[PATCH v5 1/9] Documentation: Introduce EPT based Subpage Protection
From: Yang Weijiang
Date: Tue Sep 17 2019 - 04:53:24 EST
Co-developed-by: yi.z.zhang@xxxxxxxxxxxxxxx
Signed-off-by: yi.z.zhang@xxxxxxxxxxxxxxx
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
Documentation/virtual/kvm/spp_kvm.txt | 178 ++++++++++++++++++++++++++
1 file changed, 178 insertions(+)
create mode 100644 Documentation/virtual/kvm/spp_kvm.txt
diff --git a/Documentation/virtual/kvm/spp_kvm.txt b/Documentation/virtual/kvm/spp_kvm.txt
new file mode 100644
index 000000000000..1bd1c11d0a99
--- /dev/null
+++ b/Documentation/virtual/kvm/spp_kvm.txt
@@ -0,0 +1,178 @@
+EPT-Based Sub-Page Protection (SPP) for KVM
+====================================================
+
+1.Overview
+ EPT-based Sub-Page Protection(SPP) allows VMM to specify
+ fine-grained(128byte per sub-page) write-protection for guest physical
+ memory. When it's enabled, the CPU enforces write-access permission
+ for the sub-pages within a 4KB page, if corresponding bit is set in
+ permission vector, write to sub-page region is allowed, otherwise,
+ it's prevented with a EPT violation.
+
+ *Note*: In current implementation, SPP is exclusive with nested flag,
+ if it's on, SPP feature won't work.
+
+2.SPP Operation
+ Sub-Page Protection Table (SPPT) is introduced to manage sub-page
+ write-access permission.
+
+ It is active when:
+ a) nested flag is turned off.
+ b) "sub-page write protection" VM-execution control is 1.
+ c) SPP is initialized with KVM_INIT_SPP ioctl.
+ d) Sub-page permissions are set with KVM_SUBPAGES_SET_ACCESS ioctl.
+ see below sections for details.
+
+ __________________________________________________________________________
+
+ How SPP hardware works:
+ __________________________________________________________________________
+
+ Guest write access --> GPA --> Walk EPT --> EPT leaf entry -----|
+ |---------------------------------------------------------------|
+ |-> if VMexec_control.spp && ept_leaf_entry.spp_bit (bit 61)
+ |
+ |-> <false> --> EPT legacy behavior
+ |
+ |
+ |-> <true> --> if ept_leaf_entry.writable
+ |
+ |-> <true> --> Ignore SPP
+ |
+ |-> <false> --> GPA --> Walk SPP 4-level table--|
+ |
+ |------------<----------get-the-SPPT-point-from-VMCS-filed-----<------|
+ |
+ Walk SPP L4E table
+ |
+ |---> if-entry-misconfiguration ------------>-------|-------<---------|
+ | | |
+ else | |
+ | | |
+ | |------------------SPP VMexit<-----------------| |
+ | | |
+ | |-> exit_qualification & sppt_misconfig --> sppt misconfig |
+ | | |
+ | |-> exit_qualification & sppt_miss --> sppt miss |
+ |---| |
+ | |
+ walk SPPT L3E--|--> if-entry-misconfiguration------------>------------|
+ | |
+ else |
+ | |
+ | |
+ walk SPPT L2E --|--> if-entry-misconfiguration-------->-------|
+ | |
+ else |
+ | |
+ | |
+ walk SPPT L1E --|-> if-entry-misconfiguration--->----|
+ |
+ else
+ |
+ |-> if sub-page writable
+ |-> <true> allow, write access
+ |-> <false> disallow, EPT violation
+ ______________________________________________________________________________
+
+3.IOCTL Interfaces
+
+ KVM_INIT_SPP:
+ Allocate storage for sub-page permission vectors and SPPT root page.
+
+ KVM_SUBPAGES_GET_ACCESS:
+ Get sub-page write permission vectors for given continuous guest pages.
+
+ KVM_SUBPAGES_SET_ACCESS
+ Set SPP bit in EPT leaf entries for given continuous guest pages. The
+ actual SPPT setup is triggered when SPP miss vm-exit is handled.
