[PATCH net 11/15] netfilter: x_tables: check target name before accessing verdict

[Pablo Neira Ayuso <pablo@netfilter.org>]

From: Fernando Fernandez Mancera <fmancera@suse.de>

Non-standard targets may contain only struct xt_entry_target.
mark_source_chains() nevertheless reads the target as an
xt_standard_target before checking its name, potentially reading four
bytes beyond a header-only target.

Check that the target is standard before reading its verdict.
Non-standard targets remain handled as fallthrough rules.

See the following KASAN trace:

BUG: KASAN: slab-out-of-bounds in translate_table+0x1660/0x1a40 [ip6_tables]
Read of size 4 at addr ffff88800ed56ff8 by task rep/432

Call Trace:
 kasan_report+0xfa/0x120
 translate_table+0x1660/0x1a40 [ip6_tables]
 do_ip6t_set_ctl+0x667/0xa76 [ip6_tables]
 nf_setsockopt+0x77/0xd0
 do_sock_setsockopt+0x118/0x490
 __sys_setsockopt+0xe4/0x160
 __x64_sys_setsockopt+0xbd/0x190
 do_syscall_64+0xde/0x4f0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Allocated by task 432:
 kasan_save_stack+0x2f/0x50
 __kasan_kmalloc+0x7f/0x90
 __kvmalloc_node_noprof+0x1b1/0x5c0
 xt_alloc_table_info+0x3c/0xa0
 do_ip6t_set_ctl+0x5ee/0xa76 [ip6_tables]
 nf_setsockopt+0x77/0xd0
 do_sock_setsockopt+0x118/0x490
 __sys_setsockopt+0xe4/0x160
 __x64_sys_setsockopt+0xbd/0x190
 do_syscall_64+0xde/0x4f0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: VEGA <vega@nebusec.ai>
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/ipv4/netfilter/arp_tables.c | 5 +++--
 net/ipv4/netfilter/ip_tables.c  | 5 +++--
 net/ipv6/netfilter/ip6_tables.c | 5 +++--
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c
index cf747e1e17a3..7ff3d1f0ffd4 100644
--- a/net/ipv4/netfilter/arp_tables.c
+++ b/net/ipv4/netfilter/arp_tables.c
@@ -371,11 +371,12 @@ static int mark_source_chains(const struct xt_table_info *newinfo,
 				e->counters.pcnt = pos;
 				pos += size;
 			} else {
-				int newpos = t->verdict;
+				int newpos;
 
 				if (strcmp(t->target.u.user.name,
 					   XT_STANDARD_TARGET) == 0 &&
-				    newpos >= 0) {
+				    t->verdict >= 0) {
+					newpos = t->verdict;
 					/* This a jump; chase it. */
 					if (!xt_find_jump_offset(offsets, newpos,
 								 newinfo->number))
diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
index 809441cedced..b302bd77180a 100644
--- a/net/ipv4/netfilter/ip_tables.c
+++ b/net/ipv4/netfilter/ip_tables.c
@@ -422,11 +422,12 @@ mark_source_chains(const struct xt_table_info *newinfo,
 				e->counters.pcnt = pos;
 				pos += size;
 			} else {
-				int newpos = t->verdict;
+				int newpos;
 
 				if (strcmp(t->target.u.user.name,
 					   XT_STANDARD_TARGET) == 0 &&
-				    newpos >= 0) {
+				    t->verdict >= 0) {
+					newpos = t->verdict;
 					/* This a jump; chase it. */
 					if (!xt_find_jump_offset(offsets, newpos,
 								 newinfo->number))
diff --git a/net/ipv6/netfilter/ip6_tables.c b/net/ipv6/netfilter/ip6_tables.c
index 313c4aac377a..010baaa60665 100644
--- a/net/ipv6/netfilter/ip6_tables.c
+++ b/net/ipv6/netfilter/ip6_tables.c
@@ -440,11 +440,12 @@ mark_source_chains(const struct xt_table_info *newinfo,
 				e->counters.pcnt = pos;
 				pos += size;
 			} else {
-				int newpos = t->verdict;
+				int newpos;
 
 				if (strcmp(t->target.u.user.name,
 					   XT_STANDARD_TARGET) == 0 &&
-				    newpos >= 0) {
+				    t->verdict >= 0) {
+					newpos = t->verdict;
 					/* This a jump; chase it. */
 					if (!xt_find_jump_offset(offsets, newpos,
 								 newinfo->number))
-- 
2.47.3