OvmfPkg/CpuHotplugSmm: introduce Post-SMM Pen for hot-added CPUs

Once a hot-added CPU finishes the SMBASE relocation, we need to pen it in
a HLT loop. Add the NASM implementation (with just a handful of
instructions, but much documentation), and some C language helper
functions.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1512
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200226221156.29589-12-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf

@@ -24,8 +24,11 @@
 [Sources]
   ApicId.h
   CpuHotplug.c
+  PostSmmPen.nasm
   QemuCpuhp.c
   QemuCpuhp.h
+  Smbase.c
+  Smbase.h
 
 [Packages]
   MdePkg/MdePkg.dec
@@ -34,6 +37,7 @@
 
 [LibraryClasses]
   BaseLib
+  BaseMemoryLib
   DebugLib
   MmServicesTableLib
   PcdLib

OvmfPkg/CpuHotplugSmm/PostSmmPen.nasm

@@ -0,0 +1,151 @@
+;------------------------------------------------------------------------------
+; @file
+; Pen any hot-added CPU in a 16-bit, real mode HLT loop, after it leaves SMM by
+; executing the RSM instruction.
+;
+; Copyright (c) 2020, Red Hat, Inc.
+;
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+; The routine implemented here is stored into normal RAM, under 1MB, at the
+; beginning of a page that is allocated as EfiReservedMemoryType. On any
+; hot-added CPU, it is executed after *at least* the first RSM (i.e., after
+; SMBASE relocation).
+;
+; The first execution of this code occurs as follows:
+;
+; - The hot-added CPU is in RESET state.
+;
+; - The ACPI CPU hotplug event handler triggers a broadcast SMI, from the OS.
+;
+; - Existent CPUs (BSP and APs) enter SMM.
+;
+; - The hot-added CPU remains in RESET state, but an SMI is pending for it now.
+;   (See "SYSTEM MANAGEMENT INTERRUPT (SMI)" in the Intel SDM.)
+;
+; - In SMM, pre-existent CPUs that are not elected SMM Monarch, keep themselves
+;   busy with their wait loops.
+;
+; - From the root MMI handler, the SMM Monarch:
+;
+;   - places this routine in the reserved page,
+;
+;   - clears the "about to leave SMM" byte in SMRAM,
+;
+;   - clears the last byte of the reserved page,
+;
+;   - sends an INIT-SIPI-SIPI sequence to the hot-added CPU,
+;
+;   - un-gates the default SMI handler by APIC ID.
+;
+; - The startup vector in the SIPI that is sent by the SMM Monarch points to
+;   this code; i.e., to the reserved page. (Example: 0x9_F000.)
+;
+; - The SMM Monarch starts polling the "about to leave SMM" byte in SMRAM.
+;
+; - The hot-added CPU boots, and immediately enters SMM due to the pending SMI.
+;   It starts executing the default SMI handler.
+;
+; - Importantly, the SMRAM Save State Map captures the following information,
+;   when the hot-added CPU enters SMM:
+;
+;   - CS selector: assumes the 16 most significant bits of the 20-bit (i.e.,
+;     below 1MB) startup vector from the SIPI. (Example: 0x9F00.)
+;
+;   - CS attributes: Accessed, Readable, User (S=1), CodeSegment (bit#11),
+;     Present.
+;
+;   - CS limit: 0xFFFF.
+;
+;   - CS base: the CS selector value shifted left by 4 bits. That is, the CS
+;     base equals the SIPI startup vector. (Example: 0x9_F000.)
+;
+;   - IP: the least significant 4 bits from the SIPI startup vector. Because
+;     the routine is page-aligned, these bits are zero (hence IP is zero).
+;
+;   - ES, SS, DS, FS, GS selectors: 0.
