sctp: sysctl: udp_port: avoid using current->nsproxy

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: sctp: sysctl: udp_port: avoid using current->nsproxy As mentioned in a previous commit of this series, using the 'net' structure via 'current' is not recommended for different reasons: - Inconsistency: getting info from the reader's/writer's netns vs only from the opener's netns. - current->nsproxy can be NULL in some cases, resulting in an 'Oops' (null-ptr-deref), e.g. when the current task is exiting, as spotted by syzbot [1] using acct(2). The 'net' structure can be obtained from the table->data using container_of(). Note that table->data could also be used directly, but that would increase the size of this fix, while 'sctp.ctl_sock' still needs to be retrieved from 'net' structure.

References

• https://git.kernel.org/stable/c/0a0966312ac3eedd7f5f2a766ed4702df39a9a65
• https://git.kernel.org/stable/c/e919197fb8616331f5dc81e4c3cc3d12769cb725
• https://git.kernel.org/stable/c/55627918febdf9d71107a1e68d1528dc591c9a15
• https://git.kernel.org/stable/c/5b77d73f3be5102720fb685b9e6900e3500e1096
• https://git.kernel.org/stable/c/c10377bbc1972d858eaf0ab366a311b39f8ef1b6