chipsec-chipsec/chipsec/modules/common/bios_wp.py

# CHIPSEC: Platform Security Assessment Framework
# Copyright (c) 2010-2020, Intel Corporation
#
# This program is free software; you can redistribute it and/or
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
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# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
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# Contact information:
# chipsec@intel.com
#


"""
The BIOS region in flash can be protected either using SMM-based protection or using configuration in the SPI controller. However, the SPI controller configuration is set once and locked, which would prevent writes later.

This module checks both mechanisms. In order to pass this test using SPI controller configuration, the SPI Protected Range registers (PR0-4) will need to cover the entire BIOS region.
Often, if this configuration is used at all, it is used only to protect part of the BIOS region (usually the boot block).
If other important data (eg. NVRAM) is not protected, however, some vulnerabilities may be possible.

`A Tale of One Software Bypass of Windows 8 Secure Boot <http://www.c7zero.info/stuff/Windows8SecureBoot_Bulygin-Furtak-Bazhniuk_BHUSA2013.pdf>`_
In a system where certain BIOS data was not protected, malware may be able to write to the Platform Key stored on the flash, thereby disabling secure boot.

SMM based write protection is controlled from the BIOS Control Register. When the BIOS Write Protect Disable bit is set (sometimes called BIOSWE or BIOS Write Enable), then writes are allowed. When cleared, it can also be locked with the BIOS Lock Enable (BLE) bit. When locked, attempts to change the WPD bit will result in generation of an SMI. This way, the SMI handler can decide whether to perform the write.

As demonstrated in the `Speed Racer <https://bromiumlabs.files.wordpress.com/2015/01/speed_racer_whitepaper.pdf>`_ issue, a race condition may exist between the outstanding write and processing of the SMI that is generated. For this reason, the EISS bit (sometimes called SMM_BWP or SMM BIOS Write Protection) must be set to ensure that only SMM can write to the SPI flash.

References:
    - `A Tale of One Software Bypass of Windows 8 Secure Boot <http://www.c7zero.info/stuff/Windows8SecureBoot_Bulygin-Furtak-Bazhniuk_BHUSA2013.pdf>`_
    - `Speed Racer <https://bromiumlabs.files.wordpress.com/2015/01/speed_racer_whitepaper.pdf>`_

Usage:
    ``chipsec_main -m common.bios_wp``

Examples:
    >>> chipsec_main.py -m common.bios_wp

Registers used: (n = 0,1,2,3,4)
    - BiosLockEnable (Control)
    - BiosWriteEnable (Control)
    - SmmBiosWriteProtection (Control)
    - PRn.PRB
    - PRn.RPE
    - PRn.PRL
    - PRn.WPE

.. note::
    - Module will fail if SMM-based protection is not correctly configured and SPI protected ranges (PR registers) do not protect the entire BIOS region.

"""

from chipsec.module_common import BaseModule, ModuleResult, MTAG_BIOS
from chipsec.hal.spi import BIOS, SPI


TAGS = [MTAG_BIOS]

class bios_wp(BaseModule):

    def __init__(self):
        BaseModule.__init__(self)
        self.spi = SPI(self.cs)

    def is_supported(self):
        return True

    def check_BIOS_write_protection(self):
        ble = self.cs.get_control('BiosLockEnable', with_print=True)
        bioswe = self.cs.get_control('BiosWriteEnable')
        smmbwp = self.cs.get_control('SmmBiosWriteProtection')

        # Is the BIOS flash region write protected?
        write_protected = 0
        if (1 == ble) and (0 == bioswe):
            if 1 == smmbwp:
                self.logger.log_good("BIOS region write protection is enabled (writes restricted to SMM)")
                write_protected = 1
            else:
                self.logger.log_important("Enhanced SMM BIOS region write protection has not been enabled (SMM_BWP is not used)")
        else:
            self.logger.log_bad("BIOS region write protection is disabled!")

        return write_protected == 1

    def check_SPI_protected_ranges(self):
        (bios_base, bios_limit, _) = self.spi.get_SPI_region(BIOS)
        self.logger.log( "\n[*] BIOS Region: Base = 0x{:08X}, Limit = 0x{:08X}".format(bios_base, bios_limit) )
        self.spi.display_SPI_Protected_Ranges()

        pr_cover_bios = False
        pr_partial_cover_bios = False

        areas_to_protect = [(bios_base, bios_limit)]

        for j in range(5):
            (base, limit, wpe, _, _, _) = self.spi.get_SPI_Protected_Range(j)
            if base > limit:
                continue
            if wpe == 1:
                areas = areas_to_protect[:]
                for area in areas:
                    (start, end) = area
                    if (base <= start) and (limit >= start):  # overlap bottom
                        if limit >= end:
                            areas_to_protect.remove(area)
                        else:
                            areas_to_protect.remove(area)
                            area = (limit + 1, end)
                            areas_to_protect.append(area)
                    elif (base <= end) and (limit >= end):  # overlap top
                        if base <= start:
                            areas_to_protect.remove(area)
                        else:
                            areas_to_protect.remove(area)
                            area = (start, base - 1)
                            areas_to_protect.append(area)
                    elif (base > start) and (limit < end):  # split
                        areas_to_protect.remove(area)
                        areas_to_protect.append((start, base - 1))
                        areas_to_protect.append((limit + 1, end))

        if (len(areas_to_protect) == 0):
            pr_cover_bios = True
        else:
            if (len(areas_to_protect) != 1) or (areas_to_protect[0] != (bios_base, bios_limit)):
                pr_partial_cover_bios = True

        if pr_partial_cover_bios:
            self.logger.log('')
            self.logger.log_important("SPI protected ranges write-protect parts of BIOS region (other parts of BIOS can be modified)")
        else:
            if not pr_cover_bios:
                self.logger.log('')
                self.logger.log_important("None of the SPI protected ranges write-protect BIOS region")

        return pr_cover_bios

    # --------------------------------------------------------------------------
    # run( module_argv )
    # Required function: run here all tests from this module
    # --------------------------------------------------------------------------
    def run(self, module_argv ):
        self.logger.start_test("BIOS Region Write Protection")
        wp  = self.check_BIOS_write_protection()
        spr = self.check_SPI_protected_ranges()

        self.logger.log('')
        if wp:
            if spr:
                self.logger.log_passed("BIOS is write protected (by SMM and SPI Protected Ranges)")
            else:
                self.logger.log_passed("BIOS is write protected")
        else:
            if spr:
                self.logger.log_passed("SPI Protected Ranges are configured to write protect BIOS")
            else:
                self.logger.log_important('BIOS should enable all available SMM based write protection mechanisms.')
                self.logger.log_important('Or configure SPI protected ranges to protect the entire BIOS region.')
                self.logger.log_failed("BIOS is NOT protected completely")

        if wp or spr:
            self.res = ModuleResult.PASSED
        else:
            self.res = ModuleResult.FAILED

        return self.res