Re: [PATCH V2 19/32] x86/sgx: Support adding of pages to an initialized enclave

Jarkko Sakkinen
2022-02-20T18:40:20Z

On Sat, Feb 19, 2022 at 01:01:08PM +0100, Jarkko Sakkinen wrote:
> On Sat, Feb 19, 2022 at 12:57:21PM +0100, Jarkko Sakkinen wrote:
> > On Mon, Feb 07, 2022 at 04:45:41PM -0800, Reinette Chatre wrote:
> > > With SGX1 an enclave needs to be created with its maximum memory demands
> > > allocated. Pages cannot be added to an enclave after it is initialized.
> > > SGX2 introduces a new function, ENCLS[EAUG], that can be used to add
> > > pages to an initialized enclave. With SGX2 the enclave still needs to
> > > set aside address space for its maximum memory demands during enclave
> > > creation, but all pages need not be added before enclave initialization.
> > > Pages can be added during enclave runtime.
> > >
> > > Add support for dynamically adding pages to an initialized enclave,
> > > architecturally limited to RW permission. Add pages via the page fault
> > > handler at the time an enclave address without a backing enclave page
> > > is accessed, potentially directly reclaiming pages if no free pages
> > > are available.
> > >
> > > The enclave is still required to run ENCLU[EACCEPT] on the page before
> > > it can be used. A useful flow is for the enclave to run ENCLU[EACCEPT]
> > > on an uninitialized address. This will trigger the page fault handler
> > > that will add the enclave page and return execution to the enclave to
> > > repeat the ENCLU[EACCEPT] instruction, this time successful.
> > >
> > > If the enclave accesses an uninitialized address in another way, for
> > > example by expanding the enclave stack to a page that has not yet been
> > > added, then the page fault handler would add the page on the first
> > > write but upon returning to the enclave the instruction that triggered
> > > the page fault would be repeated and since ENCLU[EACCEPT] was not run
> > > yet it would trigger a second page fault, this time with the SGX flag
> > > set in the page fault error code. This can only be recovered by entering
> > > the enclave again and directly running the ENCLU[EACCEPT] instruction on
> > > the now initialized address.
> > >
> > > Accessing an uninitialized address from outside the enclave also
> > > triggers this flow but the page will remain inaccessible (access will
> > > result in #PF) until accepted from within the enclave via
> > > ENCLU[EACCEPT].
> > >
> > > The page is added with the architecturally constrained RW permissions
> > > as runtime as well as maximum allowed permissions. It is understood that
> > > there are some use cases, for example code relocation, that requires RWX
> > > maximum permissions. Supporting these use cases require guidance from
> > > user space policy before such maximum permissions can be allowed.
> > > Integration with user policy is deferred.
> > >
> > > Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> > > ---
> > > Changes since V1:
> > > - Fix subject line "to initialized" -> "to an initialized" (Jarkko).
> > > - Move text about hardware's PENDING state to the patch that introduces
> > > the ENCLS[EAUG] wrapper (Jarkko).
> > > - Ensure kernel-doc uses brackets when referring to function.
> > >
> > > arch/x86/kernel/cpu/sgx/encl.c | 133 ++++++++++++++++++++++++++++++++
> > > arch/x86/kernel/cpu/sgx/encl.h | 2 +
> > > arch/x86/kernel/cpu/sgx/ioctl.c | 4 +-
> > > 3 files changed, 137 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> > > index a5d4a7efb986..d1e3ea86b902 100644
> > > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > > @@ -124,6 +124,128 @@ struct sgx_encl_page *sgx_encl_load_page(struct sgx_encl *encl,
> > > return entry;
> > > }
> > >
> > > +/**
> > > + * sgx_encl_eaug_page() - Dynamically add page to initialized enclave
> > > + * @vma: VMA obtained from fault info from where page is accessed
