Re: [PATCH v26 10/22] x86/sgx: Linux Enclave Driver

[Jordan Hand <jorhand@linux.microsoft.com>]

On 2/20/20 10:13 AM, Sean Christopherson wrote:
> On Tue, Feb 18, 2020 at 07:26:31PM -0800, Jordan Hand wrote:
>> During mprotect (in mm/mprotect.c line 525) the following checks if
>> READ_IMPLIES_EXECUTE and a PROT_READ is being requested. If so and
>> VM_MAYEXEC is set, it also adds PROT_EXEC to the request.
>>
>> 	if (rier && (vma->vm_flags & VM_MAYEXEC))
>> 		prot |= PROT_EXEC;
>>
>> But if we look at sgx_encl_page_alloc(), we see vm_max_prot_bits is set
>> without taking VM_MAYEXEC into account:
>>
>> 	encl_page->vm_max_prot_bits = calc_vm_prot_bits(prot, 0);
>>
>> sgx_encl_may_map() checks that the requested protection can be added with:
>>
>> 	if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
>> 		return -EACCESS
>>
>> This means that for any process where READ_IMPLIES_EXECUTE is set and
>> page where (vma->vm_flags & VM_MAYEXEC) == true, mmap/mprotect calls to
>> that request PROT_READ on a page that was not added with PROT_EXEC will
>> fail.
> 
> I could've sworn this was discussed on the SGX list at one point, but
> apparently we only discussed it internally.  Anyways...
> 
> More than likely, the READ_IMPLIES_EXECUTE (RIE) crud rears its head
> because part of the enclave loader is written in assembly.  Unless
> explicitly told otherwise, the linker assumes that any program with
> assembly code may need an executable stack, which leads to the RIE
> personality being set for the process.  Here's a fantastic write up for
> more details: https://www.airs.com/blog/archives/518
> 
> There are essentially two paths we can take:
> 
>  1) Exempt EPC pages from RIE during mmap()/mprotect(), i.e. don't add
>     PROT_EXEC for enclaves.
> 
>  2) Punt the issue to userspace.
> 
> Option (1) is desirable in some ways:
> 
>   - Enclaves will get an executable stack if and only if the loader/creator
>     intentionally configures it to have an executable stack.
> 
>   - Separates enclaves from the personality of the loader.
> 
>   - Userspace doesn't have to do anything for the common case of not
>     wanting an executable stack for its enclaves.
> 
> The big down side to (1) is that it'd require an ugly hook in architecture
> agnostic code.  And arguably, it reduces the overall security of the
> platform (more below).
> 
> For (2), userspace has a few options:
> 
>  a) Tell the linker the enclave loader doesn't need RIE, either via a .note
>     in assembly files or via the global "-z noexecstack" flag.
> 
>  b) Spawn a separate process to run/map the enclave if the enclave loader
>     needs RIE.
> 
>  c) Require enclaves to allow PROT_EXEC on all pages.  Note, this is an
>     absolutely terrible idea and only included for completeness.
> 
> As shown by the lack of a mmap()/mprotect() hook in this series to squash
> RIE, we chose option (2).  Given that enclave loaders are not legacy code
> and hopefully following decent coding practices, option (2a) should suffice
> for all loaders.  The security benefit mentioned above is that forcing
> enclave loaders to squash RIE eliminates an exectuable stack as an attack
> vector on the loader.

I see your point and I do agree that there are security benefits to (2a)
and I think we could do that for our loader. That said, it does concern
me that this breaks perfectly valid userspace behavior. If a userspace
process decides to use RIE, I don't know that the SGX driver should
disobey that decision.

So option (3) would be to just honor RIE for enclave pages and when page
permissions are set to PROT_READ in sgx_encl_page_alloc and RIE is set,
also add PROT_EXEC.

I understand your concerns that this using RIE is bad security practice
and I'm not convinced that (3) is the way to go, but from a philosophy
perspective I don't know that the kernel should be in the business of
stopping userspace from doing valid things.

If option (3) can't/shouldn't be done for some reason, option (1) at
least keeps from breaking expected userspace behavior. But I do agree
that (1) is ugly to implement.

-Jordan