[PATCH v3 1/9] Documentation: Introduce EPT based Subpage Protection

From: Yang Weijiang
Date: Thu Jun 06 2019 - 11:34:03 EST

Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
Documentation/virtual/kvm/spp_kvm.txt | 216 ++++++++++++++++++++++++++
1 file changed, 216 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
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--- /dev/null
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+EPT-Based Sub-Page Protection (SPP) for KVM
+=============================================
+
+1. Overview
+
+EPT-based Sub-Page Protection (SPP) capability to allow Virtual Machine
+Monitors to specify write-protection for guest physical memory at a
+sub-page (128 byte) granularity. When this capability is utilized, the
+CPU enforces write-access permissions for sub-page regions inside 4K pages
+as specified by the VMI tools.
+
+2. Operation of SPP
+
+Sub-Page Protection Table (SPPT) is introduced to manage sub-page
+write-access.
+
+SPPT is active when:
+a) moddule parameter spp=on is configured for kvm-intel.ko
+b) large paging is disabled on host
+c) "sub-page write protection" VM-execution control bit is set
+SPPT looks up the guest physical address to seek a 64-bit
+bitmap indicating sub-page write permission in SPPT leaf entry.
+
+When the "sub-page write protection" VM-execution control is 1, the SPPT
+is used to lookup write permission bits for the 128 byte sub-page regions
+contained in the 4KB guest physical page. EPT specifies the 4KB page
+write-protection privilege whereas SPPT defines the write permissions
+at 128-byte granularity within one 4KB page. Write accesses
+prevented due to sub-page permissions induces EPT violation VM exits.
+Similar to EPT, a logical processor uses SPPT to lookup sub-page level
+write permissions for guest-physical addresses only when those addresses
+are used to access memory.
+__________________________________________________________________________
+
+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. Interfaces
+
+* Feature enabling
+
+Add "spp=on" to KVM module parameter to enable SPP feature, default is off.
+
+* Get/Set sub-page write access permission
+
+New KVM ioctl:
+
+KVM_SUBPAGES_GET_ACCESS:
+Get sub-pages write access bitmap corresponding to given rang of continuous gfn.
+
+KVM_SUBPAGES_SET_ACCESS
+Set sub-pages write access bitmap corresponding to given rang of continuous gfn.
+
+
+/* for KVM_SUBPAGES_GET_ACCESS and KVM_SUBPAGES_SET_ACCESS */
+struct kvm_subpage_info {
+ __u64 gfn;
+ __u64 npages; /* number of 4K pages */
+ __u64 *access_map; /* sub-page write-access bitmap array */
+};
+
+#define KVM_SUBPAGES_GET_ACCESS _IOR(KVMIO, 0x49, struct kvm_subpage_info)
+#define KVM_SUBPAGES_SET_ACCESS _IOW(KVMIO, 0x4a, struct kvm_subpage_info)
+
+
+4. SPPT initialization
+
+* SPPT root page allocation
+
+ SPPT is referenced via a 64-bit control field called "sub-page
+ protection table pointer" (SPPTP, encoding 0x2030) which contains a
+ 4K-align physical address.
+
+ SPPT is a 4-level paging structure similar as EPT. When KVM
+ loads mmu, it allocates a root page for SPPT L4 table as well.
+
+* EPT leaf entry SPP bit (bit 61)
+
+ Set 0 to SPP bit to close SPP.
+
+5. Set/Get Sub-Page access bitmap for a bunch of guest physical pages
+
+* To utilize SPP feature, system admin should set sub-page access via
+ SPP KVM ioctl `KVM_SUBPAGES_SET_ACCESS`, configuring EPT and SPPT in below flow:
+
+ (1) If the target 4KB pages to be protected are there, it locates EPT leaf entries
+ via the guest physical addresses, flags the bit 61 of the corresponding entries to
+ enable sub-page protection for the pages, then setup SPPT paging structure.
+ (2) otherwise, stores the [gfn,permission] mappings in KVM data structure. When
+ EPT page-fault is generated due to target protected page accessing, it settles
+ EPT entry configureation together with SPPT build-up.
+
+ 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
+
+6. SPPT-induced vmexits
+
+* SPP VM exits
+
+Accesses using guest physical addresses may cause VM exits due to a SPPT
+misconfiguration or a SPPT missing.
+
+A SPPT misconfiguration vmexit occurs when, in the course of translating
+a guest physical address, the logical processor encounters a leaf EPT
+paging-structure entry mapping a 4KB page, with SPP enabled, during the
+SPPT lookup, a SPPT paging-structure entry contains an unsupported
+value.
+
+A SPPT missing vmexit occurs during the SPPT lookup there is no SPPT
+misconfiguration but any level of SPPT paging-structure entries are not
+present.
+
+NOTE. SPPT misconfigurations and SPPT miss can occur only due to an
+attempt to write memory with a guest physical address.
+
+* EPT violation vmexits due to SPPT
+
+EPT violations generated due to SPP sub-page
+permission are reported as EPT violation vmexits.
+
+7. SPPT-induced vmexits 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
+
+Allocate a page for the SPPT entry and set the entry correctly.
+
+* EPT violation vmexits due to SPPT
+
+While hardware traverses SPPT, If the sub-page region write
+permission bit is set, the write is allowed, otherwise it's prevented
+and an EPT violation is generated.
--
2.17.2