MdeModulePkg/Core/Dxe: limit FwVol encapsulation section recursion

The DXE Core sets up a protocol notify function in its entry point, for
instances of the Firmware Volume Block2 Protocol:

  DxeMain()           [DxeMain/DxeMain.c]
    FwVolDriverInit() [FwVol/FwVol.c]

Assume that a 3rd party UEFI driver or application installs an FVB
instance, with crafted contents. The notification function runs:

  NotifyFwVolBlock() [FwVol/FwVol.c]

installing an instance of the Firmware Volume 2 Protocol on the handle.

(Alternatively, assume that a 3rd party application calls
gDS->ProcessFirmwareVolume(), which may also produce a Firmware Volume 2
Protocol instance.)

The EFI_FIRMWARE_VOLUME2_PROTOCOL.ReadSection() member performs "a
depth-first, left-to-right search algorithm through all sections found in
the specified file" (quoting the PI spec), as follows:

  FvReadFileSection()   [FwVol/FwVolRead.c]
    GetSection()        [SectionExtraction/CoreSectionExtraction.c]
      FindChildNode()   [SectionExtraction/CoreSectionExtraction.c]
        FindChildNode() // recursive call

FindChildNode() is called recursively for encapsulation sections.

Currently this recursion is not limited. Introduce a new PCD
(fixed-at-build, or patchable-in-module), and make FindChildNode() track
the section nesting depth against that PCD.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1743
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201119105340.16225-3-lersek@redhat.com>

MdeModulePkg/MdeModulePkg.dec

@@ -1529,6 +1529,12 @@
   # @Prompt Enable Capsule On Disk support.
   gEfiMdeModulePkgTokenSpaceGuid.PcdCapsuleOnDiskSupport|FALSE|BOOLEAN|0x0000002d
 
+  ## Maximum permitted encapsulation levels of sections in a firmware volume,
+  #  in the DXE phase. Minimum value is 1. Sections nested more deeply are
+  #  rejected.
+  # @Prompt Maximum permitted FwVol section nesting depth (exclusive).
+  gEfiMdeModulePkgTokenSpaceGuid.PcdFwVolDxeMaxEncapsulationDepth|0x10|UINT32|0x00000030
+
 [PcdsPatchableInModule, PcdsDynamic, PcdsDynamicEx]
   ## This PCD defines the Console output row. The default value is 25 according to UEFI spec.
   #  This PCD could be set to 0 then console output would be at max column and max row.