net: flower: fix stack-out-of-bounds in fl_set_key_cfm()

Typical misuse of

	nla_parse_nested(array, XXX_MAX, ...);

array must be declared as

	struct nlattr *array[XXX_MAX + 1];

v2: Based on feedbacks from Ido Schimmel and Zahari Doychev,
I also changed TCA_FLOWER_KEY_CFM_OPT_MAX and cfm_opt_policy
definitions.

syzbot reported:

BUG: KASAN: stack-out-of-bounds in __nla_validate_parse+0x136/0x2bd0 lib/nlattr.c:588
Write of size 32 at addr ffffc90003a0ee20 by task syz-executor296/5014

CPU: 0 PID: 5014 Comm: syz-executor296 Not tainted 6.5.0-rc2-syzkaller-00307-gd192f5382581 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
print_address_description mm/kasan/report.c:364 [inline]
print_report+0x163/0x540 mm/kasan/report.c:475
kasan_report+0x175/0x1b0 mm/kasan/report.c:588
kasan_check_range+0x27e/0x290 mm/kasan/generic.c:187
__asan_memset+0x23/0x40 mm/kasan/shadow.c:84
__nla_validate_parse+0x136/0x2bd0 lib/nlattr.c:588
__nla_parse+0x40/0x50 lib/nlattr.c:700
nla_parse_nested include/net/netlink.h:1262 [inline]
fl_set_key_cfm+0x1e3/0x440 net/sched/cls_flower.c:1718
fl_set_key+0x2168/0x6620 net/sched/cls_flower.c:1884
fl_tmplt_create+0x1fe/0x510 net/sched/cls_flower.c:2666
tc_chain_tmplt_add net/sched/cls_api.c:2959 [inline]
tc_ctl_chain+0x131d/0x1ac0 net/sched/cls_api.c:3068
rtnetlink_rcv_msg+0x82b/0xf50 net/core/rtnetlink.c:6424
netlink_rcv_skb+0x1df/0x430 net/netlink/af_netlink.c:2549
netlink_unicast_kernel net/netlink/af_netlink.c:1339 [inline]
netlink_unicast+0x7c3/0x990 net/netlink/af_netlink.c:1365
netlink_sendmsg+0xa2a/0xd60 net/netlink/af_netlink.c:1914
sock_sendmsg_nosec net/socket.c:725 [inline]
sock_sendmsg net/socket.c:748 [inline]
____sys_sendmsg+0x592/0x890 net/socket.c:2494
___sys_sendmsg net/socket.c:2548 [inline]
__sys_sendmsg+0x2b0/0x3a0 net/socket.c:2577
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f54c6150759
Code: 48 83 c4 28 c3 e8 d7 19 00 00 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe06c30578 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f54c619902d RCX: 00007f54c6150759
RDX: 0000000000000000 RSI: 0000000020000280 RDI: 0000000000000003
RBP: 00007ffe06c30590 R08: 0000000000000000 R09: 00007ffe06c305f0
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f54c61c35f0
R13: 00007ffe06c30778 R14: 0000000000000001 R15: 0000000000000001
</TASK>

The buggy address belongs to stack of task syz-executor296/5014
and is located at offset 32 in frame:
fl_set_key_cfm+0x0/0x440 net/sched/cls_flower.c:374

This frame has 1 object:
[32, 56) 'nla_cfm_opt'

The buggy address belongs to the virtual mapping at
[ffffc90003a08000, ffffc90003a11000) created by:
copy_process+0x5c8/0x4290 kernel/fork.c:2330

Fixes: 7cfffd5 ("net: flower: add support for matching cfm fields")
Reported-by: syzbot <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Simon Horman <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Reviewed-by: Zahari Doychev <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

include/uapi/linux/pkt_cls.h

@@ -710,9 +710,11 @@ enum {
 	TCA_FLOWER_KEY_CFM_OPT_UNSPEC,
 	TCA_FLOWER_KEY_CFM_MD_LEVEL,
 	TCA_FLOWER_KEY_CFM_OPCODE,
-	TCA_FLOWER_KEY_CFM_OPT_MAX,
+	__TCA_FLOWER_KEY_CFM_OPT_MAX,
 };
 
+#define TCA_FLOWER_KEY_CFM_OPT_MAX (__TCA_FLOWER_KEY_CFM_OPT_MAX - 1)
+
 #define TCA_FLOWER_MASK_FLAGS_RANGE	(1 << 0) /* Range-based match */
 
 /* Match-all classifier */

net/sched/cls_flower.c

@@ -776,7 +776,8 @@ mpls_stack_entry_policy[TCA_FLOWER_KEY_MPLS_OPT_LSE_MAX + 1] = {
 	[TCA_FLOWER_KEY_MPLS_OPT_LSE_LABEL]    = { .type = NLA_U32 },
 };
 
-static const struct nla_policy cfm_opt_policy[TCA_FLOWER_KEY_CFM_OPT_MAX] = {
+static const struct nla_policy
+cfm_opt_policy[TCA_FLOWER_KEY_CFM_OPT_MAX + 1] = {
 	[TCA_FLOWER_KEY_CFM_MD_LEVEL]	= NLA_POLICY_MAX(NLA_U8,
 						FLOW_DIS_CFM_MDL_MAX),
 	[TCA_FLOWER_KEY_CFM_OPCODE]	= { .type = NLA_U8 },
@@ -1709,7 +1710,7 @@ static int fl_set_key_cfm(struct nlattr **tb,
 			  struct fl_flow_key *mask,
 			  struct netlink_ext_ack *extack)
 {
-	struct nlattr *nla_cfm_opt[TCA_FLOWER_KEY_CFM_OPT_MAX];
+	struct nlattr *nla_cfm_opt[TCA_FLOWER_KEY_CFM_OPT_MAX + 1];
 	int err;
 
 	if (!tb[TCA_FLOWER_KEY_CFM])