netfilter: x_tables: gpf inside xt_find_revision()

nested target/match_revfn() calls work with xt[NFPROTO_UNSPEC] lists
without taking xt[NFPROTO_UNSPEC].mutex. This can race with module unload
and cause host to crash:

general protection fault: 0000 [#1]
Modules linked in: ... [last unloaded: xt_cluster]
CPU: 0 PID: 542455 Comm: iptables
RIP: 0010:[<ffffffff8ffbd518>]  [<ffffffff8ffbd518>] strcmp+0x18/0x40
RDX: 0000000000000003 RSI: ffff9a5a5d9abe10 RDI: dead000000000111
R13: ffff9a5a5d9abe10 R14: ffff9a5a5d9abd8c R15: dead000000000100
(VvS: %R15 -- &xt_match,  %RDI -- &xt_match.name,
xt_cluster unregister match in xt[NFPROTO_UNSPEC].match list)
Call Trace:
 [<ffffffff902ccf44>] match_revfn+0x54/0xc0
 [<ffffffff902ccf9f>] match_revfn+0xaf/0xc0
 [<ffffffff902cd01e>] xt_find_revision+0x6e/0xf0
 [<ffffffffc05a5be0>] do_ipt_get_ctl+0x100/0x420 [ip_tables]
 [<ffffffff902cc6bf>] nf_getsockopt+0x4f/0x70
 [<ffffffff902dd99e>] ip_getsockopt+0xde/0x100
 [<ffffffff903039b5>] raw_getsockopt+0x25/0x50
 [<ffffffff9026c5da>] sock_common_getsockopt+0x1a/0x20
 [<ffffffff9026b89d>] SyS_getsockopt+0x7d/0xf0
 [<ffffffff903cbf92>] system_call_fastpath+0x25/0x2a

Fixes: 656caff2 ("netfilter 04/09: x_tables: fix match/target revision lookup")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/x_tables.c

@@ -330,6 +330,7 @@ static int match_revfn(u8 af, const char *name, u8 revision, int *bestp)
 	const struct xt_match *m;
 	int have_rev = 0;
 
+	mutex_lock(&xt[af].mutex);
 	list_for_each_entry(m, &xt[af].match, list) {
 		if (strcmp(m->name, name) == 0) {
 			if (m->revision > *bestp)
@@ -338,6 +339,7 @@ static int match_revfn(u8 af, const char *name, u8 revision, int *bestp)
 				have_rev = 1;
 		}
 	}
+	mutex_unlock(&xt[af].mutex);
 
 	if (af != NFPROTO_UNSPEC && !have_rev)
 		return match_revfn(NFPROTO_UNSPEC, name, revision, bestp);
@@ -350,6 +352,7 @@ static int target_revfn(u8 af, const char *name, u8 revision, int *bestp)
 	const struct xt_target *t;
 	int have_rev = 0;
 
+	mutex_lock(&xt[af].mutex);
 	list_for_each_entry(t, &xt[af].target, list) {
 		if (strcmp(t->name, name) == 0) {
 			if (t->revision > *bestp)
@@ -358,6 +361,7 @@ static int target_revfn(u8 af, const char *name, u8 revision, int *bestp)
 				have_rev = 1;
 		}
 	}
+	mutex_unlock(&xt[af].mutex);
 
 	if (af != NFPROTO_UNSPEC && !have_rev)
 		return target_revfn(NFPROTO_UNSPEC, name, revision, bestp);
@@ -371,12 +375,10 @@ int xt_find_revision(u8 af, const char *name, u8 revision, int target,
 {
 	int have_rev, best = -1;
 
-	mutex_lock(&xt[af].mutex);
 	if (target == 1)
 		have_rev = target_revfn(af, name, revision, &best);
 	else
 		have_rev = match_revfn(af, name, revision, &best);
-	mutex_unlock(&xt[af].mutex);
 
 	/* Nothing at all?  Return 0 to try loading module. */
 	if (best == -1) {