> > > + * @encl: enclave accessing the page
> > > + * @addr: address that triggered the page fault
> > > + *
> > > + * When an initialized enclave accesses a page with no backing EPC page
> > > + * on a SGX2 system then the EPC can be added dynamically via the SGX2
> > > + * ENCLS[EAUG] instruction.
> > > + *
> > > + * Returns: Appropriate vm_fault_t: VM_FAULT_NOPAGE when PTE was installed
> > > + * successfully, VM_FAULT_SIGBUS or VM_FAULT_OOM as error otherwise.
> > > + */
> > > +static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
> > > + struct sgx_encl *encl, unsigned long addr)
> > > +{
> > > + struct sgx_pageinfo pginfo = {0};
> > > + struct sgx_encl_page *encl_page;
> > > + struct sgx_epc_page *epc_page;
> > > + struct sgx_va_page *va_page;
> > > + unsigned long phys_addr;
> > > + unsigned long prot;
> > > + vm_fault_t vmret;
> > > + int ret;
> > > +
> > > + if (!test_bit(SGX_ENCL_INITIALIZED, &encl->flags))
> > > + return VM_FAULT_SIGBUS;
> > > +
> > > + encl_page = kzalloc(sizeof(*encl_page), GFP_KERNEL);
> > > + if (!encl_page)
> > > + return VM_FAULT_OOM;
> > > +
> > > + encl_page->desc = addr;
> > > + 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.
> > > + */
> > > + 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;
> > > +
> > > + epc_page = sgx_alloc_epc_page(encl_page, true);
> > > + if (IS_ERR(epc_page)) {
> > > + kfree(encl_page);
> > > + return VM_FAULT_SIGBUS;
> > > + }
> > > +
> > > + va_page = sgx_encl_grow(encl);
> > > + if (IS_ERR(va_page)) {
> > > + ret = PTR_ERR(va_page);
> > > + goto err_out_free;
> > > + }
> > > +
> > > + mutex_lock(&encl->lock);
> > > +
> > > + /*
> > > + * Copy comment from sgx_encl_add_page() to maintain guidance in
> > > + * this similar flow:
> > > + * Adding to encl->va_pages must be done under encl->lock. Ditto for
> > > + * deleting (via sgx_encl_shrink()) in the error path.
> > > + */
> > > + if (va_page)
> > > + list_add(&va_page->list, &encl->va_pages);
> > > +
> > > + ret = xa_insert(&encl->page_array, PFN_DOWN(encl_page->desc),
> > > + encl_page, GFP_KERNEL);
> > > + /*
> > > + * If ret == -EBUSY then page was created in another flow while
> > > + * running without encl->lock
> > > + */
> > > + if (ret)
> > > + goto err_out_unlock;
> > > +
> > > + pginfo.secs = (unsigned long)sgx_get_epc_virt_addr(encl->secs.epc_page);
> > > + pginfo.addr = encl_page->desc & PAGE_MASK;
> > > + pginfo.metadata = 0;
> > > +
> > > + ret = __eaug(&pginfo, sgx_get_epc_virt_addr(epc_page));
> > > + if (ret)
> > > + goto err_out;
> > > +
> > > + encl_page->encl = encl;
> > > + encl_page->epc_page = epc_page;
> > > + encl_page->type = SGX_PAGE_TYPE_REG;
> > > + encl->secs_child_cnt++;
> > > +
> > > + sgx_mark_page_reclaimable(encl_page->epc_page);
> > > +
> > > + phys_addr = sgx_get_epc_phys_addr(epc_page);
> > > + /*
> > > + * Do not undo everything when creating PTE entry fails - next #PF
> > > + * would find page ready for a PTE.
> > > + * PAGE_SHARED because protection is forced to be RW above and COW
> > > + * is not supported.
> > > + */
> > > + vmret = vmf_insert_pfn_prot(vma, addr, PFN_DOWN(phys_addr),
> > > + PAGE_SHARED);
> > > + if (vmret != VM_FAULT_NOPAGE) {
> > > + mutex_unlock(&encl->lock);
> > > + return VM_FAULT_SIGBUS;
> > > + }
> > > + mutex_unlock(&encl->lock);
> > > + return VM_FAULT_NOPAGE;
> > > +
> > > +err_out:
> > > + xa_erase(&encl->page_array, PFN_DOWN(encl_page->desc));
> > > +
> > > +err_out_unlock:
> > > + sgx_encl_shrink(encl, va_page);
> > > + mutex_unlock(&encl->lock);
> > > +
> > > +err_out_free:
> > > + sgx_encl_free_epc_page(epc_page);
> > > + kfree(encl_page);
> > > +
> > > + return VM_FAULT_SIGBUS;
> > > +}
> > > +
> > > static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
> > > {
> > > unsigned long addr = (unsigned long)vmf->address;
