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UefiCpuPkg/PiSmmCpuDxeSmm: Fix CP Exception when CET enable
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Root cause:
1. Before DisableReadonlyPageWriteProtect() is called, the return
address (coconut-svsm#1) is pushed in shadow stack.
2. CET is disabled.
3. DisableReadonlyPageWriteProtect() returns to coconut-svsm#1.
4. Page table is modified.
5. EnableReadonlyPageWriteProtect() is called, but the return
address (coconut-svsm#2) is not pushed in shadow stack.
6. CET is enabled.
7. EnableReadonlyPageWriteProtect() returns to coconut-svsm#2.
#CP exception happens because the actual return address (coconut-svsm#2)
doesn't match the return address stored in shadow stack (coconut-svsm#1).

Analysis:
Shadow stack will stop update after CET disable (DisableCet() in
DisableReadOnlyPageWriteProtect), but normal smi stack will be
continue updated with the function called and return
(DisableReadOnlyPageWriteProtect & EnableReadOnlyPageWriteProtect),
thus leading stack mismatch after CET re-enabled (EnableCet() in
EnableReadOnlyPageWriteProtect).

According SDM Vol 3, 6.15-Control Protection Exception:
Normal smi stack and shadow stack must be matched when CET enable,
otherwise CP Exception will happen, which is caused by a near RET
instruction.

CET is disabled in DisableCet(), while can be enabled in
EnableCet(). This way won't cause the problem because they are
implemented in a way that return address of DisableCet() is
poped out from shadow stack (Incsspq performs a pop to increases
the shadow stack) and EnableCet() doesn't use "RET" but "JMP" to
return to caller. So calling EnableCet() and DisableCet() doesn't
have the same issue as calling DisableReadonlyPageWriteProtect()
and EnableReadonlyPageWriteProtect().

With above root cause & analysis, define below 2 macros instead of
functions for WP & CET operation:
WRITE_UNPROTECT_RO_PAGES (Wp, Cet)
WRITE_PROTECT_RO_PAGES (Wp, Cet)
Because DisableCet() & EnableCet() must be in the same function
to avoid shadow stack and normal SMI stack mismatch.

Note: WRITE_UNPROTECT_RO_PAGES () must be called pair with
WRITE_PROTECT_RO_PAGES () in same function.

Change-Id: I4e126697efcd8dbfb4887da034d8691bfca969e3
Cc: Eric Dong <[email protected]>
Cc: Ray Ni <[email protected]>
Cc: Zeng Star <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: Rahul Kumar <[email protected]>
Cc: Laszlo Ersek <[email protected]>
Signed-off-by: Jiaxin Wu <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Reviewed-by: Ray Ni <[email protected]>
Reviewed-by: Eric Dong <[email protected]>
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@jiaxinwu @mergify
jiaxinwu authored and mergify[bot] committed Nov 10, 2023
1 parent 35c0c63 commit 589f2e4
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Showing 3 changed files with 81 additions and 58 deletions.
59 changes: 47 additions & 12 deletions UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1553,27 +1553,62 @@ SmmWaitForApArrival (
);

/**
Disable Write Protect on pages marked as read-only if Cr0.Bits.WP is 1.
Write unprotect read-only pages if Cr0.Bits.WP is 1.
@param[out] WriteProtect If Cr0.Bits.WP is enabled.
@param[out] WpEnabled If Cr0.WP is enabled.
@param[out] CetEnabled If CET is enabled.
**/
VOID
DisableReadOnlyPageWriteProtect (
OUT BOOLEAN *WpEnabled,
OUT BOOLEAN *CetEnabled
SmmWriteUnprotectReadOnlyPage (
OUT BOOLEAN *WriteProtect
);

/**
Enable Write Protect on pages marked as read-only.
Write protect read-only pages.
@param[in] WriteProtect If Cr0.Bits.WP should be enabled.
@param[out] WpEnabled If Cr0.WP should be enabled.
@param[out] CetEnabled If CET should be enabled.
**/
VOID
EnableReadOnlyPageWriteProtect (
BOOLEAN WpEnabled,
BOOLEAN CetEnabled
SmmWriteProtectReadOnlyPage (
IN BOOLEAN WriteProtect
);

