Re: [PATCH v2] UefiCpuPkg/MpInitLib: Allocate a separate SEV-ES AP reset stack area


Laszlo Ersek
 

On 05/14/21 22:28, Lendacky, Thomas wrote:
BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3324

The SEV-ES stacks currently share a page with the reset code and data.
Separate the SEV-ES stacks from the reset vector code and data to avoid
possible stack overflows from overwriting the code and/or data.

When SEV-ES is enabled, invoke the GetWakeupBuffer() routine a second time
to allocate a new area, below the reset vector and data.

Both the PEI and DXE versions of GetWakeupBuffer() are changed so that
when PcdSevEsIsEnabled is true, they will track the previous reset buffer
allocation in order to ensure that the new buffer allocation is below the
previous allocation. When PcdSevEsIsEnabled is false, the original logic
is followed.

Fixes: 7b7508ad784d16a5208c8d12dff43aef6df0835b
Is this really a *bugfix*?

I called what's being fixed "a gap in a generic protection mechanism",
in <https://bugzilla.tianocore.org/show_bug.cgi?id=3324#c9>.

I don't know if that makes this patch a feature addition (or feature
completion -- the feature being "more extensive page protections"), or a
bugfix.

The distinction matters because of the soft feature freeze:

https://github.com/tianocore/tianocore.github.io/wiki/EDK-II-Release-Planning

We still have approximately 2 hours before the SFF starts. So if we can
*finish* reviewing this in 2 hours, then it can be merged during the
SFF, even if we call it a feature. But I'd like Marvin to take a look as
well, plus I'd like at least one of Eric and Ray to check.

... I'm tempted not to call it a bugfix, because the lack of this patch
does not break SEV-ES usage, as far as I understand.

Cc: Eric Dong <eric.dong@...>
Cc: Ray Ni <ray.ni@...>
Cc: Laszlo Ersek <lersek@...>
Cc: Rahul Kumar <rahul1.kumar@...>
Cc: Marvin Häuser <mhaeuser@...>
Signed-off-by: Tom Lendacky <thomas.lendacky@...>

---

Changes since v1:
- Renamed the wakeup buffer variables to be unique in the PEI and DXE
libraries and to make it obvious they are SEV-ES specific.
- Use PcdGetBool (PcdSevEsIsEnabled) to make the changes regression-free
so that the new support is only use for SEV-ES guests.
---
UefiCpuPkg/Library/MpInitLib/DxeMpLib.c | 19 +++++++-
UefiCpuPkg/Library/MpInitLib/MpLib.c | 49 +++++++++++++-------
UefiCpuPkg/Library/MpInitLib/PeiMpLib.c | 19 +++++++-
3 files changed, 69 insertions(+), 18 deletions(-)

diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
index 7839c249760e..93fc63bf93e3 100644
--- a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
@@ -29,6 +29,11 @@ VOID *mReservedApLoopFunc = NULL;
UINTN mReservedTopOfApStack;
volatile UINT32 mNumberToFinish = 0;

+//
+// Begin wakeup buffer allocation below 0x88000
+//
+STATIC EFI_PHYSICAL_ADDRESS mSevEsDxeWakeupBuffer = 0x88000;
+
/**
Enable Debug Agent to support source debugging on AP function.

@@ -102,7 +107,14 @@ GetWakeupBuffer (
// LagacyBios driver depends on CPU Arch protocol which guarantees below
// allocation runs earlier than LegacyBios driver.
//
- StartAddress = 0x88000;
+ if (PcdGetBool (PcdSevEsIsEnabled)) {
+ //
+ // SEV-ES Wakeup buffer should be under 0x88000 and under any previous one
+ //
+ StartAddress = mSevEsDxeWakeupBuffer;
+ } else {
+ StartAddress = 0x88000;
+ }
Status = gBS->AllocatePages (
AllocateMaxAddress,
MemoryType,
@@ -112,6 +124,11 @@ GetWakeupBuffer (
ASSERT_EFI_ERROR (Status);
if (EFI_ERROR (Status)) {
StartAddress = (EFI_PHYSICAL_ADDRESS) -1;
+ } else if (PcdGetBool (PcdSevEsIsEnabled)) {
+ //
+ // Next SEV-ES wakeup buffer allocation must be below this allocation
+ //
+ mSevEsDxeWakeupBuffer = StartAddress;
}

DEBUG ((DEBUG_INFO, "WakeupBufferStart = %x, WakeupBufferSize = %x\n",
diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
index dc2a54aa31e8..b9a06747edbf 100644
--- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
@@ -1164,20 +1164,6 @@ GetApResetVectorSize (
AddressMap->SwitchToRealSize +
sizeof (MP_CPU_EXCHANGE_INFO);

- //
- // The AP reset stack is only used by SEV-ES guests. Do not add to the
- // allocation if SEV-ES is not enabled.
- //
- if (PcdGetBool (PcdSevEsIsEnabled)) {
- //
- // Stack location is based on APIC ID, so use the total number of
- // processors for calculating the total stack area.
- //
- Size += AP_RESET_STACK_SIZE * PcdGet32 (PcdCpuMaxLogicalProcessorNumber);
-
- Size = ALIGN_VALUE (Size, CPU_STACK_ALIGNMENT);
- }
-
return Size;
}