> > > @@ -145,6 +267,17 @@ static vm_fault_t sgx_vma_fault(struct vm_fault *vmf)
> > > if (unlikely(!encl))
> > > return VM_FAULT_SIGBUS;
> > >
> > > + /*
> > > + * The page_array keeps track of all enclave pages, whether they
> > > + * are swapped out or not. If there is no entry for this page and
> > > + * the system supports SGX2 then it is possible to dynamically add
> > > + * a new enclave page. This is only possible for an initialized
> > > + * enclave that will be checked for right away.
> > > + */
> > > + if (cpu_feature_enabled(X86_FEATURE_SGX2) &&
> > > + (!xa_load(&encl->page_array, PFN_DOWN(addr))))
> > > + return sgx_encl_eaug_page(vma, encl, addr);
> > > +
> > > mutex_lock(&encl->lock);
> > >
> > > entry = sgx_encl_load_page(encl, addr);
> > > diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h
> > > index 848a28d28d3d..1b6ce1da7c92 100644
> > > --- a/arch/x86/kernel/cpu/sgx/encl.h
> > > +++ b/arch/x86/kernel/cpu/sgx/encl.h
> > > @@ -123,4 +123,6 @@ void sgx_encl_free_epc_page(struct sgx_epc_page *page);
> > > struct sgx_encl_page *sgx_encl_load_page(struct sgx_encl *encl,
> > > unsigned long addr);
> > >
> > > +struct sgx_va_page *sgx_encl_grow(struct sgx_encl *encl);
> > > +void sgx_encl_shrink(struct sgx_encl *encl, struct sgx_va_page *va_page);
> > > #endif /* _X86_ENCL_H */
> > > diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> > > index 23bdf558b231..58ff62a1fb00 100644
> > > --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> > > +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> > > @@ -17,7 +17,7 @@
> > > #include "encl.h"
> > > #include "encls.h"
> > >
> > > -static struct sgx_va_page *sgx_encl_grow(struct sgx_encl *encl)
> > > +struct sgx_va_page *sgx_encl_grow(struct sgx_encl *encl)
> > > {
> > > struct sgx_va_page *va_page = NULL;
> > > void *err;
> > > @@ -43,7 +43,7 @@ static struct sgx_va_page *sgx_encl_grow(struct sgx_encl *encl)
> > > return va_page;
> > > }
> > >
> > > -static void sgx_encl_shrink(struct sgx_encl *encl, struct sgx_va_page *va_page)
> > > +void sgx_encl_shrink(struct sgx_encl *encl, struct sgx_va_page *va_page)
> > > {
> > > encl->page_cnt--;
> > >
> > > --
> > > 2.25.1
> > >
> >
> > Quickly looking through also this sequence is possible:
> >
> > 1. Enclave's run-time flow ignores the whole EACCEPT but instead a memory
> > dereference will initialize the sequence.
> > 2. This causes #PF handler to do EAUG and after the enclave is re-entered
> > the vDSO exists because the page is not EACCEPT'd.
> > 2. Enclave host enter in-enclave exception handler, which does EACCEPT.
> >
> > Can you confirm this? I'm planning to test this patch by implementing EAUG
> > support in Rust for Enarx. At this point I'm not yet sure whether I choose
> > EACCEPT initiated or memory deference initiated code path but I think it is
> > good if the kernel implementation is good enough to support both.
> >
> > Other than that, this looks super solid!
>
> I got my answer:
>
> https://lore.kernel.org/linux-sgx/32c1116934a588bd3e6c174684e3e36a05c0a4d4.1644274683.git.reinette.chatre@intel.com/
>
> I could almost give reviewed-by but I need to write the user space
> implementation first to check that this works for Enarx.

Do you know if it is possible to do EAUG, EMODPR and the do a single
EACCEPT for both? Just looking at pseudo-code, it looked doable but
I need to check this.

I.e. EAUG has this

EPCM(DS:RCX).BLOCKED := 0;
EPCM(DS:RCX).PENDING := 1;
EPCM(DS:RCX).MODIFIED := 0;
EPCM(DS:RCX).PR := 0;
(* associate the EPCPAGE with the SECS by storing the SECS identifier of DS:TMP_SECS *)
Update EPCM(DS:RCX) SECS identifier to reference DS:TMP_SECS identifier;
(* Set EPCM valid fields *)
EPCM(DS:RCX).VALID := 1;

And EMODPR only checks .VALID.

Doing two EACCEPT rounds is a bit rough as you have the page available in a
kind of "stalled' state.

/Jarkko