christopherson wrote:
> On Mon, Jun 03, 2019 at 10:19:18AM -0400, Stephen Smalley wrote:
>> On 5/31/19 7:31 PM, Sean Christopherson wrote:
>>> enclave_load() is roughly analogous to the existing file_mprotect().
>>>
>>> Due to the nature of SGX and its Enclave Page Cache (EPC), all enclave
>>> VMAs are backed by a single file, i.e. /dev/sgx/enclave, that must be
>>> MAP_SHARED.  Furthermore, all enclaves need read, write and execute
>>> VMAs.  As a result, file_mprotect() does not provide any meaningful
>>> security for enclaves since an LSM can only deny/grant access to the
>>> EPC as a whole.
>>>
>>> security_enclave_load() is called when SGX is first loading an enclave
>>> page, i.e. copying a page from normal memory into the EPC.  The notable
>>> difference from file_mprotect() is the allowed_prot parameter, which
>>> is essentially an SGX-specific version of a VMA's MAY_{READ,WRITE,EXEC}
>>> flags.  The purpose of allowed_prot is to enable checks such as
>>> SELinux's FILE__EXECMOD permission without having to track and update
>>> VMAs across multiple mm structs, i.e. SGX can ensure userspace doesn't
>>> overstep its bounds simply by restricting an enclave VMA's protections
>>> by vetting what is maximally allowed during build time.
>>>
>>> An alternative to the allowed_prot approach would be to use an enclave's
>>> SIGSTRUCT (a smallish structure that can uniquely identify an enclave)
>>> as a proxy for the enclave.  For example, SGX could take and hold a
>>> reference to the file containing the SIGSTRUCT (if it's in a file) and
>>> call security_enclave_load() during mprotect().  While the SIGSTRUCT
>>> approach would provide better precision, the actual value added was
>>> deemed to be negligible.  On the other hand, pinning a file for the
>>> lifetime of the enclave is ugly, and essentially caching LSM policies
>>> in each page's allowed_prot avoids having to make an extra LSM upcall
>>> during mprotect().
>>>
>>> Note, extensive discussion yielded no sane alternative to some form of
>>> SGX specific LSM hook[1].
>>>
>>> [1] https://lkml.kernel.org/r/CALCETrXf8mSK45h7sTK5Wf+pXLVn=Bjsc_RLpgO-h-qdzBRo5Q@mail.gmail.com
>>>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>>> ---
>>>   arch/x86/kernel/cpu/sgx/driver/ioctl.c | 14 +++++++++-----
>>>   include/linux/lsm_hooks.h              | 16 ++++++++++++++++
>>>   include/linux/security.h               |  2 ++
>>>   security/security.c                    |  8 ++++++++
>>>   4 files changed, 35 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/cpu/sgx/driver/ioctl.c b/arch/x86/kernel/cpu/sgx/driver/ioctl.c
>>> index 5f71be7cbb01..260417ecbcff 100644
>>> --- a/arch/x86/kernel/cpu/sgx/driver/ioctl.c
>>> +++ b/arch/x86/kernel/cpu/sgx/driver/ioctl.c
>>> @@ -8,6 +8,7 @@
>>>   #include <linux/highmem.h>
>>>   #include <linux/ratelimit.h>
>>>   #include <linux/sched/signal.h>
>>> +#include <linux/security.h>
>>>   #include <linux/shmem_fs.h>
>>>   #include <linux/slab.h>
>>>   #include <linux/suspend.h>
>>> @@ -580,21 +581,24 @@ static int sgx_encl_page_protect(unsigned long src, unsigned long prot,
>>>   				 unsigned long *allowed_prot)
>>>   {
>>>   	struct vm_area_struct *vma;
>>> +	int ret = 0;
>>> -	if (!(*allowed_prot & VM_EXEC))
>>> +	if (!(*allowed_prot & VM_EXEC) && !IS_ENABLED(CONFIG_SECURITY))
>>>   		goto do_check;
>>>   	down_read(&current->mm->mmap_sem);
>>>   	vma = find_vma(current->mm, src);
>>>   	if (!vma || (vma->vm_file && path_noexec(&vma->vm_file->f_path)))
>>>   		*allowed_prot &= ~VM_EXEC;
>>> +#ifdef CONFIG_SECURITY
>>> +	ret = security_enclave_load(vma, prot, allowed_prot);
>>> +#endif
>>
>> Normally you'd define a static inline stub for the hook in the #else clause
>> for CONFIG_SECURITY in include/linux/security.h and avoid any ifdef here.
> 
> Ah, right.
>   
>> What ensures that the mapping referenced by src can't be changed to an
>> entirely different one (with a different vm_file) between the time of check
>> (here) and the time of use?
> 
> Nothing.  Holding mmap_sem across copy_from_user() would suffice, correct?

I don't believe you can do that; copy_from_user() could stall 
indefinitely.  Not sure how to do what you want here or if it requires 
changing the interface.

> 
>>>   	up_read(&current->mm->mmap_sem);
>>>   do_check:
>>> -	if (prot & ~*allowed_prot)
>>> -		return -EACCES;
>>> -
>>> -	return 0;
>>> +	if (!ret && (prot & ~*allowed_prot))
>>> +		ret = -EACCES;
>>> +	return ret;
>>>   }
>>>   static int sgx_encl_add_page(struct sgx_encl *encl, unsigned long addr,