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xfrm: destroy xfrm_state synchronously on net exit path
xfrm_state_put() moves struct xfrm_state to the GC list and schedules the GC work to clean it up. On net exit call path, xfrm_state_flush() is called to clean up and xfrm_flush_gc() is called to wait for the GC work to complete before exit. However, this doesn't work because one of the ->destructor(), ipcomp_destroy(), schedules the same GC work again inside the GC work. It is hard to wait for such a nested async callback. This is also why syzbot still reports the following warning: WARNING: CPU: 1 PID: 33 at net/ipv6/xfrm6_tunnel.c:351 xfrm6_tunnel_net_exit+0x2cb/0x500 net/ipv6/xfrm6_tunnel.c:351 ... ops_exit_list.isra.0+0xb0/0x160 net/core/net_namespace.c:153 cleanup_net+0x51d/0xb10 net/core/net_namespace.c:551 process_one_work+0xd0c/0x1ce0 kernel/workqueue.c:2153 worker_thread+0x143/0x14a0 kernel/workqueue.c:2296 kthread+0x357/0x430 kernel/kthread.c:246 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352 In fact, it is perfectly fine to bypass GC and destroy xfrm_state synchronously on net exit call path, because it is in process context and doesn't need a work struct to do any blocking work. This patch introduces xfrm_state_put_sync() which simply bypasses GC, and lets its callers to decide whether to use this synchronous version. On net exit path, xfrm_state_fini() and xfrm6_tunnel_net_exit() use it. And, as ipcomp_destroy() itself is blocking, it can use xfrm_state_put_sync() directly too. Also rename xfrm_state_gc_destroy() to ___xfrm_state_destroy() to reflect this change. Fixes: b48c05ab ("xfrm: Fix warning in xfrm6_tunnel_net_exit.") Reported-and-tested-by:<syzbot+e9aebef558e3ed673934@syzkaller.appspotmail.com> Cc: Steffen Klassert <steffen.klassert@secunet.com> Signed-off-by: