netfilter: nf_log: don't hold nf_log_mutex during user access

BugLink: https://bugs.launchpad.net/bugs/1784409

commit ce00bf07 upstream.

The old code would indefinitely block other users of nf_log_mutex if
a userspace access in proc_dostring() blocked e.g. due to a userfaultfd
region. Fix it by moving proc_dostring() out of the locked region.

This is a followup to commit 266d07cb ("netfilter: nf_log: fix
sleeping function called from invalid context"), which changed this code
from using rcu_read_lock() to taking nf_log_mutex.

Fixes: 266d07cb ("netfilter: nf_log: fix sleeping function calle[...]")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

net/netfilter/nf_log.c

@@ -437,14 +437,17 @@ static int nf_log_proc_dostring(struct ctl_table *table, int write,
 		rcu_assign_pointer(net->nf.nf_loggers[tindex], logger);
 		mutex_unlock(&nf_log_mutex);
 	} else {
+		struct ctl_table tmp = *table;
+
+		tmp.data = buf;
 		mutex_lock(&nf_log_mutex);
 		logger = nft_log_dereference(net->nf.nf_loggers[tindex]);
 		if (!logger)
-			table->data = "NONE";
+			strlcpy(buf, "NONE", sizeof(buf));
 		else
-			table->data = logger->name;
-		r = proc_dostring(table, write, buffer, lenp, ppos);
+			strlcpy(buf, logger->name, sizeof(buf));
 		mutex_unlock(&nf_log_mutex);
+		r = proc_dostring(&tmp, write, buffer, lenp, ppos);
 	}
 
 	return r;