bpf: reject stores into ctx via st and xadd
Alexei found that verifier does not reject stores into context via BPF_ST instead of BPF_STX. And while looking at it, we also should not allow XADD variant of BPF_STX. The context rewriter is only assuming either BPF_LDX_MEM- or BPF_STX_MEM-type operations, thus reject anything other than that so that assumptions in the rewriter properly hold. Add test cases as well for BPF selftests. Fixes: d691f9e8 ("bpf: allow programs to write to certain skb fields") Reported-by: NAlexei Starovoitov <ast@kernel.org> Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net> Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
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