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Florian Westphal says: ==================== mptcp: reject invalid mp_join requests right away At the moment MPTCP can detect an invalid join request (invalid token, max number of subflows reached, and so on) right away but cannot reject the connection until the 3WHS has completed. Instead the connection will complete and the subflow is reset afterwards. To send the reset most information is already available, but we don't have good spot where the reset could be sent: 1. The ->init_req callback is too early and also doesn't allow to return an error that could be used to inform the TCP stack that the SYN should be dropped. 2. The ->route_req callback lacks the skb needed to send a reset. 3. The ->send_synack callback is the best fit from the available hooks, but its called after the request socket has been inserted into the queue already. This means we'd have to remove it again right away. From a technical point of view, the second hook would be best: 1. Its before insertion into listener queue. 2. If it returns NULL TCP will drop the packet for us. Problem is that we'd have to pass the skb to the function just for MPTCP. Paolo suggested to merge init_req and route_req callbacks instead: This makes all info available to MPTCP -- a return value of NULL drops the packet and MPTCP can send the reset if needed. Because 'route_req' has a 'const struct sock *', this means either removal of const qualifier, or a bit of code churn to pass 'const' in security land. This does the latter; I did not find any spots that need write access to struct sock. To recap, the two alternatives are: 1. Solve it entirely in MPTCP: use the ->send_synack callback to unlink the request socket from the listener & drop it. 2. Avoid 'security' churn by removing the const qualifier. ==================== Link: https://lore.kernel.org/r/20201130153631.21872-1-fw@strlen.de Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.