CVE-2025-21641 – Linux Kernel MPTCP Sysctl Blackhole Timeout Use-After-Free Vulnerability

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().