[U-Boot] CVE-2018-18439, CVE-2018-18440 - U-Boot verified boot bypass vulnerabilities
Simon Goldschmidt
simon.k.r.goldschmidt at gmail.com
Wed Nov 14 15:13:00 UTC 2018
On 14.11.2018 15:45, Andrea Barisani wrote:
> On Wed, Nov 14, 2018 at 01:03:12PM +0100, Simon Goldschmidt wrote:
>> 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.
>>
> I understand your point, however what does happen when we enter the 'case
> TFTP_DATA' and we are in the first block received, so we trigger
> new_transfer() that sets the tftp_block_wrap_offset to 0 *and*
> tftp_mcast_active is set?
>
> I don't see any protection for this case for the underflow, am I wrong?
>
> static void new_transfer(void)
> {
> tftp_prev_block = 0;
> tftp_block_wrap = 0;
> tftp_block_wrap_offset = 0;
> #ifdef CONFIG_CMD_TFTPPUT
> tftp_put_final_block_sent = 0;
> #endif
> }
>
> ...
> case TFTP_DATA:
>
> if (tftp_state == STATE_SEND_RRQ || tftp_state == STATE_OACK ||
> tftp_state == STATE_RECV_WRQ) {
> /* first block received */
> tftp_state = STATE_DATA;
> tftp_remote_port = src;
> new_transfer();
> ^^^^^^^^^^^^^^^
See some lines below...
>
> #ifdef CONFIG_MCAST_TFTP
> if (tftp_mcast_active) { /* start!=1 common if mcast */ <<<< HERE
> tftp_prev_block = tftp_cur_block - 1;
> } else
> #endif
> if (tftp_cur_block != 1) { /* Assertion */
If tftp_cur_block is 0 for the first block, we stop right away. No
chance to reach store_block() at that time.
> puts("\nTFTP error: ");
> printf("First block is not block 1 (%ld)\n",
> tftp_cur_block);
> puts("Starting again\n\n");
> net_start_again();
> break;
> }
> }
>
> 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);
> ^^^^^^^^^^^^^^^^^^
> This should result in having -1 and thus -512 as result of the 'offset' math
> that converted to ulong would result in a very large value.
>
>>> }
>>>
>>> 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...
>>
> As you see from our report the core issues have been fully tested and
> reproduced.
Yes. Thanks for that. I'm working on fixing them :-)
>
> It is true however that the additional remark on the `store_block' function
> has only been evaluated by code analysis, in the context of the advisory it
> seemed something worth notice in relation to the code structure but again, as
> you say we didn't practically test that specific aspect, while everything
> else was tested and reproduced.
>
> The vulnerability report highlights two (in our opinion) critical
> vulnerabilities, one of which described a secondary aspect only checked by
> means of source code analysis.
In my opinion as well these are critical, yes.
> The secondary aspect that we are discussing does not change the overall
> impact of the TFTP bugs, which remains unchanged as arbitrary code execution
> can anyway be achieved.
Of course. I'm working on fixing the actual bug and while debugging it
tried to fix the other thing you mentioned. I could not reproduce it in
a test setup (where I can freely send tftp packets). That's why I asked.
The other bugs are of course not affected by this one not being valid.
Thanks for confirming this.
Simon
>
> Thanks!
>
>>> 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
More information about the U-Boot
mailing list