Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page permissions

[Haitao Huang]

Hi all,

On Wed, 02 Mar 2022 16:57:45 -0600, Reinette Chatre  
<reinette.chatre@xxxxxxxxx> wrote:

> Hi Jarkko,
>
> On 3/1/2022 6:05 PM, Jarkko Sakkinen wrote:
> > On Tue, Mar 01, 2022 at 09:48:48AM -0800, Reinette Chatre wrote:
> > > Hi Jarkko,
> > >
> > > On 3/1/2022 5:42 AM, Jarkko Sakkinen wrote:
> > > > > With EACCEPTCOPY (kudos to Mark S. for reminding me of this version  
> > > > > of
> > > > > EACCEPT @ chat.enarx.dev) it is possible to make R and RX pages but
> > > > > obviously new RX pages are now out of the picture:
> > > > >
> > > > >
> > > > > 	/*
> > > > > 	 * 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.
> > > > > 	 */
> > > > > 	prot = PROT_READ | PROT_WRITE;
> > > > > 	encl_page->vm_run_prot_bits = calc_vm_prot_bits(prot, 0);
> > > > > 	encl_page->vm_max_prot_bits = encl_page->vm_run_prot_bits;
> > > > >
> > > > > If that TBD is left out to the final version the page augmentation  
> > > > > has a
> > > > > risk of a API bottleneck, and that risk can realize then also in the  
> > > > > page
> > > > > permission ioctls.
> > > > >
> > > > > I.e. now any review comment is based on not fully known territory,  
> > > > > we have
> > > > > one known unknown, and some unknown unknowns from unpredictable  
> > > > > effect to
> > > > > future API changes.
> > >
> > > The plan to complete the "TBD" in the above snippet was to follow this  
> > > work
> > > with user policy integration at this location. On a high level the  
> > > plan was
> > > for this to look something like:
> > >
> > >
> > >  	/*
> > >  	 * Adding a regular page that is architecturally allowed to only
> > >  	 * be created with RW permissions.
> > >  	 * Interface with user space policy to support max permissions
> > >  	 * of RWX.
> > >  	 */
> > >  	prot = PROT_READ | PROT_WRITE;
> > >  	encl_page->vm_run_prot_bits = calc_vm_prot_bits(prot, 0);
> > >
> > >         if (user space policy allows RWX on dynamically added pages)
> > > 	 	encl_page->vm_max_prot_bits = calc_vm_prot_bits(PROT_READ |  
> > > PROT_WRITE | PROT_EXEC, 0);
> > > 	else
> > > 		encl_page->vm_max_prot_bits = calc_vm_prot_bits(PROT_READ |  
> > > PROT_WRITE, 0);
> > >
> > > The work that follows this series aimed to do the integration with user
> > > space policy.
> >
> > What do you mean by "user space policy" anyway exactly? I'm sorry but I
> > just don't fully understand this.
>
> My apologies - I just assumed that you would need no reminder about this  
> contentious
> part of SGX history. Essentially it means that, yes, the kernel could  
> theoretically
> permit any kind of access to any file/page, but some accesses are known  
> to generally
> be a bad idea - like making memory executable as well as writable - and  
> thus there
> are additional checks based on what user space permits before the kernel  
> allows
> such accesses.
>
> For example,
> mm/mprotect.c:do_mprotect_pkey()->security_file_mprotect()
>
> User policy and SGX has seen significant discussion. Some notable  
> threads:
> https://lore.kernel.org/linux-security-module/CALCETrXf8mSK45h7sTK5Wf+pXLVn=Bjsc_RLpgO-h-qdzBRo5Q@xxxxxxxxxxxxxx/
> https://lore.kernel.org/linux-security-module/20190619222401.14942-1-sean.j.christopherson@xxxxxxxxx/
>
> > It's too big of a risk to accept this series without X taken care of.  
> > Patch
> > series should neither have TODO nor TBD comments IMHO. I don't want to  
> > ack
> > a series based on speculation what might happen in the future.
>
> ok
>
> > > > I think the best way to move forward would be to do EAUG's explicitly  
> > > > with
> > > > an ioctl that could also include secinfo for permissions. Then you can
> > > > easily do the rest with EACCEPTCOPY inside the enclave.
> > >
> > > SGX_IOC_ENCLAVE_ADD_PAGES already exists and could possibly be used for
