Re: [PATCH net-next] tcx: Fix splat during dev unregister

[Leon Romanovsky <leonro@nvidia.com>]

On Fri, Jul 28, 2023 at 11:47:17PM +0200, Daniel Borkmann wrote:
> From: Martin KaFai Lau <martin.lau@kernel.org>
> 
> During unregister_netdevice_many_notify(), the ordering of our concerned
> function calls is like this:
> 
>   unregister_netdevice_many_notify
>     dev_shutdown
> 	qdisc_put
>             clsact_destroy
>     tcx_uninstall
> 
> The syzbot reproducer triggered a case that the qdisc refcnt is not
> zero during dev_shutdown().
> 
> tcx_uninstall() will then WARN_ON_ONCE(tcx_entry(entry)->miniq_active)
> because the miniq is still active and the entry should not be freed.
> The latter assumed that qdisc destruction happens before tcx teardown.
> 
> This fix is to avoid tcx_uninstall() doing tcx_entry_free() when the
> miniq is still alive and let the clsact_destroy() do the free later, so
> that we do not assume any specific ordering for either of them.
> 
> If still active, tcx_uninstall() does clear the entry when flushing out
> the prog/link. clsact_destroy() will then notice the "!tcx_entry_is_active()"
> and then does the tcx_entry_free() eventually.
> 
> Fixes: e420bed02507 ("bpf: Add fd-based tcx multi-prog infra with link support")
> Reported-by: syzbot+376a289e86a0fd02b9ba@syzkaller.appspotmail.com
> Reported-by: Leon Romanovsky <leonro@nvidia.com>
> Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
> Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Tested-by: syzbot+376a289e86a0fd02b9ba@syzkaller.appspotmail.com
> ---
>  [ Sending directly to net-next given the issue was reported there by Leon. ]
> 
>  include/linux/bpf_mprog.h | 16 ++++++++++++++++
>  kernel/bpf/tcx.c          | 12 ++++++++----
>  2 files changed, 24 insertions(+), 4 deletions(-)
> 

Thanks,
Tested-by: Leon Romanovsky <leonro@nvidia.com>