CVE-2024-50256: netfilter: nf_reject_ipv6: fix potential crash in nf_send_reset6()

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nfrejectipv6: fix potential crash in nfsendreset6()

I got a syzbot report without a repro [1] crashing in nfsendreset6()

I think the issue is that dev->hardheaderlen is zero, and we attempt later to push an Ethernet header.

Use LLMAXHEADER, as other functions in net/ipv6/netfilter/nfrejectipv6.c.

[1]

skbuff: skbunderpanic: text:ffffffff89b1d008 len:74 put:14 head:ffff88803123aa00 data:ffff88803123a9f2 tail:0x3c end:0x140 dev:syztun kernel BUG at net/core/skbuff.c:206 ! Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI CPU: 0 UID: 0 PID: 7373 Comm: syz.1.568 Not tainted 6.12.0-rc2-syzkaller-00631-g6d858708d465 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024 RIP: 0010:skbpanic net/core/skbuff.c:206 [inline] RIP: 0010:skbunderpanic+0x14b/0x150 net/core/skbuff.c:216 Code: 0d 8d 48 c7 c6 60 a6 29 8e 48 8b 54 24 08 8b 0c 24 44 8b 44 24 04 4d 89 e9 50 41 54 41 57 41 56 e8 ba 30 38 02 48 83 c4 20 90 <0f> 0b 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 RSP: 0018:ffffc900045269b0 EFLAGS: 00010282 RAX: 0000000000000088 RBX: dffffc0000000000 RCX: cd66dacdc5d8e800 RDX: 0000000000000000 RSI: 0000000000000200 RDI: 0000000000000000 RBP: ffff88802d39a3d0 R08: ffffffff8174afec R09: 1ffff920008a4ccc R10: dffffc0000000000 R11: fffff520008a4ccd R12: 0000000000000140 R13: ffff88803123aa00 R14: ffff88803123a9f2 R15: 000000000000003c FS: 00007fdbee5ff6c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000000 CR3: 000000005d322000 CR4: 00000000003526f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> skbpush+0xe5/0x100 net/core/skbuff.c:2636 ethheader+0x38/0x1f0 net/ethernet/eth.c:83 devhardheader include/linux/netdevice.h:3208 [inline] nfsendreset6+0xce6/0x1270 net/ipv6/netfilter/nfrejectipv6.c:358 nftrejectineteval+0x3b9/0x690 net/netfilter/nftrejectinet.c:48 exprcallopseval net/netfilter/nftablescore.c:240 [inline] nftdochain+0x4ad/0x1da0 net/netfilter/nftablescore.c:288 nftdochaininet+0x418/0x6b0 net/netfilter/nftchainfilter.c:161 nfhookentryhookfn include/linux/netfilter.h:154 [inline] nfhookslow+0xc3/0x220 net/netfilter/core.c:626 nfhook include/linux/netfilter.h:269 [inline] NFHOOK include/linux/netfilter.h:312 [inline] brnfpreroutingipv6+0x63e/0x770 net/bridge/brnetfilteripv6.c:184 nfhookentryhookfn include/linux/netfilter.h:154 [inline] nfhookbridgepre net/bridge/brinput.c:277 [inline] brhandleframe+0x9fd/0x1530 net/bridge/brinput.c:424 netifreceiveskbcore+0x13e8/0x4570 net/core/dev.c:5562 netifreceiveskbonecore net/core/dev.c:5666 [inline] netifreceiveskb+0x12f/0x650 net/core/dev.c:5781 netifreceiveskbinternal net/core/dev.c:5867 [inline] netifreceiveskb+0x1e8/0x890 net/core/dev.c:5926 tunrxbatched+0x1b7/0x8f0 drivers/net/tun.c:1550 tungetuser+0x3056/0x47e0 drivers/net/tun.c:2007 tunchrwriteiter+0x10d/0x1f0 drivers/net/tun.c:2053 newsyncwrite fs/readwrite.c:590 [inline] vfswrite+0xa6d/0xc90 fs/readwrite.c:683 ksyswrite+0x183/0x2b0 fs/readwrite.c:736 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xf3/0x230 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7fdbeeb7d1ff Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 c9 8d 02 00 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 1c 8e 02 00 48 RSP: 002b:00007fdbee5ff000 EFLAGS: 00000293 ORIGRAX: 0000000000000001 RAX: ffffffffffffffda RBX: 00007fdbeed36058 RCX: 00007fdbeeb7d1ff RDX: 000000000000008e RSI: 0000000020000040 RDI: 00000000000000c8 RBP: 00007fdbeebf12be R08: 0000000 ---truncated---