In the Linux kernel, the following vulnerability has been resolved: ipv6: ioam: refresh hdr pointer before ioam6_event() Reported by Sashi

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

ipv6: ioam: refresh hdr pointer before ioam6_event()

Reported by Sashiko:

In ipv6_hop_ioam(), the hdr pointer is initialized to point into the
skb's linear data buffer. Later, the code calls skb_ensure_writable(),
which might reallocate the buffer:

 1if (skb_ensure_writable(skb, optoff + 2 + hdr->opt_len))
 2	goto drop;
 3 
 4/* Trace pointer may have changed */
 5trace = (struct ioam6_trace_hdr *)(skb_network_header(skb)
 6				   + optoff + sizeof(*hdr));
 7 
 8ioam6_fill_trace_data(skb, ns, trace, true);
 9 
10ioam6_event(IOAM6_EVENT_TRACE, dev_net(skb->dev),
11	    GFP_ATOMIC, (void *)trace, hdr->opt_len - 2);

If the skb is cloned or lacks sufficient linear headroom,
skb_ensure_writable() will invoke pskb_expand_head(), which reallocates
the skb's data buffer and frees the old one, invalidating pointers to
it. While the code recalculates the trace pointer immediately after the
call to skb_ensure_writable(), it fails to recalculate the hdr pointer.

This patch fixes the above by recalculating the hdr pointer before
passing hdr->opt_len to ioam6_event(), so that we avoid any UaF.