@@ -1192,6 +1178,7 @@ AllocateResetVector (
)
{
UINTN ApResetVectorSize;
+ UINTN ApResetStackSize;

if (CpuMpData->WakeupBuffer == (UINTN) -1) {
ApResetVectorSize = GetApResetVectorSize (&CpuMpData->AddressMap);
@@ -1207,9 +1194,39 @@ AllocateResetVector (
CpuMpData->AddressMap.ModeTransitionOffset
);
//
- // The reset stack starts at the end of the buffer.
+ // The AP reset stack is only used by SEV-ES guests. Do not allocate it
+ // if SEV-ES is not enabled.
//
- CpuMpData->SevEsAPResetStackStart = CpuMpData->WakeupBuffer + ApResetVectorSize;
+ if (PcdGetBool (PcdSevEsIsEnabled)) {
+ //
+ // Stack location is based on ProcessorNumber, so use the total number
sneaky documenation fix :) I vaguely remember discussing earlier that
the APIC ID reference was incorrect. OK.

+ // of processors for calculating the total stack area.
+ //
+ ApResetStackSize = (AP_RESET_STACK_SIZE *
+ PcdGet32 (PcdCpuMaxLogicalProcessorNumber));
+
+ //
+ // Invoke GetWakeupBuffer a second time to allocate the stack area
+ // below 1MB. The returned buffer will be page aligned and sized and
+ // below the previously allocated buffer.
+ //
+ CpuMpData->SevEsAPResetStackStart = GetWakeupBuffer (ApResetStackSize);
+
+ //
+ // Check to be sure that the "allocate below" behavior hasn't changed.
+ // This will also catch a failed allocation, as "-1" is returned on
+ // failure.
+ //
+ if (CpuMpData->SevEsAPResetStackStart >= CpuMpData->WakeupBuffer) {
+ DEBUG ((
+ DEBUG_ERROR,
+ "SEV-ES AP reset stack is not below wakeup buffer\n"
+ ));
+
+ ASSERT (FALSE);
+ CpuDeadLoop ();
+ }
+ }
}
BackupAndPrepareWakeupBuffer (CpuMpData);
}
diff --git a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
index 3989bd6a7a9f..90015c650c68 100644
--- a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
@@ -11,6 +11,8 @@
#include <Guid/S3SmmInitDone.h>
#include <Ppi/ShadowMicrocode.h>

+STATIC UINT64 mSevEsPeiWakeupBuffer = BASE_1MB;
+
/**
S3 SMM Init Done notification function.

@@ -220,7 +222,13 @@ GetWakeupBuffer (
// Need memory under 1MB to be collected here
//
WakeupBufferEnd = Hob.ResourceDescriptor->PhysicalStart + Hob.ResourceDescriptor->ResourceLength;
- if (WakeupBufferEnd > BASE_1MB) {
+ if (PcdGetBool (PcdSevEsIsEnabled) &&
+ WakeupBufferEnd > mSevEsPeiWakeupBuffer) {
+ //
+ // SEV-ES Wakeup buffer should be under 1MB and under any previous one
+ //
+ WakeupBufferEnd = mSevEsPeiWakeupBuffer;
+ } else if (WakeupBufferEnd > BASE_1MB) {
//
// Wakeup buffer should be under 1MB
//
@@ -244,6 +252,15 @@ GetWakeupBuffer (
}
DEBUG ((DEBUG_INFO, "WakeupBufferStart = %x, WakeupBufferSize = %x\n",
WakeupBufferStart, WakeupBufferSize));
+
+ if (PcdGetBool (PcdSevEsIsEnabled)) {
+ //
+ // Next SEV-ES wakeup buffer allocation must be below this
+ // allocation
+ //
+ mSevEsPeiWakeupBuffer = WakeupBufferStart;
+ }
+
return (UINTN)WakeupBufferStart;
}
}
The code in the patch seems sound to me, but, now that I've managed to
take a bit more careful look, I think the patch is incomplete.

Note the BackupAndPrepareWakeupBuffer() function call -- visible in the
context --, at the end of the AllocateResetVector() function! Before, we
had a single area allocated for the reset vector, which was
appropriately sized for SEV-ES stacks too, in case SEV-ES was enabled.

That was because MpInitLibInitialize() called GetApResetVectorSize()
too, and stored the result to the "BackupBufferSize" field. Thus, the
BackupAndPrepareWakeupBuffer() function, and its counterpart
RestoreWakeupBuffer(), would include the SEV-ES AP stacks area in the
backup/restore operations.

But now, with SEV-ES enabled, we'll have a separate, discontiguous area
-- and neither BackupAndPrepareWakeupBuffer(), nor its counterpart
RestoreWakeupBuffer() take that into account.

Therefore I think, while this patch is regression-free for the SEV-ES
*disabled* case, it may corrupt memory (through not restoring the AP
stack area's original contents) with SEV-ES enabled.

I think we need to turn "ApResetStackSize" into an explicit field. It
should have a defined value only when SEV-ES is enabled. And for that
case, we seem to need a *separate backup buffer* too.

FWIW, I'd really like to re-classify this BZ as a Feature Request (see
the Product field on BZ#3324), and I'd really like to delay the patch
until after edk2-stable202105. The patch is not necessary for using
SEV-ES, but it has a chance to break SEV-ES.

Thanks
Laszlo

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