Re: [PATCH V2 06/32] x86/sgx: Support VMA permissions more relaxed than enclave permissions

Jarkko Sakkinen
2022-03-08T18:47:43Z

On Tue, Mar 08, 2022 at 09:49:01AM -0800, Reinette Chatre wrote:
> Hi Jarkko,
>
> On 3/8/2022 9:00 AM, Jarkko Sakkinen wrote:
> > On Tue, Mar 08, 2022 at 08:04:33AM -0800, Reinette Chatre wrote:
> >> Hi Jarkko,
> >>
> >> On 3/8/2022 1:12 AM, Jarkko Sakkinen wrote:
> >>> On Tue, Mar 08, 2022 at 11:06:46AM +0200, Jarkko Sakkinen wrote:
> >>>> On Tue, Mar 08, 2022 at 10:14:42AM +0200, Jarkko Sakkinen wrote:
> >>>>> On Mon, Mar 07, 2022 at 09:36:36AM -0800, Reinette Chatre wrote:
> >>>>>> Hi Jarkko,
> >>>>>>
> >>>>>> On 3/7/2022 9:10 AM, Jarkko Sakkinen wrote:
> >>>>>>> On Mon, Feb 07, 2022 at 04:45:28PM -0800, Reinette Chatre wrote:
> >>>>>>>> === Summary ===
> >>>>>>>>
> >>>>>>>> An SGX VMA can only be created if its permissions are the same or
> >>>>>>>> weaker than the Enclave Page Cache Map (EPCM) permissions. After VMA
> >>>>>>>> creation this same rule is again enforced by the page fault handler:
> >>>>>>>> faulted enclave pages are required to have equal or more relaxed
> >>>>>>>> EPCM permissions than the VMA permissions.
> >>>>>>>>
> >>>>>>>> On SGX1 systems the additional enforcement in the page fault handler
> >>>>>>>> is redundant and on SGX2 systems it incorrectly prevents access.
> >>>>>>>> On SGX1 systems it is unnecessary to repeat the enforcement of the
> >>>>>>>> permission rule. The rule used during original VMA creation will
> >>>>>>>> ensure that any access attempt will use correct permissions.
> >>>>>>>> With SGX2 the EPCM permissions of a page can change after VMA
> >>>>>>>> creation resulting in the VMA permissions potentially being more
> >>>>>>>> relaxed than the EPCM permissions and the page fault handler
> >>>>>>>> incorrectly blocking valid access attempts.
> >>>>>>>>
> >>>>>>>> Enable the VMA's pages to remain accessible while ensuring that
> >>>>>>>> the PTEs are installed to match the EPCM permissions but not be
> >>>>>>>> more relaxed than the VMA permissions.
> >>>>>>>>
> >>>>>>>> === Full Changelog ===
> >>>>>>>>
> >>>>>>>> An SGX enclave is an area of memory where parts of an application
> >>>>>>>> can reside. First an enclave is created and loaded (from
> >>>>>>>> non-enclave memory) with the code and data of an application,
> >>>>>>>> then user space can map (mmap()) the enclave memory to
> >>>>>>>> be able to enter the enclave at its defined entry points for
> >>>>>>>> execution within it.
> >>>>>>>>
> >>>>>>>> The hardware maintains a secure structure, the Enclave Page Cache Map
> >>>>>>>> (EPCM), that tracks the contents of the enclave. Of interest here is
> >>>>>>>> its tracking of the enclave page permissions. When a page is loaded
> >>>>>>>> into the enclave its permissions are specified and recorded in the
> >>>>>>>> EPCM. In parallel the kernel maintains permissions within the
> >>>>>>>> page table entries (PTEs) and the rule is that PTE permissions
> >>>>>>>> are not allowed to be more relaxed than the EPCM permissions.
> >>>>>>>>
> >>>>>>>> A new mapping (mmap()) of enclave memory can only succeed if the
> >>>>>>>> mapping has the same or weaker permissions than the permissions that
> >>>>>>>> were vetted during enclave creation. This is enforced by
> >>>>>>>> sgx_encl_may_map() that is called on the mmap() as well as mprotect()
> >>>>>>>> paths. This rule remains.
