On 06.11.2018 15:51, Andrea Barisani wrote:
[..] The issue can be exploited by several means:
An excessively large crafted boot image file is parsed by the `tftp_handler` function which lacks any size checks, allowing the memory overwrite.
A malicious server can manipulate TFTP packet sequence numbers to store downloaded file chunks at arbitrary memory locations, given that the sequence number is directly used by the `tftp_handler` function to calculate the destination address for downloaded file chunks.
Additionally the `store_block` function, used to store downloaded file chunks in memory, when invoked by `tftp_handler` with a `tftp_cur_block` value of 0, triggers an unchecked integer underflow.
This allows to potentially erase memory located before the `loadAddr` when a packet is sent with a null, following at least one valid packet.
Do you happen to have more details on this suggested integer underflow? I have tried to reproduce it, but I failed to get a memory write address before 'load_addr'. This is because the 'store_block' function does not directly use the underflowed integer as a block counter, but adds 'tcp_block_wrap_offset' to this offset.
To me it seems like alternating between '0' and 'not 0' for the block counter could increase memory overwrites, but I fail to see how you can use this to store chunks at arbitrary memory locations. All you can do is subtract one block size from 'tftp_block_wrap_offset'...
Simon