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Date: Tue, 12 Oct 2021 19:57:57 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org
Cc: dave.hansen@...ux.intel.com, seanjc@...gle.com, x86@...nel.org,
yang.zhong@...el.com
Subject: Re: [PATCH v2 2/2] x86: sgx_vepc: implement SGX_IOC_VEPC_REMOVE
ioctl
On Tue, 2021-10-12 at 06:57 -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 sandbox themselves after the guest
> starts and forbid further calls to open(), in order to mitigate exploits
> from untrusted guests.
>
> 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, and the child and SECS pages could be in separate vEPC
> regions. Therefore, the ioctl returns the number of EREMOVE failures,
> telling userspace to try the ioctl again after it's done with all
> vEPC regions. A more verbose description of the correct usage and
> the possible error conditions is documented in sgx.rst.
>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> v1->v2: return EBUSY for SGX_ENCLAVE_ACT, adjust docs
> add cond_resched
>
> Documentation/x86/sgx.rst | 26 +++++++++++++++
> arch/x86/include/asm/sgx.h | 3 ++
> arch/x86/include/uapi/asm/sgx.h | 2 ++
> arch/x86/kernel/cpu/sgx/virt.c | 57 +++++++++++++++++++++++++++++++++
> 4 files changed, 88 insertions(+)
>
> diff --git a/Documentation/x86/sgx.rst b/Documentation/x86/sgx.rst
> index dd0ac96ff9ef..7bc9c3b297d6 100644
> --- a/Documentation/x86/sgx.rst
> +++ b/Documentation/x86/sgx.rst
> @@ -250,3 +250,29 @@ 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.
> +
> +Architectural behavior is to restore all EPC pages to an uninitialized
> +state also after a guest reboot. 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.
> +
> +``EREMOVE`` can fail for two reasons, which Linux relays to userspace
> +in a different manner:
> +
> +1. Page removal will always fail when any thread is running in the
> + enclave to which the page belongs. In this case the ioctl will
> + return ``EBUSY`` independent of whether it has successfully removed
> + some pages; userspace can avoid these failures by preventing execution
> + of any vcpu which maps the virtual EPC.
> +
> +2) Page removal will also fail for SGX "SECS" metadata pages which still
> + have child pages. Child pages can be removed by executing
> + ``SGX_IOC_VEPC_REMOVE_ALL`` on all ``/dev/sgx_vepc`` file descriptors
> + mapped into the guest. This means that the ioctl() must be called
> + twice: an initial set of calls to remove child pages and a subsequent
> + set of calls to remove SECS pages. The second set of calls is only
> + required for those mappings that returned a nonzero value from the
> + first call. It indicates a bug in the kernel or the userspace client
> + if any of the second round of ``SGX_IOC_VEPC_REMOVE_ALL`` calls has
> + a return code other than 0.
> diff --git a/arch/x86/include/asm/sgx.h b/arch/x86/include/asm/sgx.h
> index 05f3e21f01a7..2e5d8c97655e 100644
> --- a/arch/x86/include/asm/sgx.h
> +++ b/arch/x86/include/asm/sgx.h
> @@ -50,6 +50,8 @@ enum sgx_encls_function {
> * %SGX_NOT_TRACKED: Previous ETRACK's shootdown sequence has not
> * been completed yet.
> * %SGX_CHILD_PRESENT SECS has child pages present in the EPC.
> + * %SGX_ENCLAVE_ACT One or more logical processors are executing
> + * inside the enclave.
> * %SGX_INVALID_EINITTOKEN: EINITTOKEN is invalid and enclave signer's
> * public key does not match IA32_SGXLEPUBKEYHASH.
> * %SGX_UNMASKED_EVENT: An unmasked event, e.g. INTR, was received
> @@ -57,6 +59,7 @@ enum sgx_encls_function {
> enum sgx_return_code {
> SGX_NOT_TRACKED = 11,
> SGX_CHILD_PRESENT = 13,
> + SGX_ENCLAVE_ACT = 14,
> SGX_INVALID_EINITTOKEN = 16,
> SGX_UNMASKED_EVENT = 128,
> };
> diff --git a/arch/x86/include/uapi/asm/sgx.h b/arch/x86/include/uapi/asm/sgx.h
> index 9690d6899ad9..f4b81587e90b 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 59cdf3f742ac..81a0a0f22007 100644
> --- a/arch/x86/kernel/cpu/sgx/virt.c
> +++ b/arch/x86/kernel/cpu/sgx/virt.c
> @@ -150,6 +150,46 @@ 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) {
> + int ret = sgx_vepc_remove_page(entry);
> + switch (ret) {
> + case 0:
> + break;
> +
> + case SGX_CHILD_PRESENT:
> + failures++;
> + break;
> +
> + case SGX_ENCLAVE_ACT:
> + /*
> + * Unlike in sgx_vepc_free_page, userspace could be calling
> + * the ioctl while logical processors are running in the
> + * enclave; do not warn.
> + */
> + return -EBUSY;
> +
> + default:
> + WARN_ONCE(1, EREMOVE_ERROR_MESSAGE, ret, ret);
> + failures++;
> + break;
> + }
> + cond_resched();
> + }
> +
> + /*
> + * 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;
> @@ -235,9 +274,27 @@ 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,
> };
I went through this a few times, the code change is sound and
reasoning makes sense in the commit message.
The only thing that I think that is IMHO lacking is a simple
kselftest. I think a trivial test for SGX_IOC_VEP_REMOVE_ALL
would do.
/Jarkko
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