inet: ping: fix recent breakage

Blamed commit broke the assumption used by ping sendmsg() that
allocated skb would have MAX_HEADER bytes in skb->head.

This patch changes the way ping works, by making sure
the skb head contains space for the icmp header,
and adjusting ping_getfrag() which was desperate
about going past the icmp header :/

This is adopting what UDP does, mostly.

syzbot is able to crash a host using both kfence and following repro in a loop.

fd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_ICMPV6)
connect(fd, {sa_family=AF_INET6, sin6_port=htons(0), sin6_flowinfo=htonl(0),
		inet_pton(AF_INET6, "::1", &sin6_addr), sin6_scope_id=0}, 28
sendmsg(fd, {msg_name=NULL, msg_namelen=0, msg_iov=[
		{iov_base="\200\0\0\0\23\0\0\0\0\0\0\0\0\0"..., iov_len=65496}],
		msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0

When kfence triggers, skb->head only has 64 bytes, immediately followed
by struct skb_shared_info (no extra headroom based on ksize(ptr))

Then icmpv6_push_pending_frames() is overwriting first bytes
of skb_shinfo(skb), making nr_frags bigger than MAX_SKB_FRAGS,
and/or setting shinfo->gso_size to a non zero value.

If nr_frags is mangled, a crash happens in skb_release_data()

If gso_size is mangled, we have the following report:

lo: caps=(0x00000516401d7c69, 0x00000516401d7c69)
WARNING: CPU: 0 PID: 7548 at net/core/dev.c:3239 skb_warn_bad_offload+0x119/0x230 net/core/dev.c:3239
Modules linked in:
CPU: 0 PID: 7548 Comm: syz-executor268 Not tainted 6.0.0-syzkaller-02754-g557f050166e5 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022
RIP: 0010:skb_warn_bad_offload+0x119/0x230 net/core/dev.c:3239
Code: 70 03 00 00 e8 58 c3 24 fa 4c 8d a5 e8 00 00 00 e8 4c c3 24 fa 4c 89 e9 4c 89 e2 4c 89 f6 48 c7 c7 00 53 f5 8a e8 13 ac e7 01 <0f> 0b 5b 5d 41 5c 41 5d 41 5e e9 28 c3 24 fa e8 23 c3 24 fa 48 89
RSP: 0018:ffffc9000366f3e8 EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff88807a9d9d00 RCX: 0000000000000000
RDX: ffff8880780c0000 RSI: ffffffff8160f6f8 RDI: fffff520006cde6f
RBP: ffff888079952000 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000000000400 R11: 0000000000000000 R12: ffff8880799520e8
R13: ffff88807a9da070 R14: ffff888079952000 R15: 0000000000000000
FS: 0000555556be6300(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020010000 CR3: 000000006eb7b000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
gso_features_check net/core/dev.c:3521 [inline]
netif_skb_features+0x83e/0xb90 net/core/dev.c:3554
validate_xmit_skb+0x2b/0xf10 net/core/dev.c:3659
__dev_queue_xmit+0x998/0x3ad0 net/core/dev.c:4248
dev_queue_xmit include/linux/netdevice.h:3008 [inline]
neigh_hh_output include/net/neighbour.h:530 [inline]
neigh_output include/net/neighbour.h:544 [inline]
ip6_finish_output2+0xf97/0x1520 net/ipv6/ip6_output.c:134
__ip6_finish_output net/ipv6/ip6_output.c:195 [inline]
ip6_finish_output+0x690/0x1160 net/ipv6/ip6_output.c:206
NF_HOOK_COND include/linux/netfilter.h:291 [inline]
ip6_output+0x1ed/0x540 net/ipv6/ip6_output.c:227
dst_output include/net/dst.h:445 [inline]
ip6_local_out+0xaf/0x1a0 net/ipv6/output_core.c:161
ip6_send_skb+0xb7/0x340 net/ipv6/ip6_output.c:1966
ip6_push_pending_frames+0xdd/0x100 net/ipv6/ip6_output.c:1986
icmpv6_push_pending_frames+0x2af/0x490 net/ipv6/icmp.c:303
ping_v6_sendmsg+0xc44/0x1190 net/ipv6/ping.c:190
inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg+0xcf/0x120 net/socket.c:734
____sys_sendmsg+0x712/0x8c0 net/socket.c:2482
___sys_sendmsg+0x110/0x1b0 net/socket.c:2536
__sys_sendmsg+0xf3/0x1c0 net/socket.c:2565
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f21aab42b89
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff1729d038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f21aab42b89
RDX: 0000000000000000 RSI: 0000000020000180 RDI: 0000000000000003
RBP: 0000000000000000 R08: 000000000000000d R09: 000000000000000d
R10: 000000000000000d R11: 0000000000000246 R12: 00007fff1729d050
R13: 00000000000f4240 R14: 0000000000021dd1 R15: 00007fff1729d044
</TASK>

Fixes: 47cf8899 ("net: unify alloclen calculation for paged requests")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>