> >>>>>>>>
> >>>>>>>> One feature of SGX2 is to support the modification of EPCM permissions
> >>>>>>>> after enclave initialization. Enclave pages may thus already be part
> >>>>>>>> of a VMA at the time their EPCM permissions are changed resulting
> >>>>>>>> in the VMA's permissions potentially being more relaxed than the EPCM
> >>>>>>>> permissions.
> >>>>>>>>
> >>>>>>>> Allow permissions of existing VMAs to be more relaxed than EPCM
> >>>>>>>> permissions in preparation for dynamic EPCM permission changes
> >>>>>>>> made possible in SGX2. New VMAs that attempt to have more relaxed
> >>>>>>>> permissions than EPCM permissions continue to be unsupported.
> >>>>>>>>
> >>>>>>>> Reasons why permissions of existing VMAs are allowed to be more relaxed
> >>>>>>>> than EPCM permissions instead of dynamically changing VMA permissions
> >>>>>>>> when EPCM permissions change are:
> >>>>>>>> 1) Changing VMA permissions involve splitting VMAs which is an
> >>>>>>>> operation that can fail. Additionally changing EPCM permissions of
> >>>>>>>> a range of pages could also fail on any of the pages involved.
> >>>>>>>> Handling these error cases causes problems. For example, if an
> >>>>>>>> EPCM permission change fails and the VMA has already been split
> >>>>>>>> then it is not possible to undo the VMA split nor possible to
> >>>>>>>> undo the EPCM permission changes that did succeed before the
> >>>>>>>> failure.
> >>>>>>>> 2) The kernel has little insight into the user space where EPCM
> >>>>>>>> permissions are controlled from. For example, a RW page may
> >>>>>>>> be made RO just before it is made RX and splitting the VMAs
> >>>>>>>> while the VMAs may change soon is unnecessary.
> >>>>>>>>
> >>>>>>>> Remove the extra permission check called on a page fault
> >>>>>>>> (vm_operations_struct->fault) or during debugging
> >>>>>>>> (vm_operations_struct->access) when loading the enclave page from swap
> >>>>>>>> that ensures that the VMA permissions are not more relaxed than the
> >>>>>>>> EPCM permissions. Since a VMA could only exist if it passed the
> >>>>>>>> original permission checks during mmap() and a VMA may indeed
> >>>>>>>> have more relaxed permissions than the EPCM permissions this extra
> >>>>>>>> permission check is no longer appropriate.
> >>>>>>>>
> >>>>>>>> With the permission check removed, ensure that PTEs do
> >>>>>>>> not blindly inherit the VMA permissions but instead the permissions
> >>>>>>>> that the VMA and EPCM agree on. PTEs for writable pages (from VMA
> >>>>>>>> and enclave perspective) are installed with the writable bit set,
> >>>>>>>> reducing the need for this additional flow to the permission mismatch
> >>>>>>>> cases handled next.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> >>>>>>>> ---
> >>>>>>>> Changes since V1:
> >>>>>>>> - Reword commit message (Jarkko).
> >>>>>>>> - Use "relax" instead of "exceed" when referring to permissions (Dave).
> >>>>>>>> - Add snippet to Documentation/x86/sgx.rst that highlights the
> >>>>>>>> relationship between VMA, EPCM, and PTE permissions on SGX
> >>>>>>>> systems (Andy).
> >>>>>>>>
> >>>>>>>> Documentation/x86/sgx.rst | 10 +++++++++
> >>>>>>>> arch/x86/kernel/cpu/sgx/encl.c | 38 ++++++++++++++++++----------------
> >>>>>>>> 2 files changed, 30 insertions(+), 18 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/Documentation/x86/sgx.rst b/Documentation/x86/sgx.rst
> >>>>>>>> index 89ff924b1480..5659932728a5 100644
> >>>>>>>> --- a/Documentation/x86/sgx.rst
> >>>>>>>> +++ b/Documentation/x86/sgx.rst
> >>>>>>>> @@ -99,6 +99,16 @@ The relationships between the different permission masks are:
> >>>>>>>> * PTEs are installed to match the EPCM permissions, but not be more
> >>>>>>>> relaxed than the VMA permissions.
