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  1. Core ReactOS
  2. CORE-12623

freeldr/arch/i386/hardware.c (PnP code): incorrect check of 'PnpBufferSize' max boundary



    • Bug
    • Resolution: Fixed
    • Major
    • 0.4.5
    • Bootloader
    • 55,173


      r73488 - boot/freeldr/freeldr/arch/i386/hardware.c (appears same thru at least r73501, latest at this writing)

      (The following is from code observation/review, perhaps I am mistaken.)

      This concerns the routine DetectPnpBios() and routines it calls.

      A size for allocation is computed,
      Size = sizeof(CM_PARTIAL_RESOURCE_LIST)
      + sizeof(CM_PNP_BIOS_INSTALLATION_CHECK) + (NodeSize * NodeCount);

      It later has a tracking size variable PnpBufferSize, which it uses to track how much of a buffer has been used, with the expression
      if (PnpBufferSize + DeviceNode->Size > Size)

      but, 'Size' on the right of that expression appears to include overhead sizeof(CM_PARTIAL_RESOURCE_LIST), which the buffer walking pointer 'Ptr' has already skipped over ('Ptr = (char *)(PartialResourceList + 1);').

      Hence, it appears that the tracking variable Ptr could be moved past the end of the actually available area by as much as sizeof(CM_PARTIAL_RESOURCE_LIST).

      This likely would only ever cause a real problem if a bios did return drive information for more drives than were allowed for in the earlier calls that resulted in the expression 'NodeSize * NodeCount', or, if the cumulative 'DeviceNode->Size's is ever greater than the total computed 'NodeSize*NodeCount', which would mean DeviceNode->Size was not always equal to the 'NodeSize' value apparently returned from the call PnpBiosGetDeviceNodeCount(&NodeSize, &NodeCount);

      If it is possible that NodeSize != DeviceNode->Size, then probably a loop should be performed prior to the allocation to determine the actual sizes involved, and use that to allocate the buffer.

      The current code assumes that NodeSize*NodeCount will always be greater than or equal to the accumulation of all actual 'DeviceNode->Size's.

      (Don't appear to be any active 'assert()'s in the boot code, but some sort of check that DeviceNode->Size == NodeSize might be comforting, and error/remediation taken if not so. If this is deemed not useful, then I would also question why the check mentioned above
      'if (PnpBufferSize + DeviceNode->Size > Size)'
      is present, as it should also not be necessary if DeviceNode->Size is always less than or equal to NodeSize obtained from 'PnpBiosGetDeviceNodeCount(&NodeSize, &NodeCount);' and used in buffer size for allocation.)


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