Re: [PATCH 2/2] x86: sgx_vepc: implement SGX_IOC_VEPC_REMOVE_ALL ioctl

Kai Huang
2021-09-20T22:19:14Z

On Mon, 20 Sep 2021 08:54:01 -0400 Paolo Bonzini wrote:
> For bare-metal SGX on real hardware, the hardware provides guarantees
> SGX state at reboot. For instance, all pages start out uninitialized.
> The vepc driver provides a similar guarantee today for freshly-opened
> vepc instances, but guests such as Windows expect all pages to be in
> uninitialized state on startup, including after every guest reboot.
>
> Some userspace implementations of virtual SGX would rather avoid having
> to close and reopen the /dev/sgx_vepc file descriptor and re-mmap the
> virtual EPC. For example, they could be sandboxing themselves before the
> guest starts in order to mitigate exploits from untrusted guests,
> forbidding further calls to open().
>
> Therefore, add a ioctl that does this with EREMOVE. Userspace can
> invoke the ioctl to bring its vEPC pages back to uninitialized state.
> There is a possibility that some pages fail to be removed if they are
> SECS pages, so the ioctl returns the number of EREMOVE failures. The
> correct usage is documented in sgx.rst.
>
> Tested-by: Yang Zhong <yang.zhong@intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> Documentation/x86/sgx.rst | 14 +++++++++++++
> arch/x86/include/uapi/asm/sgx.h | 2 ++
> arch/x86/kernel/cpu/sgx/virt.c | 36 +++++++++++++++++++++++++++++++++
> 3 files changed, 52 insertions(+)
>
> diff --git a/Documentation/x86/sgx.rst b/Documentation/x86/sgx.rst
> index dd0ac96ff9ef..b855db96c6c6 100644
> --- a/Documentation/x86/sgx.rst
> +++ b/Documentation/x86/sgx.rst
> @@ -250,3 +250,17 @@ user wants to deploy SGX applications both on the host and in guests
> on the same machine, the user should reserve enough EPC (by taking out
> total virtual EPC size of all SGX VMs from the physical EPC size) for
> host SGX applications so they can run with acceptable performance.
> +
> +Some guests, such as Windows, require that all pages are uninitialized
> +at startup or after a guest reboot.

I would say this is not requirement of some windows guests, but requirement of
correctly emulating architectural behaviour. On bare-metal EPC is lost on
reboot, so on VM reboot, virtual EPC should be reset too, regardless what
guest VM is.

Because this state can be reached
> +only through the privileged ``ENCLS[EREMOVE]`` instruction, ``/dev/sgx_vepc``
> +provides the ``SGX_IOC_VEPC_REMOVE_ALL`` ioctl to execute the instruction on
> +all pages in the virtual EPC.
> +
> +The ioctl will often not able to remove SECS pages, in case their child
> +pages have not gone through ``EREMOVE`` yet; therefore, the ioctl returns the
> +number of pages that failed to be removed. ``SGX_IOC_VEPC_REMOVE_ALL`` should
> +first be invoked on all the ``/dev/sgx_vepc`` file descriptors mapped
> +into the guest; a second call to the ioctl will be able to remove all
> +leftover pages and will return 0. Any other return value on the second call
> +would be a symptom of a bug in either Linux or the userspace client.

Maybe also worth to mention userspace should guarantee there's no vcpu running
inside guest enclave when resetting guest's virtual EPC.

> diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
> index 9690d6899ad9..f79d84ce8033 100644
> --- a/arch/x86/include/uapi/asm/sgx.h
> +++ b/arch/x86/include/uapi/asm/sgx.h
> @@ -27,6 +27,8 @@ enum sgx_page_flags {
> _IOW(SGX_MAGIC, 0x02, struct sgx_enclave_init)
> #define SGX_IOC_ENCLAVE_PROVISION \
> _IOW(SGX_MAGIC, 0x03, struct sgx_enclave_provision)
> +#define SGX_IOC_VEPC_REMOVE_ALL \
> + _IO(SGX_MAGIC, 0x04)
>
> /**
> * struct sgx_enclave_create - parameter structure for the
> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
> index 59b9c13121cd..81dc186fda2e 100644
> --- a/arch/x86/kernel/cpu/sgx/virt.c
> +++ b/arch/x86/kernel/cpu/sgx/virt.c
> @@ -154,6 +154,24 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
> return 0;
> }
>
> +static long sgx_vepc_remove_all(struct sgx_vepc *vepc)
> +{
> + struct sgx_epc_page *entry;
> + unsigned long index;
> + long failures = 0;
> +
> + xa_for_each(&vepc->page_array, index, entry)
> + if (sgx_vepc_remove_page(entry))
> + failures++;

I think need to hold vepc->lock?

> +
> + /*
> + * Return the number of pages that failed to be removed, so
> + * userspace knows that there are still SECS pages lying
> + * around.
> + */
> + return failures;
> +}
> +
> static int sgx_vepc_release(struct inode *inode, struct file *file)
> {
> struct sgx_vepc *vepc = file->private_data;
> @@ -239,9 +257,27 @@ static int sgx_vepc_open(struct inode *inode, struct file *file)
> return 0;
> }
>
> +static long sgx_vepc_ioctl(struct file *file,
> + unsigned int cmd, unsigned long arg)
> +{
> + struct sgx_vepc *vepc = file->private_data;
> +
> + switch (cmd) {
> + case SGX_IOC_VEPC_REMOVE_ALL:
> + if (arg)
> + return -EINVAL;
> + return sgx_vepc_remove_all(vepc);
> +
> + default:
> + return -ENOTTY;
> + }
> +}
> +
> static const struct file_operations sgx_vepc_fops = {
> .owner = THIS_MODULE,
> .open = sgx_vepc_open,
> + .unlocked_ioctl = sgx_vepc_ioctl,
> + .compat_ioctl = sgx_vepc_ioctl,
> .release = sgx_vepc_release,
> .mmap = sgx_vepc_mmap,
> };
> --
> 2.27.0
>