CVE-2026-98287

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

pppoatm: ensure a writable skb header and linear data

In pppoatm_send(), LLC encapsulation checks whether there is sufficient
headroom for the 4-byte LLC header, but does not ensure that the skb header
is writable.

Normal transmit packets passing through ppp_start_xmit() have their header
unshared via skb_cow_head(). However, packets can also reach pppoatm_send()
via PPP channel bridging (PPPIOCBRIDGECHAN) without going through
ppp_start_xmit().

Use skb_cow_head() to ensure both sufficient headroom and a writable
header before pushing the LLC header.

While at it:
- Call pskb_may_pull(skb, 1) before inspecting skb->data[0] to prevent
  out-of-bounds reads on zero-length or non-linear frames (e.g. from
  bridging).
- Defer SC_COMP_PROT protocol compression until after pppoatm_may_send()
  succeeds. This eliminates the temporary skb allocation on admission failure
  and completely removes the fragile "undo" heuristic at the nospace label,
  avoiding any risk of reading uninitialized headroom or performing an
  unbalanced skb_push().