+;
+;   - ES, SS, DS, FS, GS attributes: same as the CS attributes, minus
+;     CodeSegment (bit#11).
+;
+;   - ES, SS, DS, FS, GS limits: 0xFFFF.
+;
+;   - ES, SS, DS, FS, GS bases: 0.
+;
+; - The hot-added CPU sets its new SMBASE value in the SMRAM Save State Map.
+;
+; - The hot-added CPU sets the "about to leave SMM" byte in SMRAM, then
+;   executes the RSM instruction immediately after, leaving SMM.
+;
+; - The SMM Monarch notices that the "about to leave SMM" byte in SMRAM has
+;   been set, and starts polling the last byte in the reserved page.
+;
+; - The hot-added CPU jumps ("returns") to the code below (in the reserved
+;   page), according to the register state listed in the SMRAM Save State Map.
+;
+; - The hot-added CPU sets the last byte of the reserved page, then halts
+;   itself.
+;
+; - The SMM Monarch notices that the hot-added CPU is done with SMBASE
+;   relocation.
+;
+; Note that, if the OS is malicious and sends INIT-SIPI-SIPI to the hot-added
+; CPU before allowing the ACPI CPU hotplug event handler to trigger a broadcast
+; SMI, then said broadcast SMI will yank the hot-added CPU directly into SMM,
+; without becoming pending for it (as the hot-added CPU is no longer in RESET
+; state). This is OK, because:
+;
+; - The default SMI handler copes with this, as it is gated by APIC ID. The
+;   hot-added CPU won't start the actual SMBASE relocation until the SMM
+;   Monarch lets it.
+;
+; - The INIT-SIPI-SIPI sequence that the SMM Monarch sends to the hot-added CPU
+;   will be ignored in this sate (it won't even be latched). See "SMI HANDLER
+;   EXECUTION ENVIRONMENT" in the Intel SDM: "INIT operations are inhibited
+;   when the processor enters SMM".
+;
+; - When the hot-added CPU (e.g., CPU#1) executes the RSM (having relocated
+;   SMBASE), it returns to the OS. The OS can use CPU#1 to attack the last byte
+;   of the reserved page, while another CPU (e.g., CPU#2) is relocating SMBASE,
+;   in order to trick the SMM Monarch (e.g., CPU#0) to open the APIC ID gate
+;   for yet another CPU (e.g., CPU#3). However, the SMM Monarch won't look at
+;   the last byte of the reserved page, until CPU#2 sets the "about to leave
+;   SMM" byte in SMRAM. This leaves a very small window (just one instruction's
+;   worth before the RSM) for CPU#3 to "catch up" with CPU#2, and overwrite
+;   CPU#2's SMBASE with its own.
+;
+; In other words, we do not / need not prevent a malicious OS from booting the
+; hot-added CPU early; instead we provide benign OSes with a pen for hot-added
+; CPUs.
+;------------------------------------------------------------------------------
+
+SECTION .data
+BITS 16
+
+GLOBAL ASM_PFX (mPostSmmPen)     ; UINT8[]
+GLOBAL ASM_PFX (mPostSmmPenSize) ; UINT16
+
+ASM_PFX (mPostSmmPen):
+  ;
+  ; Point DS at the same reserved page.
+  ;
+  mov ax, cs
+  mov ds, ax
+
+  ;
+  ; Inform the SMM Monarch that we're done with SMBASE relocation, by setting
+  ; the last byte in the reserved page.
+  ;
+  mov byte [ds : word 0xFFF], 1
+
+  ;
+  ; Halt now, until we get woken by another SMI, or (more likely) the OS
+  ; reboots us with another INIT-SIPI-SIPI.
+  ;
+HltLoop:
+  cli
+  hlt
+  jmp HltLoop
+
+ASM_PFX (mPostSmmPenSize):
+  dw $ - ASM_PFX (mPostSmmPen)

