tcp: check congestion control restriction first

tcp_set_default_congestion_control() takes a reference to an algorithm
before checking whether a non-init network namespace may select it. A
rejected request returns -EPERM without dropping the reference.

This affects module-backed algorithms too; BPF is not required. For
example, after root loads tcp_bbr, a user with CAP_NET_ADMIN in a
non-init network namespace can repeatedly request it as the default
although it is not in tcp_allowed_congestion_control. Each rejected
write leaks a module reference. Since module->refcnt is an atomic_t,
2^31 such writes can make it negative; later module_put() calls warn,
and rmmod can hit the BUG_ON() in try_release_module_ref().

Check the namespace restriction before acquiring the reference, as
tcp_set_congestion_control() already does.

Fixes: 8d432592f30f ("net: Only allow init netns to set default tcp cong to a restricted algo")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/71c142b2d45af6a2fe7fc6cd4a5ca52c53afbe23.1793075984.git.zhilinz@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

0 files changed, 0 insertions, 0 deletions