CVE-2024-26641: ip6_tunnel: make sure to pull inner header in __ip6_tnl_rcv()

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

ip6tunnel: make sure to pull inner header in ip6tnlrcv()

syzbot found ip6tnlrcv() could access unitiliazed data [1].

Call pskbinetmaypull() to fix this, and initialize ipv6h variable after this call as it can change skb->head.

[1] BUG: KMSAN: uninit-value in INETECNdecapsulate include/net/inetecn.h:253 [inline] BUG: KMSAN: uninit-value in INETECNdecapsulate include/net/inetecn.h:275 [inline] BUG: KMSAN: uninit-value in IP6ECNdecapsulate+0x7df/0x1e50 include/net/inetecn.h:321 INETECNdecapsulate include/net/inetecn.h:253 [inline] INETECNdecapsulate include/net/inetecn.h:275 [inline] IP6ECNdecapsulate+0x7df/0x1e50 include/net/inetecn.h:321 ip6ip6dscpecndecapsulate+0x178/0x1b0 net/ipv6/ip6tunnel.c:727 ip6tnlrcv+0xd4e/0x1590 net/ipv6/ip6tunnel.c:845 ip6tnlrcv+0xce/0x100 net/ipv6/ip6tunnel.c:888 grercv+0x143f/0x1870 ip6protocoldeliverrcu+0xda6/0x2a60 net/ipv6/ip6input.c:438 ip6inputfinish net/ipv6/ip6input.c:483 [inline] NFHOOK include/linux/netfilter.h:314 [inline] ip6input+0x15d/0x430 net/ipv6/ip6input.c:492 ip6mcinput+0xa7e/0xc80 net/ipv6/ip6input.c:586 dstinput include/net/dst.h:461 [inline] ip6rcvfinish+0x5db/0x870 net/ipv6/ip6input.c:79 NFHOOK include/linux/netfilter.h:314 [inline] ipv6rcv+0xda/0x390 net/ipv6/ip6input.c:310 netifreceiveskbonecore net/core/dev.c:5532 [inline] netifreceiveskb+0x1a6/0x5a0 net/core/dev.c:5646 netifreceiveskbinternal net/core/dev.c:5732 [inline] netifreceiveskb+0x58/0x660 net/core/dev.c:5791 tunrxbatched+0x3ee/0x980 drivers/net/tun.c:1555 tungetuser+0x53af/0x66d0 drivers/net/tun.c:2002 tunchrwriteiter+0x3af/0x5d0 drivers/net/tun.c:2048 callwriteiter include/linux/fs.h:2084 [inline] newsyncwrite fs/readwrite.c:497 [inline] vfswrite+0x786/0x1200 fs/readwrite.c:590 ksyswrite+0x20f/0x4c0 fs/readwrite.c:643 dosyswrite fs/readwrite.c:655 [inline] sesyswrite fs/readwrite.c:652 [inline] x64syswrite+0x93/0xd0 fs/readwrite.c:652 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0x6d/0x140 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+0x5e9/0xb10 mm/slub.c:3523 kmallocreserve+0x13d/0x4a0 net/core/skbuff.c:560 allocskb+0x318/0x740 net/core/skbuff.c:651 allocskb include/linux/skbuff.h:1286 [inline] allocskbwithfrags+0xc8/0xbd0 net/core/skbuff.c:6334 sockallocsendpskb+0xa80/0xbf0 net/core/sock.c:2787 tunallocskb drivers/net/tun.c:1531 [inline] tungetuser+0x1e8a/0x66d0 drivers/net/tun.c:1846 tunchrwriteiter+0x3af/0x5d0 drivers/net/tun.c:2048 callwriteiter include/linux/fs.h:2084 [inline] newsyncwrite fs/readwrite.c:497 [inline] vfswrite+0x786/0x1200 fs/readwrite.c:590 ksyswrite+0x20f/0x4c0 fs/readwrite.c:643 dosyswrite fs/readwrite.c:655 [inline] sesyswrite fs/readwrite.c:652 [inline] x64syswrite+0x93/0xd0 fs/readwrite.c:652 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0x6d/0x140 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x63/0x6b

CPU: 0 PID: 5034 Comm: syz-executor331 Not tainted 6.7.0-syzkaller-00562-g9f8413c4a66f #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023