linux-kernel - Re: [PATCH] x86/sgx: Enable PROT

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Date:   Mon, 7 Mar 2022 15:39:01 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
CC:     <linux-sgx@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nathaniel McCallum <nathaniel@...fian.com>,
        "H. Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/sgx: Enable PROT_EXEC for EAUG'd pages

Hi Jarkko,

On 3/7/2022 9:22 AM, Jarkko Sakkinen wrote:
> On Mon, Mar 07, 2022 at 09:13:48AM -0800, Reinette Chatre wrote:
>> Hi Jarkko,
>>
>> On 3/7/2022 8:09 AM, Jarkko Sakkinen wrote:
>>> On Mon, Mar 07, 2022 at 06:02:03PM +0200, Jarkko Sakkinen wrote:
>>>> On Mon, Mar 07, 2022 at 05:35:04PM +0200, Jarkko Sakkinen wrote:
>>>>> vm_max_permissions was created to control the pre-initialization content
>>>>> that contributes to MRSIGNATURE. It was never meant to be as a limit to
>>>>> dynamically added pages.
>>>>>
>>>>> E.g. static content could be used as a hook for LSM's to decide whether
>>>>> certain signature is qualified for EINIT. Dynamic content has nothing to
>>>>> do with that. The current mechanisms only add to the complexity on how
>>>>> to control PTE and EPCM permissions, and do not add anything else than
>>>>> obfuscity to security side of things.
>>
>> Linux has mechanisms to enforce what can be executed. For example, with SELinux
>> a process can be required to have PROCESS__EXECHEAP or PROCESS__EXECSTACK 
>> before it can be allowed to execute writable memory.
>>
>> A few SGX runtimes enables unmodified executables to be run within SGX enclaves.
>>
>> Does a change like this not enable executables prevented by existing 
>> security mechanisms to circumvent such restrictions by running within
>> a SGX enclave?
> 
> It does not open any extra exposure as the existing policies apply for
> the enclave content created before initialization.
> 
> And I'm not sure what kind of circumvention scenario we are talking
> about.
> 
>>>>> Thus add PROT_EXEC to the permissions assigned by the #PF handler.
>>>>>
>>>>> Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
>>>>> ---
>>>>>  arch/x86/kernel/cpu/sgx/encl.c | 9 ++++-----
>>>>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
>>>>> index 79e39bd99c09..0256918b2c2f 100644
>>>>> --- a/arch/x86/kernel/cpu/sgx/encl.c
>>>>> +++ b/arch/x86/kernel/cpu/sgx/encl.c
>>>>> @@ -160,12 +160,11 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
>>>>>  	encl_page->encl = encl;
>>>>>  
>>>>>  	/*
>>>>> -	 * Adding a regular page that is architecturally allowed to only
>>>>> -	 * be created with RW permissions.
>>>>> -	 * TBD: Interface with user space policy to support max permissions
>>>>> -	 * of RWX.
>>>>> +	 * Dynamic pages do not contribute to MRSIGNATURE, i.e. they are
>>>>> +	 * controlled only by PTE and EPCM permissions. Thus, the no limit
>>>>> +	 * is set here.
>>>>>  	 */
>>>>> -	prot = PROT_READ | PROT_WRITE;
>>>>> +	prot = PROT_READ | PROT_WRITE | PROT_EXEC;
>>>>>  	encl_page->vm_run_prot_bits = calc_vm_prot_bits(prot, 0);
>>>>>  	encl_page->vm_max_prot_bits = encl_page->vm_run_prot_bits;
>>>>>  
>>>>> -- 
>>>>> 2.35.1
>>>>>
>>>>
>>>> This is really a show stopper. I think here's a logical mistake on for what
>>>> purpose vm_max_prot_bits are used for. They are meant for the static and
>>>> also signed content of the enclave.
>>>>
>>>> These changes in the patch set that are related to vm_max_prot_bits only
>>>> messes up what already exists, and make incredibly hard to implement
>>>> anything decent on top of SGX2 features.
>>>
>>> I.e. once signed content has passed EINIT ioctl, and whatever checks
>>> there are now or in future (e.g. LSM hooks), the system has accepted
>>> the enclave behaviour, and it includes also the use of EACCEPT opcode.
>>>
>>> It's the exec or no-exec decision point. The thing that these patches
>>> do is making an obfuscated mess of all this. When EINIT has passed,
>>> it has been decided that the enclave can do its workload. Let's not
>>> throw stick in front of it, and make everyones life misserable.
>>
>> A common use for these dynamically added pages is to increase the heap
>> and stack. Always allowing PTEs of RWX on these pages irrespective
>> whether it will be used for heap, stack, or relocatable code does
>> not match with how the kernel manages protections.
>>
>> As I said before I am not comfortable with such a change and cannot
>> sign off on this. I would defer to the maintainers to choose the
>> direction.
>>
>> Reinette
> 
> My choice is to not use this existing mechanism for dynamically created
> pages because otherwise the implementation overally is just crippled.
> 
> Something unusable is for sure as secure as you can get.
> 

ok, I will proceed with your guidance here and include your snippet from
this patch into the next version.

One question, regarding "MRSIGNATURE" - did you perhaps mean "MRENCLAVE"?

Reinette