OvmfPkg/CpuHotplugSmm/Smbase.c

@@ -0,0 +1,110 @@
+/** @file
+  SMBASE relocation for hot-plugged CPUs.
+
+  Copyright (c) 2020, Red Hat, Inc.
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include <Base.h>                             // BASE_1MB
+#include <Library/BaseMemoryLib.h>            // CopyMem()
+#include <Library/DebugLib.h>                 // DEBUG()
+
+#include "Smbase.h"
+
+extern CONST UINT8 mPostSmmPen[];
+extern CONST UINT16 mPostSmmPenSize;
+
+/**
+  Allocate a non-SMRAM reserved memory page for the Post-SMM Pen for hot-added
+  CPUs.
+
+  This function may only be called from the entry point function of the driver.
+
+  @param[out] PenAddress   The address of the allocated (normal RAM) reserved
+                           page.
+
+  @param[in] BootServices  Pointer to the UEFI boot services table. Used for
+                           allocating the normal RAM (not SMRAM) reserved page.
+
+  @retval EFI_SUCCESS          Allocation successful.
+
+  @retval EFI_BAD_BUFFER_SIZE  The Post-SMM Pen template is not smaller than
+                               EFI_PAGE_SIZE.
+
+  @return                      Error codes propagated from underlying services.
+                               DEBUG_ERROR messages have been logged. No
+                               resources have been allocated.
+**/
+EFI_STATUS
+SmbaseAllocatePostSmmPen (
+  OUT UINT32                  *PenAddress,
+  IN  CONST EFI_BOOT_SERVICES *BootServices
+  )
+{
+  EFI_STATUS           Status;
+  EFI_PHYSICAL_ADDRESS Address;
+
+  //
+  // The pen code must fit in one page, and the last byte must remain free for
+  // signaling the SMM Monarch.
+  //
+  if (mPostSmmPenSize >= EFI_PAGE_SIZE) {
+    Status = EFI_BAD_BUFFER_SIZE;
+    DEBUG ((DEBUG_ERROR, "%a: mPostSmmPenSize=%u: %r\n", __FUNCTION__,
+      mPostSmmPenSize, Status));
+    return Status;
+  }
+
+  Address = BASE_1MB - 1;
+  Status = BootServices->AllocatePages (AllocateMaxAddress,
+                           EfiReservedMemoryType, 1, &Address);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "%a: AllocatePages(): %r\n", __FUNCTION__, Status));
+    return Status;
+  }
+
+  DEBUG ((DEBUG_INFO, "%a: Post-SMM Pen at 0x%Lx\n", __FUNCTION__, Address));
+  *PenAddress = (UINT32)Address;
+  return EFI_SUCCESS;
+}
+
+/**
+  Copy the Post-SMM Pen template code into the reserved page allocated with
+  SmbaseAllocatePostSmmPen().
+
+  Note that this effects an "SMRAM to normal RAM" copy.
+
+  The SMM Monarch is supposed to call this function from the root MMI handler.
+
+  @param[in] PenAddress  The allocation address returned by
+                         SmbaseAllocatePostSmmPen().
+**/
+VOID
+SmbaseReinstallPostSmmPen (
+  IN UINT32 PenAddress
+  )
+{
+  CopyMem ((VOID *)(UINTN)PenAddress, mPostSmmPen, mPostSmmPenSize);
+}
+
+/**
+  Release the reserved page allocated with SmbaseAllocatePostSmmPen().
+
+  This function may only be called from the entry point function of the driver,
+  on the error path.
+
+  @param[in] PenAddress    The allocation address returned by
+                           SmbaseAllocatePostSmmPen().
+
+  @param[in] BootServices  Pointer to the UEFI boot services table. Used for
+                           releasing the normal RAM (not SMRAM) reserved page.
+**/
+VOID
+SmbaseReleasePostSmmPen (
+  IN UINT32                  PenAddress,
+  IN CONST EFI_BOOT_SERVICES *BootServices
+  )
+{
+  BootServices->FreePages (PenAddress, 1);
+}

OvmfPkg/CpuHotplugSmm/Smbase.h

@@ -0,0 +1,32 @@
+/** @file
+  SMBASE relocation for hot-plugged CPUs.
+
+  Copyright (c) 2020, Red Hat, Inc.
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#ifndef SMBASE_H_
+#define SMBASE_H_
+
+#include <Uefi/UefiBaseType.h> // EFI_STATUS
+#include <Uefi/UefiSpec.h>     // EFI_BOOT_SERVICES
+
+EFI_STATUS
+SmbaseAllocatePostSmmPen (
+  OUT UINT32                  *PenAddress,
+  IN  CONST EFI_BOOT_SERVICES *BootServices
+  );
+
+VOID
+SmbaseReinstallPostSmmPen (
+  IN UINT32 PenAddress
+  );
+
+VOID
+SmbaseReleasePostSmmPen (
+  IN UINT32                  PenAddress,
+  IN CONST EFI_BOOT_SERVICES *BootServices
+  );
+
+#endif // SMBASE_H_