From:   Andrea Righi <andrea.righi@canonical.com>
To:     "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] ipv6: fix NULL pointer dereference in ip6_output()
Date:   Mon,  6 Dec 2021 17:34:47 +0100
Message-Id: <20211206163447.991402-1-andrea.righi@canonical.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

It is possible to trigger a NULL pointer dereference by running the srv6
net kselftest (tools/testing/selftests/net/srv6_end_dt46_l3vpn_test.sh):

[  249.051216] BUG: kernel NULL pointer dereference, address: 0000000000000378
[  249.052331] #PF: supervisor read access in kernel mode
[  249.053137] #PF: error_code(0x0000) - not-present page
[  249.053960] PGD 0 P4D 0
[  249.054376] Oops: 0000 [#1] PREEMPT SMP NOPTI
[  249.055083] CPU: 1 PID: 21 Comm: ksoftirqd/1 Tainted: G            E     5.16.0-rc4 #2
[  249.056328] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
[  249.057632] RIP: 0010:ip6_forward+0x53c/0xab0
[  249.058354] Code: 49 c7 44 24 20 00 00 00 00 48 83 e0 fe 48 8b 40 30 48 3d 70 b2 b5 81 0f 85 b5 04 00 00 e8 7c f2 ff ff 41 89 c5 e9 17 01 00 00 <44> 8b 93 78 03 00 00 45 85 d2 0f 85 92 fb ff ff 49 8b 54 24 10 48
[  249.061274] RSP: 0018:ffffc900000cbb30 EFLAGS: 00010246
[  249.062042] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8881051d3400
[  249.063141] RDX: ffff888104bda000 RSI: 00000000000002c0 RDI: 0000000000000000
[  249.064264] RBP: ffffc900000cbbc8 R08: 0000000000000000 R09: 0000000000000000
[  249.065376] R10: 0000000000000040 R11: 0000000000000000 R12: ffff888103409800
[  249.066498] R13: ffff8881051d3410 R14: ffff888102725280 R15: ffff888103525000
[  249.067619] FS:  0000000000000000(0000) GS:ffff88813bc80000(0000) knlGS:0000000000000000
[  249.068881] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  249.069777] CR2: 0000000000000378 CR3: 0000000104980000 CR4: 0000000000750ee0
[  249.070907] PKRU: 55555554
[  249.071337] Call Trace:
[  249.071730]  <TASK>
[  249.072070]  ? debug_smp_processor_id+0x17/0x20
[  249.072807]  seg6_input_core+0x2bb/0x2d0
[  249.073436]  ? _raw_spin_unlock_irqrestore+0x29/0x40
[  249.074225]  seg6_input+0x3b/0x130
[  249.074768]  lwtunnel_input+0x5e/0xa0
[  249.075357]  ip_rcv+0x17b/0x190
[  249.075867]  ? update_load_avg+0x82/0x600
[  249.076514]  __netif_receive_skb_one_core+0x86/0xa0
[  249.077231]  __netif_receive_skb+0x15/0x60
[  249.077843]  process_backlog+0x97/0x160
[  249.078389]  __napi_poll+0x31/0x170
[  249.078912]  net_rx_action+0x229/0x270
[  249.079506]  __do_softirq+0xef/0x2ed
[  249.080085]  run_ksoftirqd+0x37/0x50
[  249.080663]  smpboot_thread_fn+0x193/0x230
[  249.081312]  kthread+0x17a/0x1a0
[  249.081847]  ? smpboot_register_percpu_thread+0xe0/0xe0
[  249.082677]  ? set_kthread_struct+0x50/0x50
[  249.083340]  ret_from_fork+0x22/0x30
[  249.083926]  </TASK>
[  249.090295] ---[ end trace 1998d7ba5965a365 ]---

It looks like commit 0857d6f8c759 ("ipv6: When forwarding count rx stats
on the orig netdev") tries to determine the right netdev to account the
rx stats, but in this particular case it's failing and the netdev is
NULL.

Fallback to the previous method of determining the netdev interface (via
skb->dev) to account the rx stats when the orig netdev can't be
determined.

Fixes: 0857d6f8c759 ("ipv6: When forwarding count rx stats on the orig netdev")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 net/ipv6/ip6_output.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index ff4e83e2a506..7ca4719ff34c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -472,6 +472,9 @@ int ip6_forward(struct sk_buff *skb)
 	u32 mtu;
 
 	idev = __in6_dev_get_safely(dev_get_by_index_rcu(net, IP6CB(skb)->iif));
+	if (unlikely(!idev))
+		idev = __in6_dev_get_safely(skb->dev);
+
 	if (net->ipv6.devconf_all->forwarding == 0)
 		goto error;
 
-- 
2.32.0