[PATCH v1 net] ipv6: Fix null-ptr-deref in fib6_mtu().

[PATCH v1 net] ipv6: Fix null-ptr-deref in fib6_mtu().

[Kuniyuki Iwashima]

syzbot reported null-ptr-deref in fib6_mtu(). [0]

When res->f6i->fib6_pmtu is 0 in fib6_mtu(), it fetches MTU from
__in6_dev_get(nh->fib_nh_dev)->cnf.mtu6.

However, __in6_dev_get() could return NULL when the device is
being unregistered.

Let's return 0 MTU if __in6_dev_get() returns NULL in fib6_mtu().

[0]:
Oops: general protection fault, probably for non-canonical address 0xdffffc00000000bc: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x00000000000005e0-0x00000000000005e7]
CPU: 0 UID: 0 PID: 7890 Comm: syz.2.502 Tainted: G             L      syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:fib6_mtu net/ipv6/route.c:1648 [inline]
RIP: 0010:rt6_insert_exception+0x9eb/0x10a0 net/ipv6/route.c:1753
Code: 3b 14 cf f7 45 85 f6 0f 85 1d 02 00 00 e8 7d 19 cf f7 48 8d bb e0 05 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 89
RSP: 0000:ffffc9000610f120 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffc9000c001000
RDX: 00000000000000bc RSI: ffffffff8a38bc83 RDI: 00000000000005e0
RBP: ffff888052f06000 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff888042d16c00
R13: ffff888042d16cc8 R14: 0000000000000001 R15: 0000000000000500
FS:  0000000000000000(0000) GS:ffff88809717d000(0063) knlGS:00000000f540db40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00000000f73c6d50 CR3: 000000006eff0000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 __ip6_rt_update_pmtu+0x555/0xd60 net/ipv6/route.c:2982
 ip6_update_pmtu+0x34f/0x3b0 net/ipv6/route.c:3014
 icmpv6_err+0x2a2/0x3f0 net/ipv6/icmp.c:82
 icmpv6_notify+0x35e/0x820 net/ipv6/icmp.c:1087
 icmpv6_rcv+0x10bf/0x1ae0 net/ipv6/icmp.c:1228
 ip6_protocol_deliver_rcu+0xf97/0x1500 net/ipv6/ip6_input.c:478
 ip6_input_finish+0x1e4/0x4a0 net/ipv6/ip6_input.c:529
 NF_HOOK include/linux/netfilter.h:318 [inline]
 NF_HOOK include/linux/netfilter.h:312 [inline]
 ip6_input+0x105/0x2f0 net/ipv6/ip6_input.c:540
 ip6_mc_input+0x513/0xf50 net/ipv6/ip6_input.c:630
 dst_input include/net/dst.h:480 [inline]
 ip6_rcv_finish net/ipv6/ip6_input.c:119 [inline]
 NF_HOOK include/linux/netfilter.h:318 [inline]
 NF_HOOK include/linux/netfilter.h:312 [inline]
 ipv6_rcv+0x34c/0x3d0 net/ipv6/ip6_input.c:351
 __netif_receive_skb_one_core+0x12d/0x1e0 net/core/dev.c:6202
 __netif_receive_skb+0x1f/0x120 net/core/dev.c:6315
 netif_receive_skb_internal net/core/dev.c:6401 [inline]
 netif_receive_skb+0x13b/0x7f0 net/core/dev.c:6460
 tun_rx_batched.isra.0+0x3f6/0x750 drivers/net/tun.c:1511
 tun_get_user+0x1e31/0x3c20 drivers/net/tun.c:1955
 tun_chr_write_iter+0xdc/0x200 drivers/net/tun.c:2001
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:688
 ksys_write+0x12a/0x250 fs/read_write.c:740
 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:83 [inline]
 do_int80_emulation+0x141/0x700 arch/x86/entry/syscall_32.c:172
 asm_int80_emulation+0x1a/0x20 arch/x86/include/asm/idtentry.h:621
RIP: 0023:0xf715616b
Code: 57 56 53 8b 44 24 14 f6 00 08 75 23 8b 44 24 18 8b 5c 24 1c 8b 4c 24 20 8b 54 24 24 8b 74 24 28 8b 7c 24 2c 8b 6c 24 30 cd 80 <5b> 5e 5f 5d c3 5b 5e 5f 5d e9 f7 a1 ff ff 66 90 66 90 66 90 90 53
RSP: 002b:00000000f540d44c EFLAGS: 00000246 ORIG_RAX: 0000000000000004
RAX: ffffffffffffffda RBX: 00000000000000c8 RCX: 0000000080000640
RDX: 000000000000007a RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000292 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 </TASK>

Fixes: dcd1f572954f ("net/ipv6: Remove fib6_idev")
Reported-by: syzbot+01f005f9c6387ca6f6dd@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/69f83f22.170a0220.13cc2.0004.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv6/route.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 0dc0316530ca..e3d355d1fbd6 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1645,6 +1645,10 @@ static unsigned int fib6_mtu(const struct fib6_result *res)
 
 		rcu_read_lock();
 		idev = __in6_dev_get(dev);
+		if (!idev) {
+			rcu_read_unlock();
+			return 0;
+		}
 		mtu = READ_ONCE(idev->cnf.mtu6);
 		rcu_read_unlock();
 	}
-- 
2.54.0.545.g6539524ca2-goog

Re: [PATCH v1 net] ipv6: Fix null-ptr-deref in fib6_mtu().

[Ido Schimmel]

On Mon, May 04, 2026 at 06:43:13AM +0000, Kuniyuki Iwashima wrote:
> syzbot reported null-ptr-deref in fib6_mtu(). [0]
> 
> When res->f6i->fib6_pmtu is 0 in fib6_mtu(), it fetches MTU from
> __in6_dev_get(nh->fib_nh_dev)->cnf.mtu6.
> 
> However, __in6_dev_get() could return NULL when the device is
> being unregistered.
> 
> Let's return 0 MTU if __in6_dev_get() returns NULL in fib6_mtu().

[...]

> Fixes: dcd1f572954f ("net/ipv6: Remove fib6_idev")
> Reported-by: syzbot+01f005f9c6387ca6f6dd@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/69f83f22.170a0220.13cc2.0004.GAE@google.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Re: [PATCH v1 net] ipv6: Fix null-ptr-deref in fib6_mtu().

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon,  4 May 2026 06:43:13 +0000 you wrote:
> syzbot reported null-ptr-deref in fib6_mtu(). [0]
> 
> When res->f6i->fib6_pmtu is 0 in fib6_mtu(), it fetches MTU from
> __in6_dev_get(nh->fib_nh_dev)->cnf.mtu6.
> 
> However, __in6_dev_get() could return NULL when the device is
> being unregistered.
> 
> [...]

Here is the summary with links:
  - [v1,net] ipv6: Fix null-ptr-deref in fib6_mtu().
    https://git.kernel.org/netdev/net/c/5ad509c1fdad

You are awesome, thank you!
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