af_packet: fix vlan_get_tci() vs MSG_PEEK

Blamed commit forgot MSG_PEEK case, allowing a crash [1] as found
by syzbot.

Rework vlan_get_tci() to not touch skb at all,
so that it can be used from many cpus on the same skb.

Add a const qualifier to skb argument.

[1]
skbuff: skb_under_panic: text:ffffffff8a8da482 len:32 put:14 head:ffff88807a1d5800 data:ffff88807a1d5810 tail:0x14 end:0x140 dev:<NULL>
------------[ cut here ]------------
 kernel BUG at net/core/skbuff.c:206 !
Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 0 UID: 0 PID: 5880 Comm: syz-executor172 Not tainted 6.13.0-rc3-syzkaller-00762-g9268abe611b0 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
 RIP: 0010:skb_panic net/core/skbuff.c:206 [inline]
 RIP: 0010:skb_under_panic+0x14b/0x150 net/core/skbuff.c:216
Code: 0b 8d 48 c7 c6 9e 6c 26 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 3a 5a 79 f7 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:ffffc90003baf5b8 EFLAGS: 00010286
RAX: 0000000000000087 RBX: dffffc0000000000 RCX: 8565c1eec37aa000
RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
RBP: ffff88802616fb50 R08: ffffffff817f0a4c R09: 1ffff92000775e50
R10: dffffc0000000000 R11: fffff52000775e51 R12: 0000000000000140
R13: ffff88807a1d5800 R14: ffff88807a1d5810 R15: 0000000000000014
FS:  00007fa03261f6c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffd65753000 CR3: 0000000031720000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
  skb_push+0xe5/0x100 net/core/skbuff.c:2636
  vlan_get_tci+0x272/0x550 net/packet/af_packet.c:565
  packet_recvmsg+0x13c9/0x1ef0 net/packet/af_packet.c:3616
  sock_recvmsg_nosec net/socket.c:1044 [inline]
  sock_recvmsg+0x22f/0x280 net/socket.c:1066
  ____sys_recvmsg+0x1c6/0x480 net/socket.c:2814
  ___sys_recvmsg net/socket.c:2856 [inline]
  do_recvmmsg+0x426/0xab0 net/socket.c:2951
  __sys_recvmmsg net/socket.c:3025 [inline]
  __do_sys_recvmmsg net/socket.c:3048 [inline]
  __se_sys_recvmmsg net/socket.c:3041 [inline]
  __x64_sys_recvmmsg+0x199/0x250 net/socket.c:3041
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83

Fixes: 79eecf6 ("af_packet: Handle outgoing VLAN packets without hardware offloading")
Reported-by: syzbot+8400677f3fd43f37d3bc@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6772c485.050a0220.2f3838.04c6.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Chengen Du <chengen.du@canonical.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20241230161004.2681892-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/packet/af_packet.c

@@ -538,10 +538,8 @@ static void *packet_current_frame(struct packet_sock *po,
 	return packet_lookup_frame(po, rb, rb->head, status);
 }
 
-static u16 vlan_get_tci(struct sk_buff *skb, struct net_device *dev)
+static u16 vlan_get_tci(const struct sk_buff *skb, struct net_device *dev)
 {
-	u8 *skb_orig_data = skb->data;
-	int skb_orig_len = skb->len;
 	struct vlan_hdr vhdr, *vh;
 	unsigned int header_len;
 
@@ -562,12 +560,8 @@ static u16 vlan_get_tci(struct sk_buff *skb, struct net_device *dev)
 	else
 		return 0;
 
-	skb_push(skb, skb->data - skb_mac_header(skb));
-	vh = skb_header_pointer(skb, header_len, sizeof(vhdr), &vhdr);
-	if (skb_orig_data != skb->data) {
-		skb->data = skb_orig_data;
-		skb->len = skb_orig_len;
-	}
+	vh = skb_header_pointer(skb, skb_mac_offset(skb) + header_len,
+				sizeof(vhdr), &vhdr);
 	if (unlikely(!vh))
 		return 0;