Merge branch 'ipv6-flowlabel-per-netns-budget-for-unprivileged-callers'

Maoyi Xie says:

====================
ipv6: flowlabel: per-netns budget for unprivileged callers

From: Maoyi Xie <maoyi.xie@ntu.edu.sg>

This series fixes the cross-tenant DoS in net/ipv6/ip6_flowlabel.c.
v1 through v6 were single-patch postings, each in its own thread.
v6 review pointed out that the existing fl_size read in
mem_check() and the corresponding write in fl_intern() are not in
the same critical section. v7 split the work into 2 patches.

Patch 1/2 is a prerequisite. It moves spin_lock_bh(&ip6_fl_lock)
and the matching unlock from fl_intern() into its only caller
ipv6_flowlabel_get(), so the mem_check() call runs under the same
critical section as the fl_intern() insert. With all writers and
the read of fl_size under the lock, fl_size is converted from
atomic_t to plain int. This is independent of the per-netns
budget. It also makes 2/2 backportable without conflicts.

Patch 2/2 is the v6 patch, rebased on 1/2.

  - flowlabel_count is plain int rather than atomic_t, since the
    previous patch put all writers and readers under ip6_fl_lock.
  - In ip6_fl_gc(), fl_free() is now placed below the fl_size
    and flowlabel_count decrements, removing the v6 cache of
    fl->fl_net.
  - In ip6_fl_purge(), fl_free() stays in its original position.
    The function argument net is used for flowlabel_count.
  - mem_check() uses spaces around the / operator on all four
    expressions, addressing the checkpatch note in v6 review.

Numeric budget (preserved from v6):

  pre-patch:
    global non-CAP_NET_ADMIN budget = FL_MAX_SIZE - FL_MAX_SIZE/4
                                    = 4096 - 1024 = 3072
    per-actor reach                 = 3072

  post-patch:
    FL_MAX_SIZE doubled to 8192
    global non-CAP_NET_ADMIN budget = 8192 - 2048 = 6144
    per-netns ceiling               = 6144 / 2 = 3072
    per-actor reach                 = 3072 (preserved)

CAP_NET_ADMIN against init_user_ns still bypasses both caps.

Reproducer (KASAN VM, 4 cores, qemu): unprivileged netns A holds
3072 flowlabels via 100 procs. Fresh unprivileged netns B then
allocates 32 flowlabels (the FL_MAX_PER_SOCK ceiling for one
socket), the same as a clean baseline. Without the per-netns
ceiling, netns A could push fl_size past FL_MAX_SIZE - FL_MAX_SIZE
/ 4 and netns B would see allocations denied.
====================

Link: https://patch.msgid.link/20260506082416.2259567-1-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>