mptcp: sysctl: blackhole timeout: avoid using current->nsproxy

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mptcp: sysctl: blackhole timeout: avoid using current->nsproxy As mentioned in the previous commit, 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 'pernet' structure can be obtained from the table->data using container_of().

References

• https://git.kernel.org/stable/c/4c74fbdc5ab95b13945be01e6065940b68222db7
• https://git.kernel.org/stable/c/92cf7a51bdae24a32c592adcdd59a773ae149289