Re: [PATCH V3 15/30] x86/sgx: Support adding of pages to an initialized enclave

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, 2022-04-04 at 09:49 -0700, 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 at creation but allowed to
> obtain RWX permissions after enclave runs EMODPE. 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].
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Changes since V2:
> - Remove runtime tracking of EPCM permissions
>   (sgx_encl_page->vm_run_prot_bits) (Jarkko).
> - Move export of sgx_encl_{grow,shrink}() to separate patch. (Jarkko)
> - Use sgx_encl_page_alloc(). (Jarkko)
> - Set max allowed permissions to be RWX (Jarkko). Update changelog
>   to indicate the change and use comment in code as
>   created by Jarkko in:
> https://lore.kernel.org/linux-sgx/20220306053211.135762-4-jarkko@kernel.org
> - Do not set protection bits but let it be inherited by VMA (Jarkko)
> 
> 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 | 124 +++++++++++++++++++++++++++++++++
>  1 file changed, 124 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 546423753e4c..fa4f947f8496 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -194,6 +194,119 @@ struct sgx_encl_page *sgx_encl_load_page(struct sgx_encl *encl,
>         return __sgx_encl_load_page(encl, 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;
> +       u64 secinfo_flags;
> +       vm_fault_t vmret;
> +       int ret;
> +
> +       if (!test_bit(SGX_ENCL_INITIALIZED, &encl->flags))
> +               return VM_FAULT_SIGBUS;
> +
> +       /*
> +        * Ignore internal permission checking for dynamically added pages.
> +        * They matter only for data added during the pre-initialization
> +        * phase. The enclave decides the permissions by the means of
> +        * EACCEPT, EACCEPTCOPY and EMODPE.
> +        */
> +       secinfo_flags = SGX_SECINFO_R | SGX_SECINFO_W | SGX_SECINFO_X;
> +       encl_page = sgx_encl_page_alloc(encl, addr - encl->base, secinfo_flags);
> +       if (IS_ERR(encl_page))
> +               return VM_FAULT_OOM;
> +
> +       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.
> +        */
> +       vmret = vmf_insert_pfn(vma, addr, PFN_DOWN(phys_addr));
> +       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;
> @@ -213,6 +326,17 @@ 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_in_vma(encl, addr, vma->vm_flags);

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko