net: hsr: fix fill_frame_info() regression vs VLAN packets

Stephan Wurm reported that my recent patch broke VLAN support.

Apparently skb->mac_len is not correct for VLAN traffic as
shown by debug traces [1].

Use instead pskb_may_pull() to make sure the expected header
is present in skb->head.

Many thanks to Stephan for his help.

[1]
kernel: skb len=170 headroom=2 headlen=170 tailroom=20
        mac=(2,14) mac_len=14 net=(16,-1) trans=-1
        shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
        csum(0x0 start=0 offset=0 ip_summed=0 complete_sw=0 valid=0 level=0)
        hash(0x0 sw=0 l4=0) proto=0x0000 pkttype=0 iif=0
        priority=0x0 mark=0x0 alloc_cpu=0 vlan_all=0x0
        encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
kernel: dev name=prp0 feat=0x0000000000007000
kernel: sk family=17 type=3 proto=0
kernel: skb headroom: 00000000: 74 00
kernel: skb linear:   00000000: 01 0c cd 01 00 01 00 d0 93 53 9c cb 81 00 80 00
kernel: skb linear:   00000010: 88 b8 00 01 00 98 00 00 00 00 61 81 8d 80 16 52
kernel: skb linear:   00000020: 45 47 44 4e 43 54 52 4c 2f 4c 4c 4e 30 24 47 4f
kernel: skb linear:   00000030: 24 47 6f 43 62 81 01 14 82 16 52 45 47 44 4e 43
kernel: skb linear:   00000040: 54 52 4c 2f 4c 4c 4e 30 24 44 73 47 6f 6f 73 65
kernel: skb linear:   00000050: 83 07 47 6f 49 64 65 6e 74 84 08 67 8d f5 93 7e
kernel: skb linear:   00000060: 76 c8 00 85 01 01 86 01 00 87 01 00 88 01 01 89
kernel: skb linear:   00000070: 01 00 8a 01 02 ab 33 a2 15 83 01 00 84 03 03 00
kernel: skb linear:   00000080: 00 91 08 67 8d f5 92 77 4b c6 1f 83 01 00 a2 1a
kernel: skb linear:   00000090: a2 06 85 01 00 83 01 00 84 03 03 00 00 91 08 67
kernel: skb linear:   000000a0: 8d f5 92 77 4b c6 1f 83 01 00
kernel: skb tailroom: 00000000: 80 18 02 00 fe 4e 00 00 01 01 08 0a 4f fd 5e d1
kernel: skb tailroom: 00000010: 4f fd 5e cd

Fixes: b9653d1 ("net: hsr: avoid potential out-of-bound access in fill_frame_info()")
Reported-by: Stephan Wurm <[email protected]>
Tested-by: Stephan Wurm <[email protected]>
Closes: https://lore.kernel.org/netdev/Z4o_UC0HweBHJ_cw@PC-LX-SteWu/
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/hsr/hsr_forward.c

@@ -700,9 +700,12 @@ static int fill_frame_info(struct hsr_frame_info *frame,
 		frame->is_vlan = true;
 
 	if (frame->is_vlan) {
-		if (skb->mac_len < offsetofend(struct hsr_vlan_ethhdr, vlanhdr))
+		/* Note: skb->mac_len might be wrong here. */
+		if (!pskb_may_pull(skb,
+				   skb_mac_offset(skb) +
+				   offsetofend(struct hsr_vlan_ethhdr, vlanhdr)))
 			return -EINVAL;
-		vlan_hdr = (struct hsr_vlan_ethhdr *)ethhdr;
+		vlan_hdr = (struct hsr_vlan_ethhdr *)skb_mac_header(skb);
 		proto = vlan_hdr->vlanhdr.h_vlan_encapsulated_proto;
 	}