[U-Boot] CVE-2018-18439, CVE-2018-18440 - U-Boot verified boot bypass vulnerabilities
Daniele Bianco
daniele.bianco at f-secure.com
Wed Nov 14 15:35:30 UTC 2018
On Wed, Nov 14, 2018 at 04:26:17PM +0100, Andrea Barisani wrote:
> On Wed, Nov 14, 2018 at 04:13:00PM +0100, Simon Goldschmidt wrote:
> > 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.
> >
>
> CC'ing my colleague Daniele whom can better reply further on this.
Hi Simon,
the 'if (tftp_cur_block != 1)' is not triggered if 'tftp_mcast_active'
is set (and the CONFIG_MCAST_TFTP is defined).
Please note the code indentation does not help in this case as it is
misleading, but this is because of the #ifdef.
Cheers,
Daniele
>
> > > 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 :-)
> >
>
> And that's much appreciated :)
>
> > >
> > > 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.
> >
>
> Understood.
>
> Cheers
>
> > 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
> >
> >
>
> --
> Andrea Barisani Head of Hardware Security | F-Secure
> Founder | Inverse Path
>
> https://www.f-secure.com https://inversepath.com
> 0x864C9B9E 0A76 074A 02CD E989 CE7F AC3F DA47 578E 864C 9B9E
> "Pluralitas non est ponenda sine necessitate"
--
Daniele Bianco
Hardware Security | F-Secure
<daniele.bianco at f-secure.com> | https://www.f-secure.com
GPG Key fingerprint = 88A7 43F4 F28F 1B9D 6F2D 4AC5 AE75 822E 9544 A497
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