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

[Haitao Huang]

On Thu, 03 Mar 2022 17:18:33 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx>  
wrote:

> On Thu, Mar 03, 2022 at 10:08:14AM -0600, Haitao Huang wrote:
> > 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.
>
> EAUG ioctl does not give better capabilities for user space to waste
> EPC given that EADD ioctl already exists, i.e. your argument is logically
> incorrect.

The point of adding EAUG is to allow more efficient use of EPC pages.  
Without EAUG, enclaves have to EADD everything upfront into EPC, consuming  
predetermined number of EPC pages, some of which may not be used at all.  
With EAUG, enclaves should be able to load minimal pages to get started,  
pages added on #PF as they are actually accessed.

Obviously as you pointed out, some usages make more sense to pre-EAUG  
(EAUG before #PF). But your proposal of supporting only pre-EAUG here  
essentially makes EAUG behave almost the same as EADD.  If the current  
implementation with EAUG on #PF can also use MAP_POPULATE for pre-EAUG  
(seems possible based on Dave's comments), then it is flxible to cover all  
cases and allow kernel to optimize allocation of EPC pages.

Thanks
Haitao