> >>>>>>>>
> >>>>>>>> +On systems supporting SGX2 EPCM permissions may change while the
> >>>>>>>> +enclave page belongs to a VMA without impacting the VMA permissions.
> >>>>>>>> +This means that a running VMA may appear to allow access to an enclave
> >>>>>>>> +page that is not allowed by its EPCM permissions. For example, when an
> >>>>>>>> +enclave page with RW EPCM permissions is mapped by a RW VMA but is
> >>>>>>>> +subsequently changed to have read-only EPCM permissions. The kernel
> >>>>>>>> +continues to maintain correct access to the enclave page through the
> >>>>>>>> +PTE that will ensure that only access allowed by both the VMA
> >>>>>>>> +and EPCM permissions are permitted.
> >>>>>>>> +
> >>>>>>>> Application interface
> >>>>>>>> =====================
> >>>>>>>>
> >>>>>>>> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> >>>>>>>> index 48afe96ae0f0..b6105d9e7c46 100644
> >>>>>>>> --- a/arch/x86/kernel/cpu/sgx/encl.c
> >>>>>>>> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> >>>>>>>> @@ -91,10 +91,8 @@ static struct sgx_epc_page *sgx_encl_eldu(struct sgx_encl_page *encl_page,
> >>>>>>>> }
> >>>>>>>>
> >>>>>>>> static struct sgx_encl_page *sgx_encl_load_page(struct sgx_encl *encl,
> >>>>>>>> - unsigned long addr,
> >>>>>>>> - unsigned long vm_flags)
> >>>>>>>> + unsigned long addr)
> >>>>>>>> {
> >>>>>>>> - unsigned long vm_prot_bits = vm_flags & (VM_READ | VM_WRITE | VM_EXEC);
> >>>>>>>> struct sgx_epc_page *epc_page;
> >>>>>>>> struct sgx_encl_page *entry;
> >>>>>>>>
> >>>>>>>> @@ -102,14 +100,6 @@ static struct sgx_encl_page *sgx_encl_load_page(struct sgx_encl *encl,
> >>>>>>>> if (!entry)
> >>>>>>>> return ERR_PTR(-EFAULT);
> >>>>>>>>
> >>>>>>>> - /*
> >>>>>>>> - * Verify that the faulted page has equal or higher build time
> >>>>>>>> - * permissions than the VMA permissions (i.e. the subset of {VM_READ,
> >>>>>>>> - * VM_WRITE, VM_EXECUTE} in vma->vm_flags).
> >>>>>>>> - */
> >>>>>>>> - if ((entry->vm_max_prot_bits & vm_prot_bits) != vm_prot_bits)
> >>>>>>>> - return ERR_PTR(-EFAULT);
> >>>>>>>> -
> >>>>>>>> /* Entry successfully located. */
> >>>>>>>> if (entry->epc_page) {
> >>>>>>>> if (entry->desc & SGX_ENCL_PAGE_BEING_RECLAIMED)
> >>>>>>>> @@ -138,7 +128,9 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
> >>>>>>>> {
> >>>>>>>> unsigned long addr = (unsigned long)vmf->address;
> >>>>>>>> struct vm_area_struct *vma = vmf->vma;
> >>>>>>>> + unsigned long page_prot_bits;
> >>>>>>>> struct sgx_encl_page *entry;
> >>>>>>>> + unsigned long vm_prot_bits;
> >>>>>>>> unsigned long phys_addr;
> >>>>>>>> struct sgx_encl *encl;
> >>>>>>>> vm_fault_t ret;
> >>>>>>>> @@ -155,7 +147,7 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
> >>>>>>>>
> >>>>>>>> mutex_lock(&encl->lock);
> >>>>>>>>
> >>>>>>>> - entry = sgx_encl_load_page(encl, addr, vma->vm_flags);
> >>>>>>>> + entry = sgx_encl_load_page(encl, addr);
> >>>>>>>> if (IS_ERR(entry)) {
> >>>>>>>> mutex_unlock(&encl->lock);
> >>>>>>>
> >>>>>>>> @@ -167,7 +159,19 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
> >>>>>>>>
> >>>>>>>> phys_addr = sgx_get_epc_phys_addr(entry->epc_page);
> >>>>>>>>
> >>>>>>>> - ret = vmf_insert_pfn(vma, addr, PFN_DOWN(phys_addr));
> >>>>>>>> + /*
> >>>>>>>> + * Insert PTE to match the EPCM page permissions ensured to not
> >>>>>>>> + * exceed the VMA permissions.
