[PATCH v2 10/11] UefiCpuPkg, OvmfPkg: Disable interrupts when using the GHCB


Lendacky, Thomas
 

From: Tom Lendacky <thomas.lendacky@...>

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3008

The QemuFlashPtrWrite() flash services runtime uses the GHCB and VmgExit()
directly to perform the flash write when running as an SEV-ES guest. If an
interrupt arrives between VmgInit() and VmgExit(), the Dr7 read in the
interrupt handler will generate a #VC, which can overwrite information in
the GHCB that QemuFlashPtrWrite() has set. This has been seen with the
timer interrupt firing and the CpuExceptionHandlerLib library code,
UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/
Xcode5ExceptionHandlerAsm.nasm and
ExceptionHandlerAsm.nasm
reading the Dr7 register while QemuFlashPtrWrite() is using the GHCB. In
general, it is necessary to protect the GHCB whenever it is used, not just
in QemuFlashPtrWrite().

Disable interrupts around the usage of the GHCB by modifying the VmgInit()
and VmgDone() interfaces:
- VmgInit() will take an extra parameter that is a pointer to a BOOLEAN
that will hold the interrupt state at the time of invocation. VmgInit()
will get and save this interrupt state before updating the GHCB.
- VmgDone() will take an extra parameter that is used to indicate whether
interrupts are to be (re)enabled. Before exiting, VmgDone() will enable
interrupts if that is requested.

Fixes: 437eb3f7a8db7681afe0e6064d3a8edb12abb766
Cc: Eric Dong <eric.dong@...>
Cc: Ray Ni <ray.ni@...>
Cc: Laszlo Ersek <lersek@...>
Cc: Rahul Kumar <rahul1.kumar@...>
Cc: Jordan Justen <jordan.l.justen@...>
Cc: Ard Biesheuvel <ard.biesheuvel@...>
Cc: Tom Lendacky <thomas.lendacky@...>
Cc: Brijesh Singh <brijesh.singh@...>
Signed-off-by: Tom Lendacky <thomas.lendacky@...>
---
UefiCpuPkg/Include/Library/VmgExitLib.h | 14 ++++++++---
OvmfPkg/Library/VmgExitLib/VmgExitLib.c | 26 +++++++++++++++++---
OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c | 5 ++--
OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c | 5 ++--
UefiCpuPkg/Library/MpInitLib/DxeMpLib.c | 5 ++--
UefiCpuPkg/Library/MpInitLib/MpLib.c | 7 +++---
UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c | 18 ++++++++------
7 files changed, 55 insertions(+), 25 deletions(-)

diff --git a/UefiCpuPkg/Include/Library/VmgExitLib.h b/UefiCpuPkg/Include/Library/VmgExitLib.h
index ba5ea024839e..617b6cf8d2e7 100644
--- a/UefiCpuPkg/Include/Library/VmgExitLib.h
+++ b/UefiCpuPkg/Include/Library/VmgExitLib.h
@@ -50,13 +50,16 @@ VmgExit (
Performs the necessary steps in preparation for invoking VMGEXIT. Must be
called before setting any fields within the GHCB.

- @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] InterruptState A pointer to hold the current interrupt
+ state, used for restoring in VmgDone ()

**/
VOID
EFIAPI
VmgInit (
- IN OUT GHCB *Ghcb
+ IN OUT GHCB *Ghcb,
+ IN OUT BOOLEAN *InterruptState
);

/**
@@ -65,13 +68,16 @@ VmgInit (
Performs the necessary steps to cleanup after invoking VMGEXIT. Must be
called after obtaining needed fields within the GHCB.

- @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] Ghcb A pointer to the GHCB
+ @param[in] InterruptState An indicator to conditionally (re)enable
+ interrupts

**/
VOID
EFIAPI
VmgDone (
- IN OUT GHCB *Ghcb
+ IN OUT GHCB *Ghcb,
+ IN BOOLEAN InterruptState
);

/**
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitLib.c b/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
index ae86d850ba61..18b102df5f9a 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
@@ -132,15 +132,27 @@ VmgExit (
Performs the necessary steps in preparation for invoking VMGEXIT. Must be
called before setting any fields within the GHCB.

- @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] InterruptState A pointer to hold the current interrupt
+ state, used for restoring in VmgDone ()

**/
VOID
EFIAPI
VmgInit (
- IN OUT GHCB *Ghcb
+ IN OUT GHCB *Ghcb,
+ IN OUT BOOLEAN *InterruptState
)
{
+ //
+ // Be sure that an interrupt can't cause a #VC while the GHCB is
+ // being used.
+ //
+ *InterruptState = GetInterruptState ();
+ if (*InterruptState) {
+ DisableInterrupts ();
+ }
+
SetMem (&Ghcb->SaveArea, sizeof (Ghcb->SaveArea), 0);
}

@@ -150,15 +162,21 @@ VmgInit (
Performs the necessary steps to cleanup after invoking VMGEXIT. Must be
called after obtaining needed fields within the GHCB.

