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

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 266d07cb1c9a ("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: 266d07cb1c9a ("netfilter: nf_log: fix sleeping function calle[...]")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/net/netfilter/nf_log.c b/net/netfilter/nf_log.c
index 2c47f9e..a61d6df 100644 (file)
--- a/net/netfilter/nf_log.c
+++ b/net/netfilter/nf_log.c
@@ -446,14 +446,17 @@ 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;