linux-kernel - [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove

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Message-Id: <20210920125401.2389105-2-pbonzini@redhat.com>
Date:   Mon, 20 Sep 2021 08:54:00 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org, jarkko@...nel.org,
        dave.hansen@...ux.intel.com, yang.zhong@...el.com
Subject: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

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.

One way to do this is to simply close and reopen the /dev/sgx_vepc file
descriptor and re-mmap the virtual EPC.  However, this is problematic
because it prevents sandboxing the userspace (for example forbidding
open() after the guest starts, or running in a mount namespace that
does not have access to /dev; both are doable with pre-opened file
descriptors and/or SCM_RIGHTS file descriptor passing).

In order to implement this, we will need a ioctl that performs
EREMOVE on all pages mapped by a /dev/sgx_vepc file descriptor:
other possibilities, such as closing and reopening the device,
are racy.

Start the implementation by pulling the EREMOVE into a separate
function.

Tested-by: Yang Zhong <yang.zhong@...el.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kernel/cpu/sgx/virt.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
index 64511c4a5200..59b9c13121cd 100644
--- a/arch/x86/kernel/cpu/sgx/virt.c
+++ b/arch/x86/kernel/cpu/sgx/virt.c
@@ -111,7 +111,7 @@ static int sgx_vepc_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }

-static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
+static int sgx_vepc_remove_page(struct sgx_epc_page *epc_page)
 {
 	int ret;

@@ -140,11 +140,17 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
 		 */
 		WARN_ONCE(ret != SGX_CHILD_PRESENT, EREMOVE_ERROR_MESSAGE,
 			  ret, ret);
-		return ret;
 	}
+	return ret;
+}

-	sgx_free_epc_page(epc_page);
+static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
+{
+	int ret = sgx_vepc_remove_page(epc_page);
+	if (ret)
+		return ret;

+	sgx_free_epc_page(epc_page);
 	return 0;
 }

-- 
2.27.0