Re: [PATCH] UefiCpuPkg: Fix CPU stack guard support by aligning GDT buffer


Vitaly Cheptsov
 

Hi Leif,

I assume you mean the commit description, because the commit message is in the topic. I believe something like that would do:

CpuExceptionHandlerLib supplies misaligned GDT to the outer world
(e.g. ArchSetupExceptionStack) when PcdCpuStackGuard is enabled.
This happens because it uses an array of UINT8 for the mNewGdt
variable, which alignment is 1 byte versus required 8 bytes. As a result
ArchSetupExceptionStack always returns EFI_INVALID_PARAMETER in OVMF Ia32
with XCODE5 and CLANGPDB at least.

Fix this by allocating extra space in mNewGdt and then aligning the pointer
upwards.

Best wishes,
Vitaly

On 5 Nov 2021, at 22:28, Leif Lindholm <leif@...> wrote:

UefiCpuPkg maintainers - please respond.

Meanwhile, Vitaly, could you please provide a commit message?
The BZ link is needed, but it's not a substitute.

/
Leif

On Mon, Sep 20, 2021 at 17:13:47 +0300, Vitaly Cheptsov wrote:
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3639



Cc: Jiewen Yao <jiewen.yao@...>

Cc: Eric Dong <eric.dong@...>

Cc: Michael Kinney <michael.d.kinney@...>

Cc: Jian J Wang <jian.j.wang@...>

Cc: Jeff Fan <vanjeff_919@...>

Cc: Mikhail Krichanov <krichanov@...>

Cc: Marvin Häuser <mhaeuser@...>

Signed-off-by: Vitaly Cheptsov <cheptsov@...>

---

.../Library/CpuExceptionHandlerLib/DxeException.c | 12 +++++++-----

1 file changed, 7 insertions(+), 5 deletions(-)



diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c b/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c

index fd59f09ecd..12874811e1 100644

--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c

+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c

@@ -22,7 +22,7 @@ EXCEPTION_HANDLER_DATA mExceptionHandlerData;



UINT8 mNewStack[CPU_STACK_SWITCH_EXCEPTION_NUMBER *

CPU_KNOWN_GOOD_STACK_SIZE];

-UINT8 mNewGdt[CPU_TSS_GDT_SIZE];

+UINT8 mNewGdt[CPU_TSS_GDT_SIZE + IA32_GDT_ALIGNMENT];



/**

Common exception handler.

@@ -238,6 +238,7 @@ InitializeCpuExceptionHandlersEx (

CPU_EXCEPTION_INIT_DATA EssData;

IA32_DESCRIPTOR Idtr;

IA32_DESCRIPTOR Gdtr;

+ UINT8 *Gdt;



//

// To avoid repeat initialization of default handlers, the caller should pass

@@ -259,6 +260,7 @@ InitializeCpuExceptionHandlersEx (

if (PcdGetBool (PcdCpuStackGuard)) {

if (InitData == NULL) {

SetMem (mNewGdt, sizeof (mNewGdt), 0);

+ Gdt = ALIGN_POINTER (mNewGdt, IA32_GDT_ALIGNMENT);



AsmReadIdtr (&Idtr);

AsmReadGdtr (&Gdtr);

@@ -270,11 +272,11 @@ InitializeCpuExceptionHandlersEx (

EssData.X64.StackSwitchExceptionNumber = CPU_STACK_SWITCH_EXCEPTION_NUMBER;

EssData.X64.IdtTable = (VOID *)Idtr.Base;

EssData.X64.IdtTableSize = Idtr.Limit + 1;

- EssData.X64.GdtTable = mNewGdt;

- EssData.X64.GdtTableSize = sizeof (mNewGdt);

- EssData.X64.ExceptionTssDesc = mNewGdt + Gdtr.Limit + 1;

+ EssData.X64.GdtTable = Gdt;

+ EssData.X64.GdtTableSize = CPU_TSS_GDT_SIZE;

+ EssData.X64.ExceptionTssDesc = Gdt + Gdtr.Limit + 1;

EssData.X64.ExceptionTssDescSize = CPU_TSS_DESC_SIZE;

- EssData.X64.ExceptionTss = mNewGdt + Gdtr.Limit + 1 + CPU_TSS_DESC_SIZE;

+ EssData.X64.ExceptionTss = Gdt + Gdtr.Limit + 1 + CPU_TSS_DESC_SIZE;

EssData.X64.ExceptionTssSize = CPU_TSS_SIZE;



InitData = &EssData;

--

2.30.1 (Apple Git-130)







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