[PATCH] netfilter: nf_tables: always synchronize with readers before releasing tables

[Alexander Atanasov <alexander.atanasov@virtuozzo.com>]

general protection fault, probably for non-canonical
address 0xdead000000000115: 0000 [#1] PREEMPT SMP NOPTI
RIP: 0010:__nf_tables_dump_rules+0x10d/0x170 [nf_tables]

__nf_tables_dump_rules runs under rcu_read_lock while __nft_release_table
is called from nf_tables_exit_net. commit_mutex is held inside
nf_tables_exit_net but this is not enough to guard against
lockless readers. When __nft_release_table does list_del(&rule->list)
next ptr is poisoned and it crashes while walking the list.

Before calling __nft_release_tables all lockless readers must be done -
to ensure this a call to synchronize_rcu() is required.

nf_tables_exit_net does this in case there is something to abort
inside __nf_tables_abort but it does not do so otherwise.
Fix this by add the missing synchronize_rcu() call before calling
__nft_release_table in the nothing to abort case.

Fixes: 6001a930ce03 ("netfilter: nftables: introduce table ownership")
Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
---
  net/netfilter/nf_tables_api.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index d73edbd4eec4..849523ece109 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -10333,9 +10333,15 @@ static void __net_exit 
nf_tables_exit_net(struct net *net)
  	struct nftables_pernet *nft_net = nft_pernet(net);
   	mutex_lock(&nft_net->commit_mutex);
+	/* Need to call synchronize_rcu() to let any active rcu lockless
+	 * readers to finish. __nf_tables_abort does this internaly so
+	 * only call it here if there is nothing to abort.
+	 */
  	if (!list_empty(&nft_net->commit_list) ||
  	    !list_empty(&nft_net->module_list))
  		__nf_tables_abort(net, NFNL_ABORT_NONE);
+	else
+		synchronize_rcu();
  	__nft_release_tables(net);
  	mutex_unlock(&nft_net->commit_mutex);
  	WARN_ON_ONCE(!list_empty(&nft_net->tables));
-- 
2.31.1