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

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: sctp: sysctl: cookie_hmac_alg: 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, as this is the only member needed from the 'net' structure, but that would increase the size of this fix, to use '*data' everywhere 'net->sctp.sctp_hmac_alg' is used.

References

• https://git.kernel.org/stable/c/5599b212d2f4466e1832a94e9932684aaa364587
• https://git.kernel.org/stable/c/03ca51faba2b017bf6c90e139434c4117d0afcdc
• https://git.kernel.org/stable/c/86ddf8118123cb58a0fb8724cad6979c4069065b
• https://git.kernel.org/stable/c/3cd0659deb9c03535fd61839e91d4d4d3e51ac71
• https://git.kernel.org/stable/c/ad673e514b2793b8d5902f6ba6ab7e890dea23d5
• https://git.kernel.org/stable/c/f0bb3935470684306e4e04793a20ac4c4b08de0b
• https://git.kernel.org/stable/c/ea62dd1383913b5999f3d16ae99d411f41b528d4