bpf: Refuse packet slices for packet-changing helpers

The helpers which may change packet data do not take a packet pointer as a
memory argument: they have no pkt_access. They make the skb writable, or
make room in it, before they read what the argument points to, and that
can reallocate the head which it points into.

A slice from bpf_dynptr_slice() of the dynptr of the skb points into the
same head, but it is a PTR_TO_MEM, and the verifier lets it pass.
bpf_lwt_push_encap(), bpf_lwt_seg6_store_bytes() and bpf_lwt_seg6_action()
then read the old head, which has been freed. bpf_skb_store_bytes() follows
the head by now, but a slice is no better than a packet pointer there.

Refuse a slice of a packet where a packet pointer is refused for that
reason. All other helpers take slices as before. bpf_dynptr_write() and
bpf_dynptr_copy() can be used to copy inside of a packet.

Fixes: 2012c867c800 ("bpf: verifier: Accept dynptr mem as mem in helpers")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261017230950.802052-17-alexei.starovoitov@gmail.com

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