On 14.11.2018 12:52, Andrea Barisani wrote:
On Tue, Nov 13, 2018 at 09:57:23PM +0100, Simon Goldschmidt wrote:
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
Hello Simon,
the integer underflow can happen if a malicious TFTP server, able to control
the TFTP packets sequence number, sends a crafted packet with sequence number
set to 0 during a flow.
This happens because, within the store_block() function, the 'block' argument
is declared as 'int' and when it is invoked inside tftp_handler() (case
TFTP_DATA) this value is passed by doing 'tftp_cur_block - 1' (where
tftp_cur_block is the sequence number extracted from the tftp packet without
any previous check):
static inline void store_block(int block, uchar *src, unsigned len)
^^^^^^^^^ can have negative values (e.g. -1)
{
ulong offset = block * tftp_block_size + tftp_block_wrap_offset;
^^^^^
here if block is -1 the result stored onto offset would be a very
large unsigned number, due to type conversions
And this is exatclty my point. This might be bad coding style, but for
me it works: 'block' is an 'int' and is '-1', so 'block *
tftp_block_size' is '-512'. Now from the code flow in tftp_handler(),
it's clear that if we come here with tftp_cur_block == 0 (so 'block' is
-1), 'tftp_block_wrap_offset' is not 0 but some positive value 'x *
tftp_block_size' (see function 'update_block_number').
So the resulting 'offset' is '-512 + (x * 512)' where 'x > 0'. I still
fail to see how this can be a very large positive number resulting in an
effective negative offset or arbitrary write.
}
static void tftp_handler(...){
case TFTP_DATA:
...
if (tftp_cur_block == tftp_prev_block) {
/* Same block again; ignore it. */
break;
}
tftp_prev_block = tftp_cur_block;
timeout_count_max = tftp_timeout_count_max;
net_set_timeout_handler(timeout_ms, tftp_timeout_handler);
store_block(tftp_cur_block - 1, pkt + 2, len);
^^^^^^^^^^^^^^^^^^
}
For these reasons the issue does not appear to be merely a "one block size"
substraction, but rather offset can reach very large values. Unless I am
missing something that I don't see of course...
So I take it this "bug" report is from reading the code only, not from
actually testing it and seeing the arbitrary memory write? I wouldn't
have expected this in a CVE report...
You should probably prevent the underflow by placing a check against
tftp_cur_block before the store_block() invocation, but I defer to you for a
better implementation of the fix as you certainly know the overall logic much
better.
Don't get me wrong: I'm just yet another user of U-Boot and I don't know
the code better than you do. In fact, I looked at the tftp code for the
first time yesterday after reading you report on the tftp issue in detail.
Simon
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