///
/// Define macros to encapsulate the write unprotect/protect
/// read-only pages.
/// Below pieces of logic are defined as macros and not functions
/// because "CET" feature disable & enable must be in the same
/// function to avoid shadow stack and normal SMI stack mismatch,
/// thus WRITE_UNPROTECT_RO_PAGES () must be called pair with
/// WRITE_PROTECT_RO_PAGES () in same function.
///
/// @param[in,out] Wp A BOOLEAN variable local to the containing
/// function, carrying write protection status from
/// WRITE_UNPROTECT_RO_PAGES() to
/// WRITE_PROTECT_RO_PAGES().
///
/// @param[in,out] Cet A BOOLEAN variable local to the containing
/// function, carrying control flow integrity
/// enforcement status from
/// WRITE_UNPROTECT_RO_PAGES() to
/// WRITE_PROTECT_RO_PAGES().
///
#define WRITE_UNPROTECT_RO_PAGES(Wp, Cet) \
do { \
Cet = ((AsmReadCr4 () & CR4_CET_ENABLE) != 0); \
if (Cet) { \
DisableCet (); \
} \
SmmWriteUnprotectReadOnlyPage (&Wp); \
} while (FALSE)

#define WRITE_PROTECT_RO_PAGES(Wp, Cet) \
do { \
SmmWriteProtectReadOnlyPage (Wp); \
if (Cet) { \
EnableCet (); \
} \
} while (FALSE)

#endif
73 changes: 30 additions & 43 deletions UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -41,60 +41,43 @@ PAGE_TABLE_POOL *mPageTablePool = NULL;
BOOLEAN mIsReadOnlyPageTable = FALSE;

/**
Disable Write Protect on pages marked as read-only if Cr0.Bits.WP is 1.
Write unprotect read-only pages if Cr0.Bits.WP is 1.
@param[out] WriteProtect If Cr0.Bits.WP is enabled.
@param[out] WpEnabled If Cr0.WP is enabled.
@param[out] CetEnabled If CET is enabled.
**/
VOID
DisableReadOnlyPageWriteProtect (
OUT BOOLEAN *WpEnabled,
OUT BOOLEAN *CetEnabled
SmmWriteUnprotectReadOnlyPage (
OUT BOOLEAN *WriteProtect
)
{
IA32_CR0 Cr0;

*CetEnabled = ((AsmReadCr4 () & CR4_CET_ENABLE) != 0) ? TRUE : FALSE;
Cr0.UintN = AsmReadCr0 ();
*WpEnabled = (Cr0.Bits.WP != 0) ? TRUE : FALSE;
if (*WpEnabled) {
if (*CetEnabled) {
//
// CET must be disabled if WP is disabled. Disable CET before clearing CR0.WP.
//
DisableCet ();
}

Cr0.UintN = AsmReadCr0 ();
*WriteProtect = (Cr0.Bits.WP != 0);
if (*WriteProtect) {
Cr0.Bits.WP = 0;
AsmWriteCr0 (Cr0.UintN);
}
}

/**
Enable Write Protect on pages marked as read-only.
Write protect read-only pages.
@param[in] WriteProtect If Cr0.Bits.WP should be enabled.
@param[out] WpEnabled If Cr0.WP should be enabled.
@param[out] CetEnabled If CET should be enabled.
**/
VOID
EnableReadOnlyPageWriteProtect (
BOOLEAN WpEnabled,
BOOLEAN CetEnabled
SmmWriteProtectReadOnlyPage (
IN BOOLEAN WriteProtect
)
{
IA32_CR0 Cr0;

if (WpEnabled) {
if (WriteProtect) {
Cr0.UintN = AsmReadCr0 ();
Cr0.Bits.WP = 1;
AsmWriteCr0 (Cr0.UintN);

if (CetEnabled) {
//
// re-enable CET.
//
EnableCet ();
}
}
}

Expand All @@ -121,7 +104,7 @@ InitializePageTablePool (
)
{
VOID *Buffer;
BOOLEAN WpEnabled;
BOOLEAN WriteProtect;
BOOLEAN CetEnabled;

//
Expand Down Expand Up @@ -159,9 +142,11 @@ InitializePageTablePool (
// If page table memory has been marked as RO, mark the new pool pages as read-only.
//
if (mIsReadOnlyPageTable) {
DisableReadOnlyPageWriteProtect (&WpEnabled, &CetEnabled);
WRITE_UNPROTECT_RO_PAGES (WriteProtect, CetEnabled);

SmmSetMemoryAttributes ((EFI_PHYSICAL_ADDRESS)(UINTN)Buffer, EFI_PAGES_TO_SIZE (PoolPages), EFI_MEMORY_RO);
EnableReadOnlyPageWriteProtect (WpEnabled, CetEnabled);

WRITE_PROTECT_RO_PAGES (WriteProtect, CetEnabled);
}

return TRUE;
Expand Down Expand Up @@ -1011,7 +996,7 @@ SetMemMapAttributes (
IA32_MAP_ENTRY *Map;
UINTN Count;
UINT64 MemoryAttribute;
BOOLEAN WpEnabled;
BOOLEAN WriteProtect;
BOOLEAN CetEnabled;

