Question - bind(2) to local route on main routing table

Question - bind(2) to local route on main routing table

[Dhupar, Rishi]

Hi,

We recently discovered an inconsistency in the behavior of bind(2) with respect to a type local route being added to the main table. 

I was able to track down the issue and it appears it was introduced in this commit[1] which merged the local and main fib tries for performance and which are then later unmerged once the RPDB is modified.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0ddcf43d5d4a03ded1ee3f6b3b72a0cbed4e90b1

Synopsis: a user can bind(2) to any address within the prefix of a type local route that has been added to the main routing table

Short example on an untainted Ubuntu 22.04 machine
$ ip route add local 1.2.3.4/32 dev lo table main
$ nc -n -s 1.2.3.4 -l -p 9999                      # Succeeds
$ ip rule add table 100                            # This can be any change to RPDB
$ nc -n -s 1.2.3.4 -l -p 9999
Can't grab 1.2.3.4:9999 with bind : Cannot assign requested address

Note: This also impacts implicit bind behavior wrt to system calls such as connect(2).

Does this warrant further investigation and/or possibly a patch to disallow this behavior?

Regards,
Rishi Dhupar