+
+ /* for KVM_SUBPAGES_GET_ACCESS and KVM_SUBPAGES_SET_ACCESS */
+ struct kvm_subpage_info {
+ __u64 gfn; /* the first page gfn of the continuous pages */
+ __u64 npages; /* number of 4K pages */
+ __u64 *access_map; /* sub-page write-access bitmap array */
+ };
+
+ #define KVM_SUBPAGES_GET_ACCESS _IOR(KVMIO, 0x49, __u64)
+ #define KVM_SUBPAGES_SET_ACCESS _IOW(KVMIO, 0x4a, __u64)
+ #define KVM_INIT_SPP _IOW(KVMIO, 0x4b, __u64)
+
+4.Set Sub-Page Permission
+
+ * To enable SPP protection, system admin sets sub-page permission via
+ KVM_SUBPAGES_SET_ACCESS ioctl:
+ (1) It first stores the access permissions in bitmap array.
+
+ (2) Then, if the target 4KB page is mapped as PT_PAGE_TABLE_LEVEL entry in EPT,
+ it sets SPP bit of the corresponding entry to mark sub-page protection.
+ If the 4KB page is mapped as PT_DIRECTORY_LEVEL or PT_PDPE_LEVEL, it
+ zapps the hugepage entry and let following memroy access to trigger EPT
+ page fault, there the gfn is check against SPP permission bitmap and
+ proper level is selected to set up EPT entry.
+
+
+ The SPPT paging structure format is as below:
+
+ Format of the SPPT L4E, L3E, L2E:
+ | Bit | Contents |
+ | :----- | :------------------------------------------------------------------------|
+ | 0 | Valid entry when set; indicates whether the entry is present |
+ | 11:1 | Reserved (0) |
+ | N-1:12 | Physical address of 4KB aligned SPPT LX-1 Table referenced by this entry |
+ | 51:N | Reserved (0) |
+ | 63:52 | Reserved (0) |
+ Note: N is the physical address width supported by the processor. X is the page level
+
+ Format of the SPPT L1E:
+ | Bit | Contents |
+ | :---- | :---------------------------------------------------------------- |
+ | 0+2i | Write permission for i-th 128 byte sub-page region. |
+ | 1+2i | Reserved (0). |
+ Note: 0<=i<=31
+
+5.SPPT-induced VM exit
+
+ * SPPT miss and misconfiguration induced VM exit
+
+ A SPPT missing VM exit occurs when walk the SPPT, there is no SPPT
+ misconfiguration but a paging-structure entry is not
+ present in any of L4E/L3E/L2E entries.
+
+ A SPPT misconfiguration VM exit occurs when reserved bits or unsupported values
+ are set in SPPT entry.
+
+ *NOTE* SPPT miss and SPPT misconfigurations can occur only due to an
+ attempt to write memory with a guest physical address.
+
+ * SPP permission induced VM exit
+ SPP sub-page permission induced violation is reported as EPT violation
+ thesefore causes VM exit.
+
+6.SPPT-induced VM exit handling
+
+ #define EXIT_REASON_SPP 66
+
+ static int (*const kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
+ ...
+ [EXIT_REASON_SPP] = handle_spp,
+ ...
+ };
+
+ New exit qualification for SPPT-induced vmexits.
+
+ | Bit | Contents |
+ | :---- | :---------------------------------------------------------------- |
+ | 10:0 | Reserved (0). |
+ | 11 | SPPT VM exit type. Set for SPPT Miss, cleared for SPPT Misconfig. |
+ | 12 | NMI unblocking due to IRET |
+ | 63:13 | Reserved (0) |
+
+ In addition to the exit qualification, guest linear address and guest
+ physical address fields will be reported.
+
+ * SPPT miss and misconfiguration induced VM exit
+ Set up SPPT entries correctly.
+
+ * SPP permission induced VM exit
+ This kind of VM exit is left to VMI tool to handle.
--
2.17.2