[edk2-devel] [PATCH RFC v2 17/28] OvmfPkg/ResetVector: Invalidate the GHCB page

[Brijesh Singh]

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

When SEV-SNP is active, the GHCB page is mapped un-encrypted in the
initial page table built by the reset vector code. Just clearing the
encryption attribute from the page table is not enough. The page also
needs to be added as shared in the RMP table.

The GHCB page was part of the pre-validated memory range specified
through the SnpBootBlock GUID. To maintain the security guarantees,
we must invalidate the GHCB page before clearing the encryption
attribute from the page table, and add the page shared in the RMP
table.

Cc: James Bottomley <jejb at linux.ibm.com>
Cc: Min Xu <min.m.xu at intel.com>
Cc: Jiewen Yao <jiewen.yao at intel.com>
Cc: Tom Lendacky <thomas.lendacky at amd.com>
Cc: Jordan Justen <jordan.l.justen at intel.com>
Cc: Ard Biesheuvel <ardb+tianocore at kernel.org>
Cc: Laszlo Ersek <lersek at redhat.com>
Cc: Erdem Aktas <erdemaktas at google.com>
Signed-off-by: Brijesh Singh <brijesh.singh at amd.com>
---
 OvmfPkg/ResetVector/Ia32/PageTables64.asm | 82 ++++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/OvmfPkg/ResetVector/Ia32/PageTables64.asm b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
index 6bf4a3524a..9949fcdfba 100644
--- a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
+++ b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
@@ -70,9 +70,78 @@ BITS    32
 %define GHCB_HYPERVISOR_FEATURES_REQUEST    128
 %define GHCB_HYPERVISOR_FEATURES_RESPONSE   129
 
+; GHCB Page Invalidate request and response protocol values
+;
+%define GHCB_PAGE_STATE_CHANGE_REQUEST    20
+%define GHCB_PAGE_STATE_CHANGE_RESPONSE   21
+%define GHCB_PAGE_STATE_SHARED            2
+
 ; GHCB request to terminate protocol values
 %define GHCB_GENERAL_TERMINATE_REQUEST    255
 
+; If its an SEV-SNP guest then use the page state change VMGEXIT to invalidate
+; the GHCB page.
+;
+; Modified:  EAX, EBX, ECX, EDX
+;
+InvalidateGHCBPage:
+    ; Check if it is SEV-SNP guest.
+    cmp     byte[SEV_ES_WORK_AREA_SNP], 1
+    jne     InvalidateGHCBPageDone
+
+    ; Check whether hypervisor features has SEV-SNP (BIT0) set to indicate that
+    ; hypervisor supports the page state change.
+    mov     eax, dword[SEV_ES_WORK_AREA_HYPERVISOR_FEATURES]
+    bt      eax, 0
+    jnc     TerminateSevGuestLaunch
+
+    ; Use PVALIDATE instruction to invalidate the page
+    mov     eax, GHCB_BASE
+    mov     ecx, 0
+    mov     edx, 0
+    DB      0xF2, 0x0F, 0x01, 0xFF
+    cmp     eax, 0
+    jnz     TerminateSevGuestLaunch
+
+    ; Ask hypervisor to change the page state to shared using the
+    ; Page State Change VMGEXIT.
+    ;
+    ; Setup GHCB MSR
+    ;   GHCB_MSR[55:52] = Page Operation
+    ;   GHCB_MSR[51:12] = Guest Physical Frame Number
+    ;   GHCB_MSR[11:0]  = Page State Change Request
+    ;
+    mov     eax, (GHCB_BASE >> 12)
+    shl     eax, 12
+    or      eax, GHCB_PAGE_STATE_CHANGE_REQUEST
+    mov     edx, (GHCB_PAGE_STATE_SHARED << 20)
+    mov     ecx, 0xc0010130
+    wrmsr
+
+    ;
+    ; Issue VMGEXIT - NASM doesn't support the vmmcall instruction in 32-bit
+    ; mode, so work around this by temporarily switching to 64-bit mode.
+    ;
+BITS    64
+    rep     vmmcall
+BITS    32
+
+    ;
+    ; Response GHCB MSR
+    ;   GHCB_MSR[51:12] = Guest Physical Frame Number
+    ;   GHCB_MSR[11:0]  = Page State Change Response
+    ;
+    mov     ecx, 0xc0010130
+    rdmsr
+    and     eax, 0xfff
+    cmp     eax, GHCB_PAGE_STATE_CHANGE_RESPONSE
+    jnz     TerminateSevGuestLaunch
+    cmp     edx, 0
+    jnz     TerminateSevGuestLaunch
+
+InvalidateGHCBPageDone:
+    OneTimeCallRet InvalidateGHCBPage
+
 ; Check if Secure Encrypted Virtualization (SEV) features are enabled.
 ;
 ; Register usage is tight in this routine, so multiple calls for the
@@ -450,6 +519,19 @@ clearGhcbMemoryLoop:
     ;
     OneTimeCall   CheckHypervisorFeatures
 
+    ;
+    ; The page table built above cleared the memory encryption mask from the
+    ; GHCB_BASE (aka made it shared). When SEV-SNP is enabled, to maintain
+    ; the security guarantees, the page state transition from private to
+    ; shared must go through the page invalidation steps. Invalidate the
+    ; memory range before loading the page table below.
+    ;
+    ; NOTE: the invalidation must happen after zeroing the GHCB memory. This
+    ;       is because, in the 32-bit mode all the access are considered private.
+    ;       The invalidation before the zero'ing will cause a #VC.
+    ;
+    OneTimeCall  InvalidateGHCBPage
+
 SetCr3:
     ;
     ; Set CR3 now that the paging structures are available
-- 
2.17.1



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