netfilter: nft_log: perform module load from nf_tables

modprobe calls from the nf_logger_find_get() API causes deadlock in very
special cases because they occur with the nf_tables transaction mutex held.

In the specific case of nf_log, deadlock is via:

 A nf_tables -> transaction mutex -> nft_log -> modprobe -> nf_log_syslog \
	    -> pernet_ops rwsem -> wait for C
 B netlink event -> rtnl_mutex -> nf_tables transaction mutex -> wait for A
 C close() -> ip6mr_sk_done -> rtnl_mutex -> wait for B

Earlier patch added NFLOG/xt_LOG module softdeps to avoid the need to load
the backend module during a transaction.

For nft_log we would have to add a softdep for both nfnetlink_log or
nf_log_syslog, since we do not know in advance which of the two backends
are going to be configured.

This defers the modprobe op until after the transaction mutex is released.

Tested-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/net/netfilter/nft_log.c b/net/netfilter/nft_log.c
index a06a46b039c5ba..54f6c2035e84d7 100644
--- a/net/netfilter/nft_log.c
+++ b/net/netfilter/nft_log.c
@@ -128,6 +128,20 @@ static const struct nla_policy nft_log_policy[NFTA_LOG_MAX + 1] = {
 	[NFTA_LOG_FLAGS]	= { .type = NLA_U32 },
 };
 
+static int nft_log_modprobe(struct net *net, enum nf_log_type t)
+{
+	switch (t) {
+	case NF_LOG_TYPE_LOG:
+		return nft_request_module(net, "%s", "nf_log_syslog");
+	case NF_LOG_TYPE_ULOG:
+		return nft_request_module(net, "%s", "nfnetlink_log");
+	case NF_LOG_TYPE_MAX:
+		break;
+	}
+
+	return -ENOENT;
+}
+
 static int nft_log_init(const struct nft_ctx *ctx,
 			const struct nft_expr *expr,
 			const struct nlattr * const tb[])
@@ -197,8 +211,12 @@ static int nft_log_init(const struct nft_ctx *ctx,
 		return 0;
 
 	err = nf_logger_find_get(ctx->family, li->type);
-	if (err < 0)
+	if (err < 0) {
+		if (nft_log_modprobe(ctx->net, li->type) == -EAGAIN)
+			err = -EAGAIN;
+
 		goto err1;
+	}
 
 	return 0;