> >>>>>>>> + */
> >>>>>>>> + vm_prot_bits = vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC);
> >>>>>>>> + page_prot_bits = entry->vm_max_prot_bits & vm_prot_bits;
> >>>>>>>> + /*
> >>>>>>>> + * Add VM_SHARED so that PTE is made writable right away if VMA
> >>>>>>>> + * and EPCM are writable (no COW in SGX).
> >>>>>>>> + */
> >>>>>>>> + page_prot_bits |= (vma->vm_flags & VM_SHARED);
> >>>>>>>> + ret = vmf_insert_pfn_prot(vma, addr, PFN_DOWN(phys_addr),
> >>>>>>>> + vm_get_page_prot(page_prot_bits));
> >>>>>>>> if (ret != VM_FAULT_NOPAGE) {
> >>>>>>>> mutex_unlock(&encl->lock);
> >>>>>>>>
> >>>>>>>> @@ -295,15 +299,14 @@ static int sgx_encl_debug_write(struct sgx_encl *encl, struct sgx_encl_page *pag
> >>>>>>>> * Load an enclave page to EPC if required, and take encl->lock.
> >>>>>>>> */
> >>>>>>>> static struct sgx_encl_page *sgx_encl_reserve_page(struct sgx_encl *encl,
> >>>>>>>> - unsigned long addr,
> >>>>>>>> - unsigned long vm_flags)
> >>>>>>>> + unsigned long addr)
> >>>>>>>> {
> >>>>>>>> struct sgx_encl_page *entry;
> >>>>>>>>
> >>>>>>>> for ( ; ; ) {
> >>>>>>>> mutex_lock(&encl->lock);
> >>>>>>>>
> >>>>>>>> - entry = sgx_encl_load_page(encl, addr, vm_flags);
> >>>>>>>> + entry = sgx_encl_load_page(encl, addr);
> >>>>>>>> if (PTR_ERR(entry) != -EBUSY)
> >>>>>>>> break;
> >>>>>>>>
> >>>>>>>> @@ -339,8 +342,7 @@ static int sgx_vma_access(struct vm_area_struct *vma, unsigned long addr,
> >>>>>>>> return -EFAULT;
> >>>>>>>>
> >>>>>>>> for (i = 0; i < len; i += cnt) {
> >>>>>>>> - entry = sgx_encl_reserve_page(encl, (addr + i) & PAGE_MASK,
> >>>>>>>> - vma->vm_flags);
> >>>>>>>> + entry = sgx_encl_reserve_page(encl, (addr + i) & PAGE_MASK);
> >>>>>>>> if (IS_ERR(entry)) {
> >>>>>>>> ret = PTR_ERR(entry);
> >>>>>>>> break;
> >>>>>>>> --
> >>>>>>>> 2.25.1
> >>>>>>>>
> >>>>>>>
> >>>>>>> If you unconditionally set vm_max_prot_bits to RWX for dynamically created
> >>>>>>> pags, you would not need to do this.
> >>>>>>>
> >>>>>>> These patches could be then safely dropped then:
> >>>>>>>
> >>>>>>> - [PATCH V2 06/32] x86/sgx: Support VMA permissions more relaxed than enclave permissions
> >>>>>>> - [PATCH V2 08/32] x86/sgx: x86/sgx: Add sgx_encl_page->vm_run_prot_bits for dynamic permission changes
> >>>>>>> - [PATCH V2 15/32] x86/sgx: Support relaxing of enclave page permissions
> >>>>>>>
> >>>>>>> And that would also keep full ABI compatibility without exceptions to the
> >>>>>>> existing mainline code.
