netfilter: log: work around missing softdep backend module

iptables/nftables has two types of log modules:

1. backend, e.g. nf_log_syslog, which implement the functionality
2. frontend, e.g. xt_LOG or nft_log, which call the functionality
provided by backend based on nf_tables or xtables rule set.

Problem is that the request_module() call to load the backed in
nf_logger_find_get() might happen with nftables transaction mutex held
in case the call path is via nf_tables/nft_compat.

This can cause deadlocks (see 'Fixes' tags for details).

The chosen solution as to let modprobe deal with this by adding 'pre: '
soft dep tag to xt_LOG (to load the syslog backend) and xt_NFLOG (to
load nflog backend).

Eric reports that this breaks on systems with older modprobe that
doesn't support softdeps.

Another, similar issue occurs when someone either insmods xt_(NF)LOG
directly or unloads the backend module (possible if no log frontend
is in use): because the frontend module is already loaded, modprobe is
not invoked again so the softdep isn't evaluated.

Add a workaround: If nf_logger_find_get() returns -ENOENT and call
is not via nft_compat, load the backend explicitly and try again.

Else, let nft_compat ask for deferred request_module via nf_tables
infra.

Softdeps are kept in-place, so with newer modprobe the dependencies
are resolved from userspace.

Fixes: cefa31a9d461 ("netfilter: nft_log: perform module load from nf_tables")
Fixes: a38b5b56d6f4 ("netfilter: nf_log: add module softdeps")
Reported-and-tested-by: Eric Dumazet <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nft_compat.c

···
 #include <linux/netfilter_bridge/ebtables.h>
 #include <linux/netfilter_arp/arp_tables.h>
 #include <net/netfilter/nf_tables.h>
+#include <net/netfilter/nf_log.h>
 
 /* Used for matches where *info is larger than X byte */
 #define NFT_MATCH_LARGE_THRESH	192
···
 	nft_compat_wait_for_destructors();
 
 	ret = xt_check_target(&par, size, proto, inv);
-	if (ret < 0)
+	if (ret < 0) {
+		if (ret == -ENOENT) {
+			const char *modname = NULL;
+
+			if (strcmp(target->name, "LOG") == 0)
+				modname = "nf_log_syslog";
+			else if (strcmp(target->name, "NFLOG") == 0)
+				modname = "nfnetlink_log";
+
+			if (modname &&
+			    nft_request_module(ctx->net, "%s", modname) == -EAGAIN)
+				return -EAGAIN;
+		}
+
 		return ret;
+	}
 
 	/* The standard target cannot be used */
 	if (!target->target)

net/netfilter/xt_LOG.c

···
 static int log_tg_check(const struct xt_tgchk_param *par)
 {
 	const struct xt_log_info *loginfo = par->targinfo;
+	int ret;
 
 	if (par->family != NFPROTO_IPV4 && par->family != NFPROTO_IPV6)
 		return -EINVAL;
···
 		return -EINVAL;
 	}
 
-	return nf_logger_find_get(par->family, NF_LOG_TYPE_LOG);
+	ret = nf_logger_find_get(par->family, NF_LOG_TYPE_LOG);
+	if (ret != 0 && !par->nft_compat) {
+		request_module("%s", "nf_log_syslog");
+
+		ret = nf_logger_find_get(par->family, NF_LOG_TYPE_LOG);
+	}
+
+	return ret;
 }
 
 static void log_tg_destroy(const struct xt_tgdtor_param *par)

net/netfilter/xt_NFLOG.c

···
 static int nflog_tg_check(const struct xt_tgchk_param *par)
 {
 	const struct xt_nflog_info *info = par->targinfo;
+	int ret;
 
 	if (info->flags & ~XT_NFLOG_MASK)
 		return -EINVAL;
 	if (info->prefix[sizeof(info->prefix) - 1] != '\0')
 		return -EINVAL;
 
-	return nf_logger_find_get(par->family, NF_LOG_TYPE_ULOG);
+	ret = nf_logger_find_get(par->family, NF_LOG_TYPE_ULOG);
+	if (ret != 0 && !par->nft_compat) {
+		request_module("%s", "nfnetlink_log");
+
+		ret = nf_logger_find_get(par->family, NF_LOG_TYPE_ULOG);
+	}
+
+	return ret;
 }
 
 static void nflog_tg_destroy(const struct xt_tgdtor_param *par)