On 22.10.20 17:46, Vincent MAILHOL wrote:
On 22.10.20 22:28, Oliver Hartkopp wrote:
On 22.10.20 14:23, Vincent MAILHOL wrote:
The above pseudo code together with CAN_CTRLMODE_RAW_DLC seems to be a
pretty safe option to me. Even if 'legacy' applications with
uninitialized raw_dlc send CAN frames or AF_PACKET users enable
CAN_CTRLMODE_RAW_DLC we always end up with a proper can_dlc == 8 and a
fitting valid raw_dlc.
But what if that "fitting valid raw_dlc" is greater than 8?
This is the plan!
The kernel
the CAN driver
would see that frame as valid, a frame with a DLC greater than 8 will
be sent on the wire and received by the other devices than might be
connected on the CAN bus.
Exactly.
And this is my point. The legacy application intended to send a frame
of 8 bytes with a DLC of 8 but on the wire, there is a frame of 8
bytes with a DLC greater than 8. The other devices on the CAN bus
might ignore it (if the device has a DLC filter) or might crash if
there is a vulnerability in DLC handling.
But this is your intention, right?
You wanted to test this behaviour. This is no normal operation - it's a
*test* mode to detect said vulnerabilities in DLC handling.
And what do you mean with "if the device has a DLC filter"?
You told me the DLCs from 9..15 are correct from the ISO standpoint.
That a good programmer checks the DLC and makes sure he only processes
max. 8 bytes is a common thing and no 'filtering'.
You might introduce CAN_RAW_RAW_DLC sockopt for CAN_RAW sockets but when
you use packet sockets e.g. with Wireshark and forge some CAN frames
there your only chance to have proper 0..8 DLCs is to disable
CAN_CTRLMODE_RAW_DLC.
Btw. do you really see any legacy SocketCAN applications (*together*
with your testing application on the same Linux host) where you don't
have the source code to check whether they properly initialize the
reserved/padding bytes?
You can also use the can-gw to let 'malicious' CAN apps run on a private
virtual CAN. Forwarded modified CAN frames definitely initialize the
reserved/padding bytes.
Basically, that would replace the use of the can_len2dlc() function.
I also think it should be extended to CAN-FD, but no need to elaborate
more on this at the moment: let's keep the discussion focused on
classical CAN and tackle this later.
Why do you want this?
You get of the bike and push your bicycle for what reason?
No.
Pushing the bike: yes, that was actually my motivation. One more time,
I am not telling you are wrong, just that I would do it in a different
way. I explained my view and you read it. I do not have more arguments
to bring.
/me either ...
https://youtu.be/aKnX5wci404?t=34
Ok, let me rephrase: Filling can_dlc with something else than a plain
length information 0..8 ;-)
Got it :-)
From my preliminary study, not so much changes or sanity check would
need to be added:
1. The current can_dlc sanity checks in can_send() and can_rcv() in
net/can/af_can.c definitely needs to be adjusted (but this would
also be the case for the raw_dlc field solution).
2. In the kernel Rx and Tx paths: the length should not be accessed
directly anymore but through a getter function.
3. In the kernel: drivers "DLC aware" need to adapt their code and
use the length getter function.
4. In user land: new "DLC aware" code should always use struct
can_frame for classical CAN and check whether can_dlc is greater
that CAN_MAX_DLEN before accessing data.
That's mostly all that would have to be adjusted.
No, it is not.
You have to go through all protocols, e.g. raw.c, bcm.c, gw.c, j1939,
isotp.c which get a skb to be (sometimes) cloned by them. And when they
need to modify the can_dlc value to send it to the userspace or
whatever, you need to modify skb->data - and then you can't clone it
anymore but need to skbcopy() them.
HERE you will get a really big effort. Occasionally I was also thinking
using can_dlc would be nice but then I looked into above code and your
use-case does not justify that effort/ risk and performance impact.
I naively thought this part would not get impacted. I have yet to
understand the full impact in detail but you made your point clear.
You won :-)
Sorry for the long exchange and thank you for your patience.
I really don't want to 'win'. But by the time the features and
functionalities have been grown and many people rely on its
functionality and performance.
The discussion helps to find the hopefully best solution and brings all
of us to new insights.
The difference is to make a new door into a house or to replace its
entire water system. You need a VERY good reason to replace the water
system ... when you want a new door.
The approach with CAN_CTRLMODE_RAW_DLC and the described testing would
be even robust against unintended miss-use.
Exception: can_dlc == 8 and a non-initialed raw_dlc gets a garbage
value between 9 and 15 from the stack (c.f. above comment).
Right, and what would be the effect of this?
You know the answer: Nothing breaks.
A frame of 8 bytes and DLC between 9 and 15 is sent on the wire and
this has the potential to crash the other devices on the bus as
explained in my initial e-mails.
Yes. But that is what you wanted to archive when you enable
CAN_CTRLMODE_RAW_DLC ¯\_(ツ)_/¯
(see my comment above)
Regards,
Oliver