> > > this purpose. It already includes SECINFO which may also be useful if
> > > needing to later support EAUG of PT_SS* pages.
> >
> > You could also simply add SGX_IOC_ENCLAVE_AUGMENT_PAGES and call it a  
> > day.
>
> I could, yes.
>
> > And if there is plan to extend SGX_IOC_ENCLAVE_ADD_PAGES what is this  
> > weird
> > thing added to the #PF handler? Why is it added at all then?
>
> I was just speculating in my response, there is no plan to extend
> SGX_IOC_ENCLAVE_ADD_PAGES (that I am aware of).
>
> > > How this could work is user space calls SGX_IOC_ENCLAVE_ADD_PAGES
> > > after enclave initialization on any memory region within the enclave  
> > > where
> > > pages are planned to be added dynamically. This ioctl() calls EAUG to  
> > > add the
> > > new pages with RW permissions and their vm_max_prot_bits can be set to  
> > > the
> > > permissions found in the included SECINFO. This will support later  
> > > EACCEPTCOPY
> > > as well as SGX_IOC_ENCLAVE_RELAX_PERMISSIONS
> >
> > I don't like this type of re-use of the existing API.
>
> I could proceed with SGX_IOC_ENCLAVE_AUGMENT_PAGES if there is consensus  
> after
> considering the user policy question (above) and performance trade-off  
> (more below).
>
> > > The big question is whether communicating user policy after enclave  
> > > initialization
> > > via the SECINFO within SGX_IOC_ENCLAVE_ADD_PAGES is acceptable to all?  
> > > I would
> > > appreciate a confirmation on this direction considering the  
> > > significant history
> > > behind this topic.
> >
> > I have no idea because I don't know what is user space policy.
>
> This discussion is about some enclave usages needing RWX permissions
> on dynamically added enclave pages. RWX permissions on dynamically added  
> pages is
> not something that should blindly be allowed for all SGX enclaves but  
> instead the user
> needs to explicitly allow specific enclaves to have such ability. This  
> is equivalent
> to (but not the same as) what exists in Linux today with LSM. As seen in
> mm/mprotect.c:do_mprotect_pkey()->security_file_mprotect() Linux is able  
> to make
> files and memory be both writable and executable, but it would only do  
> so for those
> files and memory that the LSM (which is how user policy is communicated,  
> like SELinux)
> indicates it is allowed, not blindly do so for all files and all memory.
>
> > > > Putting EAUG to the #PF handler and implicitly call it just too  
> > > > flakky and
> > > > hard to make deterministic for e.g. JIT compiler in our use case (not  
> > > > to
> > > > mention that JIT is not possible at all because inability to do RX  
> > > > pages).
>
> I understand how SGX_IOC_ENCLAVE_AUGMENT_PAGES can be more deterministic  
> but from
> what I understand it would have a performance impact since it would  
> require all memory
> that may be needed by the enclave be pre-allocated from outside the  
> enclave and not
> just dynamically allocated from within the enclave at the time it is  
> needed.
>
> Would such a performance impact be acceptable?

User space won't always have enough info to decide whether the pages to be  
EAUG'd immediately. In some cases (shared libraries, JVM for example) lots  
of code/data pages can be mapped but never actually touched. One  
enclave/process does not know if any other more important enclave/process  
would need the EPC.

It should be for kernel to make the final decision as it has overall  
picture of the system EPC usage and availability.

User space can provide a hint (similar to MAP_POPULATE) to kernel that the  
mmap'd area will soon be needed and kernel should EAUG as soon as it sees  
fit based on current system usage. Or kernel implement some policy to  
avoid #PF triggered by EACCEPT, for example, if the system has ton of free  
EPC relative to the requested by mmap at the time.

BR
Haitao