Hello,

On 27.02.23 14:44, Florian Westphal wrote:

Alexander Atanasov [off-list ref] wrote:

quoted

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 <redacted>
---
  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();

Wouldn't it be better to just drop those list_empty() checks?
AFAICS __nf_tables_abort will DTRT in that case.

Ok, i will drop the checks.

You can still add a comment like the one you added above to make
it clear that we also need to wait for those readers to finish.

Ok.

Lastly, that list_del() in __nft_release_basechain should probably
be list_del_rcu()?

I am still in process of untwisting that place but so far.
Simple change to list_del_rcu wouldn't help as it wouldn't in 
__nft_release_table:

list_del(&rule->list);
ctx->chain->use--;
nf_tables_rule_release(ctx, rule) {
	nft_rule_expr_deactivate(ctx, rule, NFT_TRANS_RELEASE);
	nf_tables_rule_destroy(ctx, rule) {
		kfree(rule); <-- freed here
	}
}

List traversal would work but instead of crash it would become use after 
free.
Adding synchronize_rcu() before list iterattion there will probably do, 
it is already under commit_mutex when called from nf_tables_netdev_event.


-- 
Regards,
Alexander Atanasov