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

[Jarkko Sakkinen <jarkko@kernel.org>]

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.

BR, Jarkko