- @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] Ghcb A pointer to the GHCB
+ @param[in] InterruptState An indicator to conditionally (re)enable
+ interrupts

**/
VOID
EFIAPI
VmgDone (
- IN OUT GHCB *Ghcb
+ IN OUT GHCB *Ghcb,
+ IN BOOLEAN InterruptState
)
{
+ if (InterruptState) {
+ EnableInterrupts ();
+ }
}

/**
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
index 9bf9d160179c..1671db3a01b1 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
@@ -1568,6 +1568,7 @@ VmgExitHandleVc (
SEV_ES_INSTRUCTION_DATA InstructionData;
UINT64 ExitCode, Status;
EFI_STATUS VcRet;
+ BOOLEAN InterruptState;

VcRet = EFI_SUCCESS;

@@ -1578,7 +1579,7 @@ VmgExitHandleVc (
Regs = SystemContext.SystemContextX64;
Ghcb = Msr.Ghcb;

- VmgInit (Ghcb);
+ VmgInit (Ghcb, &InterruptState);

ExitCode = Regs->ExceptionData;
switch (ExitCode) {
@@ -1662,7 +1663,7 @@ VmgExitHandleVc (
VcRet = EFI_PROTOCOL_ERROR;
}

- VmgDone (Ghcb);
+ VmgDone (Ghcb, InterruptState);

return VcRet;
}
diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
index f9b21b54137d..1b0742967f71 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
@@ -52,6 +52,7 @@ QemuFlashPtrWrite (
if (MemEncryptSevEsIsEnabled ()) {
MSR_SEV_ES_GHCB_REGISTER Msr;
GHCB *Ghcb;
+ BOOLEAN InterruptState;

Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
Ghcb = Msr.Ghcb;
@@ -63,12 +64,12 @@ QemuFlashPtrWrite (
// #VC exception. Instead, use the the VMGEXIT MMIO write support directly
// to perform the update.
//
- VmgInit (Ghcb);
+ VmgInit (Ghcb, &InterruptState);
Ghcb->SharedBuffer[0] = Value;
Ghcb->SaveArea.SwScratch = (UINT64) (UINTN) Ghcb->SharedBuffer;
VmgSetOffsetValid (Ghcb, GhcbSwScratch);
VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, (UINT64) (UINTN) Ptr, 1);
- VmgDone (Ghcb);
+ VmgDone (Ghcb, InterruptState);
} else {
*Ptr = Value;
}
diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
index 2c00d72ddefe..7839c249760e 100644
--- a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
@@ -171,6 +171,7 @@ GetSevEsAPMemory (
EFI_PHYSICAL_ADDRESS StartAddress;
MSR_SEV_ES_GHCB_REGISTER Msr;
GHCB *Ghcb;
+ BOOLEAN InterruptState;

//
// Allocate 1 page for AP jump table page
@@ -192,9 +193,9 @@ GetSevEsAPMemory (
Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
Ghcb = Msr.Ghcb;

- VmgInit (Ghcb);
+ VmgInit (Ghcb, &InterruptState);
VmgExit (Ghcb, SVM_EXIT_AP_JUMP_TABLE, 0, (UINT64) (UINTN) StartAddress);
- VmgDone (Ghcb);
+ VmgDone (Ghcb, InterruptState);

return (UINTN) StartAddress;
}
diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
index 07426274f639..4f4b26a7c196 100644
--- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
@@ -884,6 +884,7 @@ ApWakeupFunction (
GHCB *Ghcb;
UINT64 Status;
BOOLEAN DoDecrement;
+ BOOLEAN InterruptState;

DoDecrement = (BOOLEAN) (CpuMpData->InitFlag == ApInitConfig);

@@ -891,7 +892,7 @@ ApWakeupFunction (
Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
Ghcb = Msr.Ghcb;

- VmgInit (Ghcb);
+ VmgInit (Ghcb, &InterruptState);

if (DoDecrement) {
DoDecrement = FALSE;
@@ -905,11 +906,11 @@ ApWakeupFunction (

Status = VmgExit (Ghcb, SVM_EXIT_AP_RESET_HOLD, 0, 0);
if ((Status == 0) && (Ghcb->SaveArea.SwExitInfo2 != 0)) {
- VmgDone (Ghcb);
+ VmgDone (Ghcb, InterruptState);
break;
}

- VmgDone (Ghcb);
+ VmgDone (Ghcb, InterruptState);
}

//
diff --git a/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c b/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c
index b000232c472e..24defd624c63 100644
--- a/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c
+++ b/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c
@@ -57,15 +57,16 @@ VmgExit (
Performs the necessary steps in preparation for invoking VMGEXIT. Must be
called before setting any fields within the GHCB.

- The base library function does nothing.
-
- @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] InterruptState A pointer to hold the current interrupt
+ state, used for restoring in VmgDone ()

**/
VOID
EFIAPI
VmgInit (
- IN OUT GHCB *Ghcb
+ IN OUT GHCB *Ghcb,
+ IN OUT BOOLEAN *InterruptState
)
{
}
@@ -76,15 +77,16 @@ VmgInit (
Performs the necessary steps to cleanup after invoking VMGEXIT. Must be
called after obtaining needed fields within the GHCB.

- The base library function does nothing.
-
- @param[in, out] Ghcb A pointer to the GHCB
+ @param[in, out] Ghcb A pointer to the GHCB
+ @param[in] InterruptState An indicator to conditionally (re)enable
+ interrupts

**/
VOID
EFIAPI
VmgDone (
- IN OUT GHCB *Ghcb
+ IN OUT GHCB *Ghcb,
+ IN BOOLEAN InterruptState
)
{
}
--
2.28.0

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