SmmGetSystemConfigurationTable (&gEdkiiPiSmmMemoryAttributesTableGuid, (VOID **)&MemoryAttributesTable);
Expand Down Expand Up @@ -1057,7 +1042,7 @@ SetMemMapAttributes (

ASSERT_RETURN_ERROR (Status);

DisableReadOnlyPageWriteProtect (&WpEnabled, &CetEnabled);
WRITE_UNPROTECT_RO_PAGES (WriteProtect, CetEnabled);

MemoryMap = MemoryMapStart;
for (Index = 0; Index < MemoryMapEntryCount; Index++) {
Expand Down Expand Up @@ -1087,7 +1072,8 @@ SetMemMapAttributes (
MemoryMap = NEXT_MEMORY_DESCRIPTOR (MemoryMap, DescriptorSize);
}

EnableReadOnlyPageWriteProtect (WpEnabled, CetEnabled);
WRITE_PROTECT_RO_PAGES (WriteProtect, CetEnabled);

FreePool (Map);

PatchSmmSaveStateMap ();
Expand Down Expand Up @@ -1394,14 +1380,14 @@ SetUefiMemMapAttributes (
UINTN MemoryMapEntryCount;
UINTN Index;
EFI_MEMORY_DESCRIPTOR *Entry;
BOOLEAN WpEnabled;
BOOLEAN WriteProtect;
BOOLEAN CetEnabled;

PERF_FUNCTION_BEGIN ();

DEBUG ((DEBUG_INFO, "SetUefiMemMapAttributes\n"));

DisableReadOnlyPageWriteProtect (&WpEnabled, &CetEnabled);
WRITE_UNPROTECT_RO_PAGES (WriteProtect, CetEnabled);

if (mUefiMemoryMap != NULL) {
MemoryMapEntryCount = mUefiMemoryMapSize/mUefiDescriptorSize;
Expand Down Expand Up @@ -1481,7 +1467,7 @@ SetUefiMemMapAttributes (
}
}

EnableReadOnlyPageWriteProtect (WpEnabled, CetEnabled);
WRITE_PROTECT_RO_PAGES (WriteProtect, CetEnabled);

//
// Do not free mUefiMemoryAttributesTable, it will be checked in IsSmmCommBufferForbiddenAddress().
Expand Down Expand Up @@ -1872,7 +1858,7 @@ SetPageTableAttributes (
VOID
)
{
BOOLEAN WpEnabled;
BOOLEAN WriteProtect;
BOOLEAN CetEnabled;

if (!IfReadOnlyPageTableNeeded ()) {
Expand All @@ -1886,7 +1872,7 @@ SetPageTableAttributes (
// Disable write protection, because we need mark page table to be write protected.
// We need *write* page table memory, to mark itself to be *read only*.
//
DisableReadOnlyPageWriteProtect (&WpEnabled, &CetEnabled);
WRITE_UNPROTECT_RO_PAGES (WriteProtect, CetEnabled);

// Set memory used by page table as Read Only.
DEBUG ((DEBUG_INFO, "Start...\n"));
Expand All @@ -1895,7 +1881,8 @@ SetPageTableAttributes (
//
// Enable write protection, after page table attribute updated.
//
EnableReadOnlyPageWriteProtect (TRUE, CetEnabled);
WRITE_PROTECT_RO_PAGES (TRUE, CetEnabled);

mIsReadOnlyPageTable = TRUE;

//
Expand Down
7 changes: 4 additions & 3 deletions UefiCpuPkg/PiSmmCpuDxeSmm/SmmProfile.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -594,7 +594,7 @@ InitPaging (
UINT64 Limit;
UINT64 PreviousAddress;
UINT64 MemoryAttrMask;
BOOLEAN WpEnabled;
BOOLEAN WriteProtect;
BOOLEAN CetEnabled;

PERF_FUNCTION_BEGIN ();
Expand All @@ -606,7 +606,8 @@ InitPaging (
Limit = (IsRestrictedMemoryAccess ()) ? LShiftU64 (1, mPhysicalAddressBits) : BASE_4GB;
}

DisableReadOnlyPageWriteProtect (&WpEnabled, &CetEnabled);
WRITE_UNPROTECT_RO_PAGES (WriteProtect, CetEnabled);

//
// [0, 4k] may be non-present.
//
Expand Down Expand Up @@ -672,7 +673,7 @@ InitPaging (
ASSERT_RETURN_ERROR (Status);
}

EnableReadOnlyPageWriteProtect (WpEnabled, CetEnabled);
WRITE_PROTECT_RO_PAGES (WriteProtect, CetEnabled);

//
// Flush TLB
Expand Down

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