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

From: Jarkko Sakkinen
Date: Tue Mar 08 2022 - 04:13:42 EST

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@xxxxxxxxx>
> > > >> ---
> > > >> 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.

BR, Jarkko