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


Marvin Häuser
 

Good day Thomas,

Thank you very much for the quick patch. Comments inline.

Best regards,
Marvin

On 11.05.21 22:50, 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 to track
the previous reset buffer allocation in order to ensure that the new
buffer allocation is below the previous allocation.

Fixes: 7b7508ad784d16a5208c8d12dff43aef6df0835b
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
UefiCpuPkg/Library/MpInitLib/DxeMpLib.c | 12 ++++-
UefiCpuPkg/Library/MpInitLib/MpLib.c | 48 +++++++++++++-------
UefiCpuPkg/Library/MpInitLib/PeiMpLib.c | 14 ++++--
3 files changed, 54 insertions(+), 20 deletions(-)

diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
index 7839c249760e..fdfa0755d37a 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
+//
This definitely is not an issue of your patch at all, but I find the comments of the behaviour very lacking. Might this be a good opportunity to briefly document it? It took me quite a bit of git blame to fully figure it out, especially due to the non-descriptive commit message. The constant is explained very well in the description: https://github.com/tianocore/edk2/commit/e4ff6349bf9ee4f3f392141374901ea4994e043e

+STATIC EFI_PHYSICAL_ADDRESS mWakeupBuffer = 0x88000;
Hmm, I wonder whether a global variable is the best solution here. With an explanation from the commit above, a top-down allocator makes good sense for this scenario. However, a "GetWakeupBuffer(Size, MaxAddress)" function might be more self-descriptive. What do you think?

+
/**
Enable Debug Agent to support source debugging on AP function.
@@ -102,7 +107,7 @@ GetWakeupBuffer (
// LagacyBios driver depends on CPU Arch protocol which guarantees below
// allocation runs earlier than LegacyBios driver.
//
- StartAddress = 0x88000;
+ StartAddress = mWakeupBuffer;
Status = gBS->AllocatePages (
AllocateMaxAddress,
MemoryType,
@@ -112,6 +117,11 @@ GetWakeupBuffer (
ASSERT_EFI_ERROR (Status);
if (EFI_ERROR (Status)) {
StartAddress = (EFI_PHYSICAL_ADDRESS) -1;
+ } else {
+ //
+ // Next wakeup buffer allocation must be below this allocation
+ //
+ mWakeupBuffer = 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..a76dae437606 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;
}
@@ -1207,9 +1193,39 @@ 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)) {
+ UINTN ApResetStackSize;
Personally, I do not mind this at all, but I think the code style prohibits declaring variables in inner scopes. Though preferably I would like to see this, nowadays, arbitrary restriction lifted rather than the patch be changed. Do the package maintainers have comments on this?

+
+ //
+ // Stack location is based on ProcessorNumber, so use the total number
+ // 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);
Should the ASSERT not only catch the broken "allocate below" behaviour, i.e. not trigger on failed allocation?

+ CpuDeadLoop ();
+ }
+ }
}
BackupAndPrepareWakeupBuffer (CpuMpData);
}
diff --git a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
index 3989bd6a7a9f..4d09e89b4128 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 mWakeupBuffer = BASE_1MB;
+
/**
S3 SMM Init Done notification function.
@@ -220,11 +222,11 @@ GetWakeupBuffer (
// Need memory under 1MB to be collected here
//
WakeupBufferEnd = Hob.ResourceDescriptor->PhysicalStart + Hob.ResourceDescriptor->ResourceLength;
- if (WakeupBufferEnd > BASE_1MB) {
+ if (WakeupBufferEnd > mWakeupBuffer) {
//
- // Wakeup buffer should be under 1MB
+ // Wakeup buffer should be under 1MB and under the previous one
//
- WakeupBufferEnd = BASE_1MB;
+ WakeupBufferEnd = mWakeupBuffer;
}
while (WakeupBufferEnd > WakeupBufferSize) {
//
@@ -244,6 +246,12 @@ GetWakeupBuffer (
}
DEBUG ((DEBUG_INFO, "WakeupBufferStart = %x, WakeupBufferSize = %x\n",
WakeupBufferStart, WakeupBufferSize));
+
+ //
+ // Next wakeup buffer allocation must be below this allocation
+ //
+ mWakeupBuffer = WakeupBufferStart;
+
return (UINTN)WakeupBufferStart;
}
}

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