CVE-2024-26633: ip6_tunnel: fix NEXTHDR_FRAGMENT handling in ip6_tnl_parse_tlv_enc_lim()

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

ip6tunnel: fix NEXTHDRFRAGMENT handling in ip6tnlparsetlvenclim()

syzbot pointed out [1] that NEXTHDRFRAGMENT handling is broken.

Reading fragoff can only be done if we pulled enough bytes to skb->head. Currently we might access garbage.

[1] BUG: KMSAN: uninit-value in ip6tnlparsetlvenclim+0x94f/0xbb0 ip6tnlparsetlvenclim+0x94f/0xbb0 ipxip6tnlxmit net/ipv6/ip6tunnel.c:1326 [inline] ip6tnlstartxmit+0xab2/0x1a70 net/ipv6/ip6tunnel.c:1432 netdevstartxmit include/linux/netdevice.h:4940 [inline] netdevstartxmit include/linux/netdevice.h:4954 [inline] xmitone net/core/dev.c:3548 [inline] devhardstartxmit+0x247/0xa10 net/core/dev.c:3564 devqueuexmit+0x33b8/0x5130 net/core/dev.c:4349 devqueuexmit include/linux/netdevice.h:3134 [inline] neighconnectedoutput+0x569/0x660 net/core/neighbour.c:1592 neighoutput include/net/neighbour.h:542 [inline] ip6finishoutput2+0x23a9/0x2b30 net/ipv6/ip6output.c:137 ip6finishoutput+0x855/0x12b0 net/ipv6/ip6output.c:222 NFHOOKCOND include/linux/netfilter.h:303 [inline] ip6output+0x323/0x610 net/ipv6/ip6output.c:243 dstoutput include/net/dst.h:451 [inline] ip6localout+0xe9/0x140 net/ipv6/outputcore.c:155 ip6sendskb net/ipv6/ip6output.c:1952 [inline] ip6pushpendingframes+0x1f9/0x560 net/ipv6/ip6output.c:1972 rawv6pushpendingframes+0xbe8/0xdf0 net/ipv6/raw.c:582 rawv6sendmsg+0x2b66/0x2e70 net/ipv6/raw.c:920 inetsendmsg+0x105/0x190 net/ipv4/afinet.c:847 socksendmsgnosec net/socket.c:730 [inline] socksendmsg net/socket.c:745 [inline] syssendmsg+0x9c2/0xd60 net/socket.c:2584 syssendmsg+0x28d/0x3c0 net/socket.c:2638 syssendmsg net/socket.c:2667 [inline] dosyssendmsg net/socket.c:2676 [inline] sesyssendmsg net/socket.c:2674 [inline] x64syssendmsg+0x307/0x490 net/socket.c:2674 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0x44/0x110 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x63/0x6b

Uninit was created at: slabpostallochook+0x129/0xa70 mm/slab.h:768 slaballocnode mm/slub.c:3478 [inline] kmemcacheallocnode+0x5c9/0x970 mm/slub.c:3517 dokmallocnode mm/slabcommon.c:1006 [inline] kmallocnodetrackcaller+0x118/0x3c0 mm/slabcommon.c:1027 kmallocreserve+0x249/0x4a0 net/core/skbuff.c:582 pskbexpandhead+0x226/0x1a00 net/core/skbuff.c:2098 pskbpulltail+0x13b/0x2310 net/core/skbuff.c:2655 pskbmaypullreason include/linux/skbuff.h:2673 [inline] pskbmaypull include/linux/skbuff.h:2681 [inline] ip6tnlparsetlvenclim+0x901/0xbb0 net/ipv6/ip6tunnel.c:408 ipxip6tnlxmit net/ipv6/ip6tunnel.c:1326 [inline] ip6tnlstartxmit+0xab2/0x1a70 net/ipv6/ip6tunnel.c:1432 netdevstartxmit include/linux/netdevice.h:4940 [inline] netdevstartxmit include/linux/netdevice.h:4954 [inline] xmitone net/core/dev.c:3548 [inline] devhardstartxmit+0x247/0xa10 net/core/dev.c:3564 devqueuexmit+0x33b8/0x5130 net/core/dev.c:4349 devqueuexmit include/linux/netdevice.h:3134 [inline] neighconnectedoutput+0x569/0x660 net/core/neighbour.c:1592 neighoutput include/net/neighbour.h:542 [inline] ip6finishoutput2+0x23a9/0x2b30 net/ipv6/ip6output.c:137 ip6finishoutput+0x855/0x12b0 net/ipv6/ip6output.c:222 NFHOOKCOND include/linux/netfilter.h:303 [inline] ip6output+0x323/0x610 net/ipv6/ip6output.c:243 dstoutput include/net/dst.h:451 [inline] ip6localout+0xe9/0x140 net/ipv6/outputcore.c:155 ip6sendskb net/ipv6/ip6output.c:1952 [inline] ip6pushpendingframes+0x1f9/0x560 net/ipv6/ip6output.c:1972 rawv6pushpendingframes+0xbe8/0xdf0 net/ipv6/raw.c:582 rawv6sendmsg+0x2b66/0x2e70 net/ipv6/raw.c:920 inetsendmsg+0x105/0x190 net/ipv4/afinet.c:847 socksendmsgnosec net/socket.c:730 [inline] socksendmsg net/socket.c:745 [inline] syssendmsg+0x9c2/0xd60 net/socket.c:2584 syssendmsg+0x28d/0x3c0 net/socket.c:2638 syssendmsg net/socket.c:2667 [inline] dosyssendms ---truncated---