> >>>>>>>
> >>>>>>
> >>>>>> Dropping these changes do not just impact dynamically created pages. Dropping
> >>>>>> these patches would result in EPCM page permission restriction being supported
> >>>>>> for all pages, those added before enclave initialization as well as dynamically
> >>>>>> added pages, but their PTEs will not be impacted.
> >>>>>>
> >>>>>> For example, if a RW enclave page is added via SGX_IOC_ENCLAVE_ADD_PAGES and
> >>>>>> then later made read-only via SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS then Linux
> >>>>>> would keep allowing and installing RW PTEs to this page.
> >>>>>
> >>>>> I think that would be perfectly fine, if someone wants to do that. There is
> >>>>> no corrateral damage on doing that. Kernel does not get messed because of
> >>>>> that. It's a use case that does not make sense in the first place, so it'd
> >>>>> be stupid to build anything extensive around it to the kernel.
> >>>>>
> >>>>> Shooting yourself to the foot is something that kernel does and should not
> >>>>> protect user space from unless there is a risk of messing the state of the
> >>>>> kernel itself.
> >>>>>
> >>>>> Much worse is that we have e.g. completely artificial ioctl
> >>>>> SGX_IOC_ENCLAVE_RELAX_PERMISSIONS to support this scheme, which could e.g.
> >>>>> cause extra roundtrips for simple EMODPE.
> >>>>>
> >>>>> Also this means not having to include 06/32, which keeps 100% backwards
> >>>>> compatibility in run-time behaviour to the mainline while not restricting
> >>>>> at all dynamically created pages. And we get rid of complex book keeping
> >>>>> of vm_run_prot_bits.
> >>>>>
> >>>>> And generally the whole model is then very easy to understand and explain.
> >>>>> If I had to keep presentation of the current mess in the patch set in a
> >>>>> conference, I can honestly say that I would be in serious trouble. It's
> >>>>> not clean and clear security model, which is a risk by itself.
> >>>>
> >>>> I.e.
> >>>>
> >>>> 1. For EADD'd pages: stick what has been the invariant 1,5 years now. Do
> >>>> not change it by any means (e.g. 06/32).
> >>>> 2. For EAUG'd pages: set vm_max_prot_bits RWX, which essentially means do
> >>>> what ever you want with PTE's and EPCM.
> >>>>
> >>>> It's a clear and understandable model that does nothing bad to the kernel,
> >>>> and a run-time developer can surely find away to get things on going. For
> >>>> user space, the most important thing is the clarity in kernel behaviour,
> >>>> and this does deliver that clarity. It's not perfect but it does do the
> >>>> job and anyone can get it.
> >>>
> >>> Also a quantitive argument for this is that by simplifying security model
> >>> this way it is one ioctl less, which must be considered as +1. We do not
> >>> want to add new ioctls unless it is something we absolutely cannnot live
> >>> without. We absolutely can live without SGX_IOC_ENCLAVE_RELAX_PERMISSIONS.
> >>>
> >>
> >> ok, with the implications understood and accepted I will proceed with a new
> >> series that separates EPCM from PTEs and make RWX PTEs possible by default
> >> for EAUG pages. This has broader impact than just removing
> >> the three patches you list. "[PATCH 07/32] x86/sgx: Add pfn_mkwrite() handler
> >> for present PTEs" is also no longer needed and there is no longer a need
> >> to flush PTEs after restricting permissions. New changes also need to
> >> be considered - at least the current documentation. I'll rework the series.
> >
> > Yes, I really think it is a solid plan. Any possible LSM hooks would most
> > likely attach to build product, not the dynamic behaviour.
> >
> > As far as the page fault handler goes, Haitao is correct after the all
> > discussions that it makes sense. The purpose of MAP_POPULATE series is
> > not to replace it but instead complement it. Just wanted to clear this
> > up as I said otherwise earlier this week.
> >
>
> Understood. I will keep the implementation where EAUG is done in page fault
> handler. I do plan to pick up your patch "x86/sgx: Export sgx_encl_page_alloc()"
> since a consequence of the other changes is that this can now be shared.

Yeah, I think we might be able to get this polished for v5.19. I'd expect
a revision or few for polishing the corners but other than that this looks
to be going on right tracks now.

> Reinette

BR, Jarkko